Published on July 13th, 2015 | by Guest Contributor


What It Takes To Create An Off-Grid Household In The Bay Area (California) Using Rooftop Solar & Battery Storage Only (Exclusive)

July 13th, 2015 by  

By Indradeep Ghosh, PhD
Cupertino, CA

Indradeep Photo thumbWith the recent announcement of the Tesla Powerwall battery pack, many articles have been written about the possibility of homeowners using it to defect from the grid and live off the grid using rooftop solar generation and battery storage for the times when the sun doesn’t shine. Though such an idea seems very attractive in theory, it comes with significant real-life challenges, especially in a world envisioned to be powered by renewable energy only. This article takes a closer look.

In order to investigate such a scenario, let us start with the energy usage profile of a net-positive household in Bay Area, California – the Energy+ household. This house has been covered in a local newspaper article few years back. It is a standard, all-electric, energy-efficient, 2200 sq. ft. house built to current code. It houses a family of four – two working adults and two kids. For the last four years, this household has remained net energy positive in day-to-day living using an 11 kW rooftop solar array and two EVs – a Chevy Volt which maintains 96% EV mode and a Ford Focus Electric. All EV charging is done at home. As a result, for four years running, this household has not burnt any natural gas, propane, wood, or gasoline in daily life. The local utility company PG&E sends a check to the family at the end of each yearly billing cycle for the excess energy generated.

Ener0 Ener1

The complete yearly energy usage and generation profile of the above house on an hourly basis is available for download from PG&E’s customer account using their green button facility. The monthly and average hourly usage data for 2014 are shown in the above graphs. The data does have some unevenness to account for the five weeks when the family was away on vacations. This data is left unchanged as this also comprises the usual energy usage profile of the family on a yearly basis. The average hourly usage profile for the various months very closely resembles the “duck curve” that California ISO is so worried about. It is apparent from the graph that there are two different problems of energy storage that have to be solved here. The first is the daily storage problem whereby the solar energy generated during the day has to be stored for use during the night. The second more significant problem is the seasonal energy storage problem where the excess solar energy generated during summer is banked with PG&E to be used during winter. The instantaneous peak load of the house is 25 kW, which a couple of single-phase inverters should have no problem handling.

To figure out how much battery storage will be needed, a simple software program is implemented that simulates a battery over this usage data. The program simply accumulates the negative generation numbers in the battery until it is fully charged. When the load turns positive, it starts using up the battery until the charge goes to zero. Then this cycle is repeated. To take a house off the grid, the battery state of charge should never go to zero throughout the whole year. Otherwise, the house runs out of energy to use. The battery simulations are started from summer and the year is wrapped around to give the battery enough time to charge fully before winter use.

Ener2Since the household used about 13,500 kWh for the whole year, the daily household energy use is averaged and the simulations are first run using the average daily storage requirement of 40 kWh. The resulting hourly usage profile for the various months is shown in the adjacent graph. It is clear from the curve that though this daily storage solves the problem for all the summer months from May to October, it is completely inadequate to serve the winter where considerable power is still drawn from the grid. To investigate how much storage is needed so that no power is drawn from the grid anytime during the whole year, the battery size is gradually increased and the simulations rerun. It turns out that the battery size needed to go off grid is a whopping 3018 kWh. Such a battery from Tesla will cost close to a million dollars and probably occupy a couple of two-car garages by volume! Clearly, this brute force approach is impractical as it stands now. So, is going off grid only with rooftop solar even feasible in the Bay Area?


The obvious problem over here is winter, when the solar generation is inadequate to service the daily needs of the household. The only way to solve this problem is to reduce winter consumption and increase winter generation. It turns out there is a lot of scope for reducing winter consumption in the household. About 60% of the winter consumption goes into space and water heating for which electric heat pumps are used. The current ones in the house were installed in 2010. Since then, more efficient heat pumps have arrived on the market. Particularly, the efficiency of the heat pump water heater has increased from 240% to 324% (loosely speaking). The average efficiency of the heat pump space heater has improved from 300% to 400%. Also, the heat pump space heater has a nasty stand-by load of 60 W which makes it waste almost 500 kWh every year. However, the current mini-split heat pumps have practically reduced the stand-by load to zero. Finally, the electric clothes dryer in the house can be replaced with a heat pump dryer recently introduced to the US market. Such a dryer will reduce dryer energy consumption by 50%. If the above three appliances are replaced in the house, the winter energy consumption can be reduced by 25% and the summer consumption by about 15%.


Satellite image of the maxed-out roof.

To increase generation, the solar array has to be oversized. Up till now, solar panels were expensive and the idea was to just cancel out yearly energy consumption completely. From the above discussion, this sizing strategy will not work as it will be deficient in winter generation. Since winter insolation in the Bay Area is half the summer insolation, for an off-grid configuration, the solar array should be sized as double what was necessary before, to balance consumption on a monthly basis. This will lead to huge over-generation in summer, where generation will have to be curtailed by disconnecting the array at times. Also, there can be the issue of running out of roof area. As can be seen from the adjacent picture, the roof is maxed out. However, if the current 15% efficient solar modules are replaced with the 22% efficient ones currently available on the market, the solar array size can be increased to 16 kW.

These two strategies are implemented in the simulator whereby the load is decreased and the generation increased in the usage profile according to the above possible scenarios and the resulting hourly usage profile for the various months is shown again without any battery. It can be observed now that there is substantial solar generation even in winter to offset the consumption on a monthly basis and huge over-generation in summer.


Next, the battery simulations are again run on this new load profile until there is no power drawn from the grid at any time. This time, the battery requirement is a much more manageable 349 kWh. The final resulting hourly load profile is shown in the adjacent graph. The estimated current cost of such a system with a 16 kW highly efficient solar array is about $150,000 and hence out of reach of all but a few.

However, within 10 years, installed solar costs are projected to drop to $1/watt and Li-on battery costs to about $100/kWh. In such a scenario, about $50,000 will yield a system that will provide all household energy practically for life throughout the year and will not need a grid. The current 5,000 cycle capacity in the battery will be adequate for more than a 50-year life as most of the battery will remain unused most of time.

Also, an intelligent battery charge/discharge controller is needed to use the battery cells evenly over time. Such a system will be much more feasible and cheaper as we go towards the tropics, where seasonal insolation variation is less. Conversely, it will be much more difficult to pull this off as we go more towards the northern latitudes and alternative strategies for winter renewable energy generation and seasonal energy storage will be needed. However, based on the above analysis, the possibility of going off the grid with only rooftop solar and battery for a Bay Area household is a dream that seems to be within reach in less than 10 years.

