Clean Power

Published on September 30th, 2013 | by Guest Contributor


Is PV Better Than Thermal Solar Water Heating?

September 30th, 2013 by  

Originally published on the Hal Slater website.

Solar thermal water heating is a temperamental thing. Water weighs a lot, it expands when it freezes, and it can cause scaling damage to pipes when it boils. Solar thermal systems are wonderfully efficient, and some systems work just fine for decades, but even these need regular inspection. When a solar thermal system fails, however, it sets about destroying itself, and it has been clear for some time that solar thermal water heating is not the way of the future except for very low-end heat usage, like swimming pools.

For a long time now, the wisdom has been that the relative efficiency advantage of solar thermal technology for water-heating more than outweighs the convenience of electric water heating. The ability of solar thermal to collect more energy per square foot means that a solar electric system powering a conventional electric water heater alone will never compete with a solar thermal system.

Recently, however, reductions in solar electric (PV) costs and maturation of air-to-water heat pump technology have provided a new model: solar-electric assisted heat pump water heating (HPWH). HPWH comes with fewer drawbacks than solar thermal, with a smaller price tag for residential applications.

Middlebury College's Solar Array.

Middlebury College house’s solar array combines solar panels with a solar water heater on the roof of their entry in the U.S. Department of Energy Solar Decathlon 2011 in Washington, D.C., Monday, Sept. 26, 2011. (Credit: Stefano Paltera/U.S. Department of Energy Solar Decathlon)

The information below assumes the use of a heat pump water heater with an efficiency factor (EF) of 2.5 and an 1,800 kWh per year rating, with 1 to 1.3 kW of grid-tied PV added to existing installation or system in a region where the PV produces at least 1400 kWh/kW/year.

PV Advantages

Lower upfront cost – Given that lower-cost open systems have proven to be unsuitable for domestic water heating, the installed cost of solar thermal should be based upon a closed loop (glycol or drainback), two-tank (or storage plus tankless) system, fully installed. The average price for such a system, designed for a family of four, is between $7,000 and $10,000 before incentives. The PV powered heat pump water heater will cost between $1,000 and $2,000 for the heat pump plus labor and between $3,500 and $6,000 for the additional PV (to an existing grid-tied system), thus a total installed cost of between $5,000 and $8,500 before incentives.

Easier to install – Replacing a water heater with another single tank and adding three to five additional modules to a PV system is far easier than replacing a single tank with two tanks and piping heat transfer fluid to heavy rooftop panels that must be pressure tested and charged after installation. This results in fewer opportunities for installer error.

Uses less space – To avoid having the solar thermal system compete with the backup source (which limits the solar fraction to about 60%), two tanks are required: one for the backup, and one for the solar. It is possible to save space, at great expense, with the use of a tankless heater as long as the tankless heater can modulate the heat flow down to a very low point while being able to also meet maximum demand.

Needs no maintenance – The Achilles Heel of solar thermal is that if the system stops working, it does not just fail to produce energy: it sets about its own self-destruction. Without flow the panels can freeze or stagnate and overheat (see below). The electronic differential controller and circulator pump(s) must be inspected yearly to assure they are functioning properly and that no scale or corrosion have begun that will lead to system failure. The piping should also be checked, especially for drainback systems in older buildings that may settle over time and trap fluid in the lines. These annual inspections must be performed by a professional, and will cost half of the yearly gas savings.

Cannot freeze – Since a solar thermal panel can freeze at temperatures as high as 42ºF, freeze protection is required throughout the mainland US for solar thermal systems. With the exception of drainback systems, freeze protection systems are “active”. This means they require a device to operate in response to low temperature. As a consequence, and since they are rarely required to function, freeze protection failures are both common and catastrophic, resulting in thousands of dollars of damage to the collector array.

Cannot overheat – Overheating is a frequently overlooked problem with solar thermal systems. There is approximately twice as much solar energy delivered in July as in January. Thus, any system that will make a significant difference in hot water cost in January will over-perform in July. This results in periods of stagnation where there is no use for the solar heat and no flow through the panel(s). Under this condition, the panels will heat to around 400ºF inside. This can result in damage and accelerates the deterioration of the collector parts. There are radiator systems that have been added to panels to mitigate this effect, but there are no solid data on how much radiator is required to cool a stagnant collector on a hot day.

No scale build up – Scale is the #1 enemy of water heaters of any type. Heat makes dissolved solids precipitate from water where they collect on the hot surface. Even with the use of a transfer fluid on the collector side, scale can be a problem with the heat exchanger by clogging the tubes the water flows through to gain heat. The lower temperatures used to heat water with a heat pump reduces the tendency of scale to build up in the tank.