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  • plainview2

    You technical guys; I love you.
    Now would you do a projection on a monthly power bill of the future for an average residential customer who utilizes community utility generated solar.
    In today’s fossil fuel vs solar wars; I only see a solar electric future with the same consumer power bills.
    Without centralized generation;avoided costs(which should be figured in),line losses, etc, etc; will we see an average consumer power bill of 50-70% less for the “free” sunshine or will we see the same as the investors laugh all the way to the bank?
    This is the conversation which will also contribute to the progression of solar produced energy.

    • Vensonata

      U.S. “Sunshot” goals for PV. Utility 6cents kwh. Commercial 7.5c, residential 9cents kwh. That is for 2020. Utility has achieved it already. Neighborhood solar should be about commercial. That means $1.25 kw installed. Not quite yet, but even within a year you may see that. On your bill they will average that up to 10Cents kwh because of seasonal flucuations and some community storage costs. Any price below 14cents kwh should be considered a steal in the U.S. In Europe it would be considered “free energy”!

      • plainview2

        In my community currently have been at .10 to..12 on purchased fossil fuel but in a cooperative. Sorry without avoided cost including oil wars and foreign entanglements costing taxpayers cannot accept less than a 40-60% reduction for community solar.
        The people need a break. Solar is democracy not plutocracy.

  • nakedChimp

    I know exactly why I want a flat roof on my house.. and concrete block walls with insulation to the outside world.

    What is the white rectangle above the left solar array (~0.6×0.6 m2) – get rid of it!
    Should also be possible to squeeze in another row of panels at the top ridge there on each side and the small arrays at the backyard facing roof (facing south I assume). For this alone you could put in at least 12 more panels.. probably even twice that amount if one is really determined at what he does (accessibility might be totaly fubar then though 😉
    The placement of the panels just doesn’t look maxed-out to me, unless you got some rules that there must be 1 m space between roof edges and the array?!

    60 panels without trouble.. a tad more if you’re determined:

    11kWp / 40 panels gives 275 Wp per panel.. not to shabby. Even with Sunpowers you’re not going to get much more than 320 Wp per panel at a similar footprint.
    We got 42 panels at 250 Wp each were I am, but we didn’t leave that much space to the roof edge, although we’re in ‘cyclone country’ and need to account for winds up to 90 m/s.

    Anyhow, 60 panels at 275 Wp each should give you 16.5 kWp already.. if you’d go to 320 Wp per panel you’re at 19.2 kWp

    • Yes your estimates are correct. That spacing is required due to fire code. There is really no more area unless I move the vents to the north roof. That rectangle is a skylight and cannot be moved. The Sunpower 345 watts X series will do the trick. They are slightly smaller than these panels.

      • nakedChimp

        Skylights can be removed and replaced with LED lighting 😉

        And as per fire code, talk with your local man there and ask him if in a fire event the fighters would be hacking a hole in the street facing part of your north facing roof anyway (the one you wont plaster with panels) or if the firies will climb over your roof to the south backyard facing roof thats not visible from the street and hack a hole into that?!

    • GCO

      FWIW, I have close to 300 m2 of flat roof (including garage and overhangs), so one might I think I could put like 150+ PV modules up there.

      In reality, I max out at about 1/5 of that, plus a ~3 m2 thermal collector for hot water.

      First, PV modules shouldn’t be laid horizontally: they’d accumulate too much dirt. Tilting them means you need some clearance between rows, or go with a more costly east-west layout (this wasn’t easily doable in my case anyway due to the direction the roof support beams were going, and going south-north obviously isn’t interesting).

      Then, at least where I live in California, the fire dpt requires a 1m easement minimum on every edge: this shrinks the roof quite a bit and cuts big 2m gaps around any level changes (and my roof over the garage and bedrooms isn’t at the same height as the rest of the house).
      Skylights, chimneys, kitchen and bathrooms exhausts, pipes vents above all sinks and toilet further fragment the available space.

      Finally, in my case, the neighbor’s trees shade the last ~1/3 of my roof most of the time.

      I used an optimizers-based system (SolarEdge), to solve partial shading issues and have the flexibility to mix different modules orientations and even sizes (I use both 60 and 72-cell in the same system), but even with that, there’s no miracle, 33 is the max I can reasonably put on that seemingly large flat roof.

      And as @Indra mentioned, installing PV modules not hugging the roof required not just more-expensive racking, but detailed wind loading calculations to get the thing permitted; that was a pain.

      • nakedChimp

        At some point I would get rid of the hot water collector and replace the area with solar panels that will via heat exchanger deliver the same amount of hot water.. with electricity you got a more viable and flexible energy than just heat.

        I’m sorry for your 1m setback rule – in Oz we have bush fires + cyclones too, but I don’t remember needing any setback if the mountings are certified you can even go over the edge of the roof.
        What is the setback good for?
        Got it.. they want to walk around the pv modules?!!? For what?
        Sounds like BS to me.. never seen firefighters run around the edges of a roof for anything.. especially not residential.
        You might need to get this overthrown..

        As for your flat roof not being adequate designed for solar an installation – exactly right there, that’s what I’m talking about – solar is on the books for 5-10 years now as a way to get your electricity – any house that has been built since then without consideration of solar on the roof is a design failure in my eyes.
        Overhangs, crevices, hacked-up-roofs, chimneys, vents, antenna masts – all that stuff that make it hard to place a lot of solar on your roof – it’s absolutely idiotic in my eyes.

        Re red tape for custom private solar mounts – I’m sorry for you Americans – we in Oz got it better.
        We got hoops to jump through too, but not as high ones as you.. good luck.

        • Bob_Wallace

          Walking on the center of a roof whose integrity may have been compromised by a fire burning underneath is frowned on by those who wish to continue living.

          Solar panels tend to provide slippery footing which could lead to an unscheduled building dismounting.

          • nakedChimp

            So it’s better to run around the edges of a roof that has been compromised?
            If there has been a fire and the roof has been compromised, no one should go up there and walk around except for dismantling the roof and any attachments, but then the situation isn’t much different from building one and you have to have harnesses on anyway.
            I’m pretty sure if there has been a fire underneath one has to take off the attachments and cladding from a roof anyway to repair any damage.. don’t see how a 1m setback is going to help there.

            Also, if there is a fire in a house with pv on the roof in Australia it’s usually let to burn as no firefighter wants to be electrocuted anyway.. and even if they would fight the fire, then you don’t have fire damage water damage and with all the plastics and stuff in your house it will need to be completely refurbished as otherwise it’s a health hazard.

            I was a volunteer fireman for over 10 years. You know what I say is true so don’t pull an imagined rank on me buddy. Fire fighting 90% of the time is protecting adjacent structures.

            From here:

          • Bob_Wallace

            One can look at the overhang and see if it is burning. One can’t see beneath the ‘skin’ of a roof.

            It’s standard practice, when fighting a building fire, to chop a vent hole in the roof.

          • nakedChimp

            Well, here you go for Oz (page 13, 200 mm for this particular mounting system):

            And again, firefighters let it burn down as worst case is they’re being electrocuted, no matter if you got a main switch on the wall next to your inverter or not.
            If your house got a north facing sloped roof let them hack that one open and use the south facing roof space for panels to the max.. I’m sure if you talk with the local code man they will agree on this and allow your maxed out installation.

          • Bob_Wallace

            Apparently fires are fought differently here on the top half of the planet.

          • GCO

            I’m sure if you talk with the local code man …

            Ha ha, you’re funny.
            Ok, in case this wasn’t a joke: no, certainly not here.

            There is a set of rules, clearly spelled out and unambiguous. Everyone, and especially the people whose job it is to verify compliance, stick to them. As they should.

            Btw, I like that. I wouldn’t see why my neighbor would be allowed to do something deemed dangerous enough to be denied to anyone else, just for “talking to the right person”.

            Making rules bendable by some opens the door to all kinds of backroom deals, corruption etc, suspected or real. Definitely not the kind of environment in which I’d want the city/county/state to operate.

        • GCO

          That solar thermal collector produces at least as much as I could with PV + heat pump. It’s as simple as it can get, has no moving part, is completely silent, and will certainly last longer than any heat pump.

          Sorry if it’s not as high-tech as you might like, but in my situation (mild climate and limited usable roof space), thermal is a no-brainer.

          Re setbacks: firefighters here want to be able to safely vent buildings, and this requires access to the roof ridge line.
          To give you an idea:

          Last, I didn’t use custom mounts, just off-the-shelf racking. Regardless, the installer had to show that all the hardware used could resist whatever winds may possibly be expected, before the project even got permitted. And compliance was later verified.
          Yes it caused some delays, but in the end, I feel better knowing that all installers, including DIYers, are held to decent minimum standards.

          In the US (and fortunately probably nowhere else), you can be sure that some people would turn any accident involving solar into political rhetoric, so it’s good that risks are kept low.

      • nakedChimp

        “On the bright side, there is a provision in the code for alternate means of compliance. So if you are reasonable with the code official and present a viable option for fire access and vertical ventilation, they CAN approve it. Meaning, you could mount panels up to the ridge, where they will most often have the best exposure to the sun. Setting them down from the ridge can also cause problems in places where it snows a lot.”

        Last post from here:

        You probably have to jump through a couple more loops and hoops, but if you’re determined enough and want to max out your solar installation there is a way.

  • nakedChimp

    a 11 kW pv array? On a medium size house in the US? Are you frikking kidding me here?
    I live in a very tiny house at the moment and we already installed 10.5 kW solar and there would be space for 6 kW more..

    The morale of the story: trying to go offgrid with too small a supply (that just covers the day-to-day usage) is IDIOTIC at best..

    OVERBUILD SOLAR, get 30kW or more installed and run the numbers again. All of a sudden you don’t need that 1 Million Dollar battery anymore..


  • GCO

    Nice analysis. Now the part I don’t understand, why call going off-grid “a dream”?

    Manufacturing some 350 kW⋅h of batteries sure seems like an environmental nightmare, and over 100k$ today, or 35k$ within the next decade, a financial one.

    I realize that the above was just a thought experiment at this point, but even when prices drop to the level the author forecasts over the next decade, wouldn’t staying on grid be much greener, for both meanings of “green”?
    Not seeing half the production of those expensive high-efficiency PV modules go wasted would sure be nice too…

    • Mike Dill

      As Vensonata and I have noted elsewhere, a 4KW gas generator will fill the winter solar ‘holes’ for about $2000.00 up front and about $200.00 a year at the current gas price. Not the best ‘green’ answer, but it is mostly affordable now.
      Oh, and staying on the grid is ‘greener’ than wasting that battery on a once-a-year weather event.

    • This article as you said purely answers the question from a technical point of view. It does not say anything as to whether it should be done. I wrote it mainly to put a perspective on all this off-grid hype around the Powerwall battery. However, having said that $50K for energy for life does not look so bad – payback with utilities and gas is about 10 years only. There is also some perverse pleasure is not being beholden to utilities. I think the main reason is to have a choice, an alternative to the utility monopolies so that they can be kept in check and do not find ways to gouge customers with unreasonable grid connection fees. This is already happening in Arizona.

      • GCO

        Assuming 100$/kW⋅h storage (BMS/charger/inverter and installation included), 22%-efficient PV modules for 1$/watt (incl installation; tough because 1$/W is what the two installers I know of count just for labor, and I don’t think this will go down), the premium to go off-grid, vs a grid-tied system covering 100% of your energy usage, would cost you over 40k$.

        Toss the same 40k$ on your 3% mortgage, that’s 100$/month right there.

        A non-infinite battery life, say 20 years, would add another ~150$/month.

        By contrast, the fee Arizona’s largest utility, APS, now charges to customers going solar: 0.7$/month per kW of capacity. 70 cents.

        Sorry, I can’t see how going off-grid in a situation like yours can possibly make financial sense, even with all the extremely optimistic assumptions above.

        The environmental aspect is even worse. As Nissan’s life-cycle analysis about its Leaf shows [link], manufacturing batteries is energy- and resources-intensive. When then used on a vehicle, it’s offset by the gasoline it saves; not so when merely time-shifting solar, obviously.

        • The SRP in Arizona is proposing a $50-100 monthly grid connection fee. Also winter insolation is better in AZ and heat requirement lower. So battery can be cut in half. It will immediately make sense there.

          • GCO

            Even 100$ < 250$/2. So no, still not interesting. Even if the numbers were more favorable, again, the huge environmental footprint of such massive batteries would absolutely turn me away.

            Second, utilities are regulated. The ridiculous fee proposal you mentioned never got to pass; from what I could find, it averages ~5$/month [link].

            Last, selfishly going off-grid by effectively prepaying decades of imaginary sky-high grid fees is not something many people can do, let alone will. It’s an ineffective, yet very expensive, way of getting attention.

          • I cant post the link. Google “Salt River Project SolarCity lawsuit”. These fees are real and coming as utilities get alarmed by solar. The environmental footprint of batteries will only go down with time. The Giga factory is supposed to be energy neutral during operations. Grid upkeep, expansion, and maintenance and centralized generation also has an environmental footprint. With grid fees you are still at the mercy of utilities. Some people like to be in control of their energy. This same argument was made against solar when net metering was set in play. There are pros and cons on each side.

      • vensonata

        The average household energy cost in U.S. is $5500 per year. $2200 house energy and $3300 gas car. That gives you a lot of money to invest in solar and storage for house and car. Even $100,000 would be 18 year, payback.

        • GCO

          Now please tell us, how do you get those 100k$ in the first place? Borrow?

          Investing in a plug-in vehicle and/or PV system brings quick payback, plus the immediate satisfaction of doing something for the planet.

          On the other hand, I can’t see how buying massive batteries for no other reason than some bizarre obsession with cutting an existing grid connection, like in this article, makes any sense.

          • vensonata

            The reason is this: The entire grid must go renewable with storage. That means storage of some sort. Electrical storage will be one form, heat and cooling storage through water will be another. The house is merely a microcosm of the grid. It is like an electric car vs electric subway. Both are currently used. The off grid house which is purpose built will not involve “massive batteries”, it will just be well built to minimize power demand, and heating and cooling will use some water or ice storage. Nightime electrical use will be trivial, perhaps 3 kwh (This morning with 12 people in residence, my overnight electric demand was 3.7kwh…I check every morning! In winter on the longest night of the year at latitude 51…16 hours, the overnight use is 7 kwh. This does not include heat or hot water, which is not required at all through the night since the house stores that energy during the day.)
            A lithium battery at present requires 10% of its lifetime storage capacity to manufacture total from mining and transport to shipping etc. (Info from Argonne labs 2012) That percentage is falling.

            You regard an electric car as appropriate technology vs gasmobile, why not an electric house vs a gas house?

          • GCO

            I don’t understand your analogy, I don’t know of any house running on gasoline (maybe yours, and even then that’s probably only a tiny fraction of the time).
            But continuing this train of thought anyway: going off-grid is akin to digging its own oil well. In urban settings, it’s environmentally disastrous.

            You mentioned Li-ion requiring about 10% of its lifetime storage to manufacture. In the above case, that’d be 175 MW⋅h, or 13 years of @Indra’s family consumption.
            Going off-grid also requires oversizing the PV array 2 to 3×; ditto for the inverter(s) etc. More impact.

            For what? What would be the point?
            Why would anyone sane currently connected to the grid incur such high financial and environmental costs to cut that connection?

            Of course we need to go renewable, but it’s much more easily achieved by leveraging resources where they are (wind, hydro, geothermal, CSP, wave/tidal…), instead of only where the consumption is taking place (in cities, solar).

            With a mix of renewables, some dispatchable, the need for electrical storage is actually quite small, orders of magnitude lower than if everyone was trying to store their own PV like is imagined in the above article.
            Many European countries see very high renewable penetration rates already, with about zero batteries.
            Those guys explain it better than I can:


          • vensonata

            Yep, you are mostly right. What is happening though, is grid connected with PV and storage. Any place where the peak prices are high it will be used. Germany and Australia, maybe even California, certainly Hawaii and possibly New England. But in places like Germany you are talking about a dinky little battery of about 4 kwh! Their energy use is so low it makes the U.S. look ridiculous. About 3500kwh year. One third of U.S. Their night time use is perhaps 2-3 kwh. Their average monthly bills are also slightly less than American at about $92 vs $110. So Price per kwh is high and justifies battery on grid with PV (much cheaper PV though!)
            As far as off grid suburban the only reason is if the grid is terribly dirty and not likely to change. And even then you have to be motivated by a desire for independence and just having fun with the project. It has never been possible before about 2015, so the thought is just beginning to occur. And this article is the first speculation among many to come. As we see the tricky part is how to manage the winter. And what to do with all that summer excess. Can it be used somehow? Stored somehow? This is the puzzle.

          • GCO

            The grid being dirty would be an excellent reason (like we needed one more) to stay on it!

            Cutting the cord requires not only massive amounts of storage (in batteries and other means you described), but also a PV array sized for the worst week, and which will therefore overproduce like crazy the rest of the year.

            By staying connected to the grid, that overproduction won’t be wasted, but will help displace other, dirtier sources instead.
            (And, you get paid for it).

            I have neighbors who wouldn’t go solar, possibly for political reasons (like the sun would care). They don’t realize that a couple hours per day on average, the power they’re using comes from my roof anyway. 🙂

            My PV array cleans “their” grid too.

          • vensonata

            Yes it is true, exporting cleans the grid. As does efficiency, by reducing the demand. PV with battery makes the house disappear all together. It is simply a matter of seasonal balance to use that electricity rather than waste it.
            I suppose we have to imagine the grid as 100 houses. If they all have pv but no storage they overproduce and do not share then they need fossil fuel for the deficit. If they all have PV and battery, there is no need for a fossil fuel supply. If they have a communal battery that works too, they can export and import all clean energy. Or what if 30% have suitable roofs and 70% do not? That is more like our reality. Then at least 70% need some community solar and storage. In the end you need storage. Of course if you have hydro and wind, then less storage. I would far rather have no batteries but they are necessary somewhere in the formula.

  • Mike Dill

    For now, I think the 300 KWH battery is not feasible. In my situation, I am studying how to use my car battery as an emergency power supply for those days when there is not enough solar. I can charge the car elsewhere, at a cost, which might defer a significant amount of storage costs.

  • StefanoR99

    What year was your house built? Did you already maximise the efficiencies possible in terms of heating and cooling? What sort of glazing do you have on the house? What’s the r value of your ductwork and wall insulation? I’d love to go off grid but with my 1988 build house it would require sorting all that before I could even consider the investment…

    Also I find that a lot of energy consumption in the winter depends on how cold you can tolerate it. We are in the north bay – we will set to 68f / 20c (with minor grumbling from the wife) in the winter during the day and 66f / 18c at night before the heat kicks in. Makes a big difference. In the summer the ac won’t come on until it’s past 80f / 26.5c in the house.

    • It is in the link of the house. House is built to code in 1997. Added more insulation to walls and attic. Walls R30, attic is R50. Could not do anything in floor as it is slab. Ducts are insulated inside attic. Windows are double pane. All apparent leaks were sealed. Cooling is really minor. Heating is main issue. We use 70F in winter and 80F in summer.

      • nakedChimp

        Uh, you Americans and your imperial stuff.. you really had me there for a moment with your R30/50 values (any metric reading this look here: One thing you need to be careful with once you start insulating and air-tightening your old building is condensation. Especially the bathroom and kitchen exhausts need to be properly designed to avoid health-affecting mould.
        But even just wall/ceiling corners that are badly heated and ventilated and are exposed to the cold on the outisde (no matter how good you insulate) will cause problems..
        In the house back in Germany we had the water heating circulation feeding lines laid along the outside corners of the house, so this wasn’t a problem..

        • StefanoR99

          That’s not such a big problem in California but in the UK I remember with a modern build circa 2001 house even with all the added vents there would still be a build up of condensation and eventually mold on the windows. It’s such a damp country!

      • StefanoR99

        Had a look at the link, it’s very interesting. Wondering where the money would be better spent for an existing home at this point. Do you invest in upping the insulation of the home (triple glazing, better wall and roof insulation) or just stick a massive solar array and let that deal with the inefficiencies. You seem to have gone for the latter as you have a fairly modern build?

        • GCO

          I ran some numbers for my home and came up with a mix of both. Improving insulation on the roof/ceilings was easily done when reroofing.
          At least some walls contain ancient fiberglass batts, so I can’t easily add blown-in insulation. I’d need a proper audit to figure out whether it’s worth taking walls apart, but my guess is it won’t be, not in this mild climate.

          Better windows would help, as I have some large ones, but those are also very expensive; I put that money on an EV and solar PV instead.
          I’ll make up for the poor windows with better, insulating blinds, and a heat pump.

        • First the low hanging fruits to reduce energy should be done as they are cheap. From experience LEDs, reducing vampires, blow in cellulose insulation in walls and attic, double pane windows (may be even triple), leak and duct sealing all make sense. After that reducing energy gets into the area of diminishing returns and generating becomes cheaper. So after you have done all that you can monitor your usage and size the solar accordingly.

          • StefanoR99

            Yes, I would like to see what kw/h array would be needed for a house built with latest LEED standards would need to go offgrid…

      • Steven F

        Did you get a blow test done. That test can find all air leak including non apparent leaks. In my place one of the bigest leaks I found was in the new bouble pane windows installed by the previous owner. The weaping holes in the frame was a big air leak. My windows are well protected from rain so I sealed the weaping holes.

        My brother didn’t do much work sealing his walls but did add insulation in the sealing. That didn’t notice any change. He then unwrapped the duct insulation in the attic and then sealed all the seams in the metal duct work. He then re wrapped the ducts with the orignial insulation and noticed the Air conditioner ran less often sealing the ducts.

        What I have learned is that non apparent can be the biggest air leaks. A blow test could help find these leaks.

        • No did not do the blow test. One thing to note is that if the house is super airtight and approaches passive house standards then you will need a ventilator as otherwise air inside will become stale in winter. This will add to energy consumption. So there are many aspects to consider.

  • Steven F

    This home does not have solar thermal hot water heating. There is enough roof space for a hot water collector. With enough hot water storage most of the heating of water and air heating could be done by the solar thermal. Even in the winter there are a lot of sunny days there. The existing hot water heat pump would become the backup on cloudy days. Solar hot water heating is easily 70% efficient verses replacing all the solar panels with 22%efficient solar panels. Solar hot water heating to me looks a lot more cost effective than proposed changes.

    Additionally homes built to code in California and in many other states leak a lot of air through the walls. Reducing air leakage by better sealing of the walls would likely cut the home heating requirement in half.

    • The roof space remaining is North facing. It will receive almost no direct sunlight in winter. A solar thermal system is more expensive than PV + heat pump and requires more maintenance. 22% efficiency of the PV panels is multiplied 3.24X by the heat pump water heater to get almost 70% efficiency. The leakage, sealing and insulation was already done. Remember once you get a air source heat pump with 400% efficiency if you reduce direct heating energy by 1000 kWh you are just effectively reducing 250 kWh from the yearly consumption.

      • GCO

        There are many types of solar thermal collectors. Climates without hard freeze (like around Cupertino) can use simple setups like thermosiphons or integral collector storage (ICS) systems [link].

        Those have no moving part, require absolutely no maintenance and can last decades, much longer than heat pumps, so the economics look quite good actually.

        ICS are neither very large nor well insulated though, and so won’t stay hot over stretches of bad weather; they may not be adequate as only heat source in an off-grid, zero-fuel setup like imagined above.
        Passive solar heaters can be combined with extra storage and/or other heat sources however, including a heat pump. ICS are often simply added in front of the existing water heater.

      • Steven F

        “The roof space remaining is North facing. It will receive almost no direct sunlight in winter.”

        How much light does your south outside wall get in the winter? If there are no large trees in your neighbors property there is a good chance solar thermal panel will work mounted vertically.

        A relative of mine installed a solar hot water system in the 80’s. The existing gas hot water heater was then used as a backup. She lived in Oregon. The system had one small electric water pump. the system worked fine for fine for about 20 years with as fare as I know no maintenance. If a modern thermosiphons will work for you you would not need a water pump.

        • No free lunch. The south facing walls are almost all windows which I utilize to the hilt in winter to heat up my home during day time by opening all the curtains and blinds. In summer the blinds and curtains remain in place.

  • vensonata

    First of all, thanks for this article. I read about Ghosh’s home a while back and wondered whether he had done the numbers for off-grid. And of course he has, being an electrical engineer, with a Phd! Now some of the commenters have hit on some interesting points already, but this house really should be thoroughly explored as a model of what can be done. It is the jump from “net zero energy” to “100% electric off grid” that requires considerable re-thinking. I have run a large off grid community for the last 15 years and so have had to give this all considerable thought. The “off grid house” is actually a delightful project but there is much practical knowledge available that is extremely useful to this new possibility in the modern world.
    It is interesting especially that he is including two electric cars. His actual house usage is 9000kwh year which is not bad, so the car demand is an extra delicious challenge which I am glad he has taken up.
    I imagine the driving is about even throughout the year at 375 kwh/month. It is the high demand in winter for heating that seems to make a problem…no surprise to any off gridder, except those in the cooling dominated climate. Ghosh says he could reduce that load with better heat pumps by 25%. That is good. But it seems to be the “time of use” on a daily basis that makes his battery estimate so high. Ghosh needs to store energy in the form of heat during the day rather than as battery electricity for evening use.

    The family comes home and raises the temperature of the house and takes showers, which draw off the battery at that time in winter. They need to have a full tank of hot water from the direct solar in the day and turn off the heating element while they use the 80 gallons at night. The next day the tank heats while they are away. Also the house should be heated during the day when they are out, it is the opposite of ordinary grid use where you turn down the thermostat while you are out and turn it up when you come home from work. In other words, maximize energy use in “real time” while the sun is shining, minimize the use of “stored energy”. Unfortunately space for a full consideration of this new paradigm house is limited in the commentary section but this topic really deserves an extended forum and I hope some such format will arrive through Cleantechnica in the near future.

    • Thanks for the tips Vensonata. I am not sure though it will impact the battery size as the size seems to be dominated by the cloudy spells in winter when there is no solar. If there is good solar everyday then 70 kWh storage is enough even in the deep of winter. On a deep cloudy/rainy day the solar can generate as little as 4 kWh. However, I need to think about this more.

      • vensonata

        It is about storing every possible kwh, with 16 kw you will certainly overproduce on certain days even in December. That thermal battery will carry you for a week of bad weather. Then it begins recharging. Plug it that 400 kwh into your nice daily weather charts for the winter months and see how often you get overproduction.
        The other thing is panel angle. On Pv watts I see that your 11 kw pv array will produce 742 kwh more through the months of November to February at 50 degrees tilt than it will at 20 degrees. Example: in December 999 kwh vs 787, and in January 1052kwh vs 856. These are significant, but I do not know your present roof angle, though obviously tilt needs to be optimised for winter.

        • The roof pitch is about 22 degrees and the panels hug the pitch. Tilting on the roof is very difficult as then all the wind shear calculations come into effect making permitting a nightmare. Also we use about 80 gallons of hot water per day in winter. So we need to store 560 gallons to tide over a week of clouds. This means a big underground tank in the backyard and then heavily insulating that to maintain heat for 7 days – is that even possible in winter?

          • vensonata

            I have a 3000 gallon super insulated tank in the ground. That stores 800 kwh of heat. The heat loss over a month even in winter is very small because of surface to volume, it is only 8 feet in diameter. This is common with people who use outdoor or indoor wood boilers to provide a large storage so you just operate the stove a few hours per day. This is the same for solar and heat pump, you need to dump it all into a thermal battery. The tank is buried with styrofoam insulation to R 40 but it can also be in a basement, Thorsten Chlupp has 5000 gallon tank built into the house. As a storage medium water and insulation is cheapest. The heat pumps and solar are maximizing production but the storage is necessary. But electrical storage beyond about 50 kwh is probably unnecessary. By the way, notice that it is the last 5% that is tricky to get for energy! the first 95% is quite easy. That is why most people supply that last 3-5% by high efficiency furnace if they are off grid, it is trivial, but still not quite perfect is it?

          • Vensonata, I will talk to my friend in the city planning divisions to see what city codes are applicable in burying a huge underground tank in the backyard. As far as I know this is not used by anyone in Cupertino. However, codes can be amended along with future needs. I have a feeling retrofitting this wont be easy or cheap. From the satellite image you can see the backyard is not large either.

          • Vensonata

            About that water tank in your backyard. How many people in your neighborhood have swimming pools? A small pool is 40,000 gallons! How about hot tubs?
            Remember you don’t need a full seasons heat storage, just your deficit for winter. Or you can simply shave off the November and February heat deficit with a modest tank and s find another solution for December and January. At least we have “shrunk” the problem. But certainly water batteries and ice batteries are far cheaper than chemical batteries. I use both, by the way.

          • Another problem is that the heat pump water heater comes with an integrated 50 gallon tank and the heat pump operation is fine tuned based on temperature of the tank water. With this external 1000 gallon tank say there has to be come data communication between the water temperature in the tank, the solar panels, and the heat pump which will be kind of stand alone unit circulating the coolant. This will determine whether heat will be stored from the excess panel electricity or heat will be taken from the tank only in case of cloudy days. I do not know if this type of control system already exists. Do you have some links?

          • vensonata

            It might be best to dump all heat into the large tank with an air to water heat pump. Then both space heat and domestic hot water can be taken from that central source. Many people do it that way.
            If your winter deficit is a total of 2500kwh you could supply that with a large tank (of about 1000kwh at 180degrees. Plus a small ground mount pv rack on a 2 axis tracker devoted exclusively to the tank year round. A 3kw array on a single pole produces 6318 kwh a year in Cupertino. It produces 1853 kwh from October1 to feb 28. Through a heat pump multiplier of 3 it produces 5559 kwh exclusively in your winter heating season. Of course it also starts October 100% full and will remain as a producer of domestic hot water through March to September.
            Question: Does your utility give you a check for your excess power or only allow you to draw the same amount back during the year you used it? And do they cancel your excess at the end of each year if not used? That is a bad deal if they do it that way, and perhaps another reason to use as much of your own production as possible.

          • Ground mount PV in the city is usually not possible. Not enough open space and shade from adjacent structures. The tank temperature still needs to be controlled and monitored as it might superheat during vacations or summer. Blind dumping of heat into the tank will probably have some issues. The utility does give some money for excess solar generation but it is a pittance – 4c/kWh. They argue this is their wholesale electricity acquisition costs from elsewhere. The kWh counter is reset end of each year. Another problem is the space heating with the stored thermal hot water means radiant floor heating has to be used. My house uses forced air heating. Retrofitting radiant floor heating into slab foundation means effectively tear down and rebuild.

          • vensonata

            So the picture becomes clearer. You are losing 2300 kwh year in overproduction.The grid does not pay you for that. That overproduction is approximately your heating deficit per year, yes? So that needs to go into a tank of water. The tank will never overheat, it has a thermostat. Cut off 180 F. No need for radiant heat. Water to air is really simple.

            Basically it is almost there without any addition of solar, just storage in a water battery, and a small two day (48 kwh) lithium battery. You would virtually never run short.
            Take a look at calculations made yesterday by “Solarchoice. au” (The calculations are from July 15, 2015 on their website) they show similar graphs to yours for a “typical Sydney house which uses 25 kwh/day and will be grid independent. It requires 10 kwh array and 20 kwh battery with an average of 9kwh overproduction per day. They are warmer than San Francisco at 1800 heating degree days per year vs 2700. But interesting that this thought is almost simultaneous in Australia. They say a purpose built house requires only 15 kwh per day and a 7kw array plus 15 kwh battery would do it!

          • Not so fast. Those guys are only talking average day. They do not account for cloudy spells in winter and advise keeping a diesel generator connected for full off-grid. The 10 kW PV in Sydney will produce 35 kWh in winter. On some very cold winter days in SFO the daily consumption of my house shoots up to 50 kWh as the heat pumps go into overdrive. Similar stuff will happen in Sydney too. The 2300 kWh surplus I have happens in May-Aug. The deficit starts to happen in early Nov to Feb. However efficient the thermal tank, it cannot store heat for 5 months. More over sizing of the PV array is necessary.

          • vensonata

            “Even though it might seem possible for this household to disconnect from
            the grid and be fully energy independent, this would not generally be
            advisable without installing surplus solar and energy storage capacity
            to cover long periods of bad weather –” Solar Choice. So yes, there can be a cloudy week, and if you want absolute certainty you must install more storage…But how much? If they are covering 98% with 20 kwh. You see what I mean by the last 3% is the tricky part. I am at 97% with a 40 kwh AGM battery of which I can use 20. In order to get the last 3% I need about 30 more kwh battery. I am merely waiting for the Tesla to be available. But last winter, in a far worse climate than yours I ran the generator 35 hours. on about 20 occasions in December and January. I also have 12 kw pv. So we are closing in. But your case is special in that you are trying for 100% electric supplied total energy house and cars. That is the moon landing. But tantalizingly close with water storage plus battery and PV.

        • Formerly_Nom_De_Plume

          I am interested in how solar arrays perform in winter weather. How much of its rated capacity will an array produce on an overcast winter day? I have read estimates as high as 20%, which would mean a 16kwh array would be still be cranking out over 3kwh even on a gloomy day. Assuming 5 hours of peak daylight, that would be over 15kwh per day.

          Are my numbers even in the ballpark?

          • No not true. On a really overcast and rainy day I found my 11 KW array to produce only 3 kWh throughout the day. In winter we get about 30 kWh from the array on a bright sunny day. In summer it can go as high as 70 kWh.

          • nakedChimp

            Can you remount the panels to run horizontal and tilt them to become more winter optimized? This way the wind effect might be less of a problem (read it somewhere further up)?

            But yeah, Vensonata is absolutely right.. get a big thermal tank to store heat when it’s being produced and make it as big as you can. It’s the cheapest form of energy storage (thermal in water) and the biggest hog in domestic building energy usage (heating).

            And if you do the system right you can even use it in summer to cool the house.

      • Pawan Sharma

        So if this model house had been in say Arizona would it have been much different?

        • Yes definitely as winter insolation is higher and heat requirement is lower. So battery and excess solar can both be cut in half I think.

    • juxx0r

      That’s the best point yet on this thread. One needs to make hay whilst the sun shines.

      Additionally you can get up to 100% (of the input power) increase in efficiency from running your heat pump during the day when it’s 60F outside compared to at night when it’s 35F.

  • Deserttrek

    one must believe in the fraudulent climate change models to buy into any of this except for those in rural and/or wilderness areas .. even then a fossil fueled backup is rational.

    • vensonata

      Here we go…send in the clowns, cue the deniers entrance.

      • Deserttrek

        send in the bigots and name callers ….. can’t refute so you smear and name call …. grow up

        • GCO

          Most readers here understand at least basic science, so posts about the Earth being flat, the center of the universe and/or whatever other non-sense will be (deservedly) ridiculed.

          If you were instead expecting a point-by-point rebuttal/explanation, look e.g. here [this is a link].

          • Deserttrek

            Insults and a closed minded herd mentality prove nothing. In fact is shows you have no real basis for you fraud so you need to insult and twist facts.
            Good science and open minded researchers not tied to an agenda has shown the models to be incorrect, the data does not match the models …. now the predictions are for reduced solar activity.
            You can accuse others of non-sense and push your fraudulent agenda, that will not stop debate nor will it stop real science from looking deeper and further exposing a political agenda.

    • Bob_Wallace

      Sorry, this site is science based. Take your fantasy world somewhere else.

      • Deserttrek

        ok bob .. sadly you don’t want to look at real research and of course looking into the costs of solar integration and how net metering hurts many rate payers .. live in your bigoted world .. i would advise children and animals to stay away from you

  • BigWu

    I’m surprised by two things here:

    1. Why not switch from a heat pump to a ground source heap pump (GSHP, aka geothermal)? The latest easily best 500% efficiency for space heating (an 80% reduction compared to resistance heat and 40% from their current system)? GSHPs can also halve their hot water power demands.

    2. There’s no mention of time shifting hot water heating! A great deal of evening and morning loads are due to electric hot water heaters. Cooking up the water during mid day would slash both battery draws and daytime overproduction.

    • I tried going GSHP but it cost 50K to install and will save only 500 kWh per year over an air source one. Cheaper to slap on a solar panel to get the extra energy. Time shifting is not applicable here as the proposed house is off grid. It just reduces peak house load. This is not a botleneck. The battery storage is.

      • vensonata

        Absolutely correct. Air source has really replaced Ground source. In your climate especially air source heat pumps are very effective and economical. Word needs to get out about this. I would suggest some water storage with heat pump for your space heating. It doesn’t have to be huge, 1500 gallons can store 400 kwh of heat at 180 degrees F. down to 70, (110x 8.3 x 1500 = 1,369,500 BTU =401 kwh) This is really the “evening” effect that you need for heat through the winter/ The heat pump can of course quadruple your solar electricity for that purpose.
        There is a man in Alaska called Thorsten Chlupp who has come close to Passive house standards at 64 degrees north with about the same size house plus water storage.

  • Carsten Pedersen

    I am surprised to see that that it is proposed to replace an electric clothes dryer in the house with a heat pump dryer without discussing the possibility of simply using a clothes-line which uses no energy at all. I know that clothes-lines are frowned upon or even forbidden in some neighbourhoods but the obvious energy saving advantages ought to suffice in overcoming this.

    • Again great idea if time permits. With 4 people and 2 kids in house for us it does not work. It will save 300 kWh per year over the heat pump dryer.

      • Carsten Pedersen

        According to the Buildings Energy Data Book average consumption of a standard clothes dryer is 1000 kWh/year so instead of investing in a heat pump dryer and save 700 kwh/year you can save 1000 kWh/year by “investing” in clothes line and spend limited additional time.

      • GCO

        Well, I guess it depends on everyone’s specific situation.

        FWIW, I live just a bit further North, with spouse and 2 kids too, and doing perfectly fine with a few clothes drying racks (each the size of a small table, small enough to easily be moved around through doors and hallways).
        A decent washer surely helps: ours is a high-efficiency front-loader (like yours, I imagine), and its spin cycle is way more effective than the top-load it replaced.

        Another reason I tend to skip the dryer: I feel that it wears down clothes faster. I don’t know weather such extra wear would measurably add to our environmental footprint however.

      • nakedChimp

        I really have to wonder how my parents did this then.. (I don’t have a wife or kids yet).. they managed to dry our clothes although BOTH were working full time without a dryer and we were 2 kids.

  • Leonardo Amigoni

    Why not supplement with a $800, 400W Wind turbine on the roof. Which would supplement and 438kwh/year could get rid of the need of lots of the batteries needed during the winter. It would drive the cost down quite a bit. I guess the wind patterns of the area would need to be factored for a detailed analysis.

    • This is right inside a city. City codes do not permit. Would definitely help if allowed.

    • Steven F

      According to this map:

      Cupertino has essentially no wind resource at a 50 meter hub height. A wind turbine attached to the home would not produce 438kwh/year. Cupertino is in the wind shadow of the costal mountains.

  • George Harvey

    There should be some treatment of insulation, air sealing, window inserts, and other efficiencies in this article.
    In Vermont, where I live, it is possible to reduce heating costs to nearly nothing through efficiencies, while increasing comfort. Local contractors tell me they can go 90% of the way to passive at no additional cost over conventional heating systems, because there is no central heating unit and no chimney. The trick is great insulation and really great air sealing with a heat exchanger for ventilation.
    You can retrofit an old house to the same standard, but it costs a bit more. The house I live in is just a bit shy of being that good. My heating bill was cut 80% through efficiencies, some of which were very inexpensive (my landlady made the window inserts herself out of scrap lumber).
    Cutting 80% of the cost of heating might make the numbers change, I would guess.

    • This is there in the link of the house. I did all that. The space heating can be cut to about 2500 kWh per year. With a passive house it is about 700 kWh per year. However a passive house needs a heat recovery ventilator that draws almost 800 kWh per year. So effectively it is 1000 kWh more per year. Retrofitting existing houses to passive house standards is very expensive. Cheaper to slap on 2-3 more solar panels if space permits.

  • juxx0r

    First we have to get over the giant leap that is 24.6kWh a day for a house in what is a mild climate.

    In roughly the same climate, i use 3kWh/day.

    Also there was no optimisation of the angle of the solar panels for winter, just bolt some more on.

    • Word. x2

    • Leonardo Amigoni

      We use so much less than 24.6kWh in europe and we nearly have the same lifestyles. Also new homes here have manditory geothermal heating/cooling systems installed, brings heating and cooling energy down so much.

      • wattleberry

        Whereabouts in Europe,please? There is a huge climate and regulation variation here.

        • Leonardo Amigoni

          Italy, Lombardy Region

      • plainview2

        I guess our banksters operate in a different welfare system from the taxpayer. From what I understand your life style may actually have a higher happiness factor. I’m from Utah.

    • newnodm

      You don’t live on 3kWh with two EVs like he does.

      • juxx0r

        he doesn’t live on 24.6kW with 2 EVs, he lives on 24.6kW PLUS two EVs

        • Calamity_Jean

          I got the impression from the story that the 24.6 kWh included the EVs. It says, “All EV charging is done at home.”

          • juxx0r

            I suggest you scroll up to the first graph at the top, get out a calculator and prove it one way or the other.

          • juxx0r is right. It is 13500 kWh total for the year with EVs. So 37 kWh per day. 24.6 is for house only. Out of that 5 kWh per day is water heating, 8 kWh per day is space heating. 2 is wash/dry, 2 is cooking, 1.6 is refrigerator, 4 is unstoppable vampire loads, 1 lighting, 1 is electronics and appliances. Please refer to the house link for more details. The data there is bit old but in the ballpark.

          • Calamity_Jean

            You’re right, I was wrong. The 24.6 kWh per day is the average for the year, however. The house is electrically heated, so the use is much larger in winter and much lower in summer.

    • You have to consider house size – 2200 sq ft. This is median in US. House members – 4 and no other fuel source. That is no natural gas, gasoline, lpg or wood. Then 3Kwh per day is impossible.

      • juxx0r

        I’ve got a two story, 300m2 five bedroom, two bathroom house with two living areas, a billiard room, and a tennis court. Two people, 3kWh/day. Gas water heating. 3kWh/day. i use a piece of rope to dry my clothes.

        • Gas is measured in therms, You have to multiply by 30 to get kWh. Also there is cooking and space heating which you may not need but I do. Drying is 1 kWh per day – not much savings using clothes line.

          • juxx0r

            Round these parts we use Nm3 for gas.

            The fact is, you don’t NEED 24.6kWh, you make choices that require 24.6kWh a day.

            Make different choices.

          • Many people in India survive just fine without electricity. Even 3 kWh is not needed.

          • juxx0r

            Use those good people as your model.

          • Pawan Sharma

            I am from india. I live in a three bed room appartment. I don’t need heating but i have an energy efficient AC and a fridge and a tv. Plus other conveniences. I have a LPG connection for cooking but since giving up my subsidy for LPG i use my induction cooker predominantly. Our average daily usage is 10 kWh.

          • That is right in the ballpark of an efficient home. If you look at my energy profile you will see 60% of the house energy goes in water and space heating – 2 sinks that are almost not applicable in India.

          • juxx0r

            I just turned my heater on. It’s 18 degrees C outside, about 22 on the verandah. Just opened the doors to the verandah. That’s my heater.

          • Leonardo Amigoni

            Indra, have you considered solar water heaters then as an option? They are cheap and wildly used in Europe. Was wondering if that could bring the requirements down.

          • Yes a comparison chart is in the house link as to why I chose PV + heat pump water heater instead. The battery requirement will not go down as the bottleneck is cloudy spells in winter when that solar collector will have zero gain. The PV oversizing might be reduced a bit as now the passive collector assists the PV on sunny days.

    • Steven F

      I live a little north of that area. I live in a condo so I cannot install solar. Heat and water are gas. My electrical consumption was about 4to 5 Kwh a day before I got a volt. The Volt pushed my electrical usage to 11kwh a day. Two electric cars plus electric heat and hot water and the larger home could esily push the home to 24.6kwh a day

    • GCO

      In the same area, with the same size family, a smaller home and just one EV (but which travels ~14’000 miles/year), we’re in the same ballpark. The EV alone eats roughly 12 kW⋅h/day already.

      But not being off-grid, we’re easily “net zero” with half the number of solar modules (and a thermal collector for hot water).

      • juxx0r

        The EVs are additional to the 24.6kWh/day

        • GCO

          Aha, thanks for pointing that out, I was wondering where the discrepancy from the ~37 kW⋅h mentioned in the article was coming from.

          That changes things indeed: so our family is using less than half @Indra’s, for the same number of people and in the same area, however some of our heating, ~4 MW⋅h thermal, was from natural gas. Last winter was just crazy warm though.
          That’d add an average of 3 kW⋅h/day with an efficient heat pump, 11 with dumb resistive heater(s); that 2nd option would close the gap somewhat…

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