100% solar fraction attainable – Due to the vagaries of weather and the impracticality of storing large volumes of hot water, no solar thermal system that offers 100% reliability can have a 100% solar fraction. The systems most highly rated under the SRCC OG300 protocol have a 90% solar fraction. Using grid-tied PV as the solar source for the heat pump water heater allows the system to “store” power in the grid for use up to one year later. The price comparison above is based upon a thermal system with an 80% solar fraction versus a 100% PV offset for the water heating.

Grid demand management – Although heat pump water heating adds a load to the grid when used to replace a gas or propane unit, the PV adds power to the grid during peak daylight hours where it is most likely to be needed by the community. Most household hot water is used early in the morning and evening when there is less community-wide electric demand. If the utility elects to use this advantage, it could also add the ability to overheat the water heater through the smart meter when excess electricity is available on the grid. Used in conduction with a mixing valve to protect the house from scalding water, it effectively “banks” hot water and can delay the need for the heat pump to turn on.

No CO2 emissions – Any use of natural gas or propane, regardless of how efficient or cheap, results in the addition of CO2 to the atmosphere which is the #1 risk factor facing civilization today. A heat pump water heater that is 100% powered (or offset) by PV makes no contribution to that problem.


Net grid efficiency v. direct gas use – The standard presumption when comparing gas use with electric use is that, after accounting for conversion and transmission losses, it takes three units of fossil fuel energy (gas, oil, coal) to deliver one unit of electric energy. Thus the rationale that if gas can be delivered to the point of use, it is more efficient to use the gas than to use electricity. Since most fossil-fueled water heaters are only about 60% efficient, this effect is only half as significant as it appears. Additionally, fossil-fueled water heaters fail to take advantage of Renewable Portfolio Standards that further reduce the ratio of gas used to electricity delivered.

Warm air required – The efficiency of the heat pump water heater depends upon the available heat source which is usually the air in the space in which the heater is placed. Installed in unheated spaces in temperate climates, this presents no problem. However, if the water heater space is heated or drops below 55º-60ºF much of the year, the backup element will be needed and efficiency will suffer. Conversely, the heat pump water heater will cool and dehumidify the space in which it is located. This may be a desirable feature.

Newer on the market – Though air-to-water heat pump water heating uses only tried-and-true concepts, household HPWH has only had about twenty years of development on the consumer market: long enough to be confident in its efficiency and ease, but not long enough to be widespread. While there are about five hundred solar thermal models and six hundred tankless (“instant”) water heaters recognized by the DOE’s Energy Star system, there are currently only 23 recognized HPWH models.

Noise level – Since heat pumps use a compressor and air-source heat pumps need a fan to assure adequate air flow across the evaporator, heat pump water heater make about as much noise as a modern refrigerator. As with the air’s heat source issue, if the unit is located inside, the sound may be a minor nuisance.

For what it was, solar thermal technology represented an improvement. It does still have some legitimate applications, even. However, household-level solar water heating comes with so many unnecessary drawbacks that it is clear the future lies in another direction. Solar photovoltaic is a highly-effective source for a heat-pump water-heating system. Soon, that water-to-water heat pumps may be available on the market, but today’s air-to-water systems are the optimal selection for many households, depending on climate and configuration.

September 9, 2013 – I posted a link to this article on a number of LinkedIn group discussions and this is the best response received thus far:

This is a discussion that will have as many opinions as there are readers. One reason that thermal gets a bad rap is our past history in the 80s where open loop systems with freeze valves or sunspool valves failed and systems froze. Then we have had instances of glycol stagnation issues turning acidic and into a jello like substance which plugged the heat exchangers. Here in Florida, we are seeing issues with water quality for various reasons; scaling from calcium carbonate and copper corrosion from salt water intrusion to fresh water sources. Normally, calcium and chlorine are not huge issues in residential plumbing but when you pipe the water in an open loop system and run it through a collector that might stagnate to 250F, you accelerate the reaction rates dramatically.
Yet, we now have strategies that can handle these, ie closed loop drainback systems.
These take a bit more thought in the piping and pump placement but a properly designed system is almost maintenance free. Consider using the HTP stainless steel tank which has a lifetime warranty. The copper absorber collector in a drainback system can go for over 30 years since you are using the same water and not adding more contaminates.
Plus, consider that you need much less roof area for collectors. My home has a 4.86kW PV array which requires 27 panels. My thermal array is 2qty 4×8 panels and generates the same equivalent. Why would I use such a precious and sophisticated product like electricity, just to warm up water? The use of both technologies is really the best.
With regards to heat pump water heaters, New data suggests that there are some significant considerations including but not limited to:
1. Weight of the unit, in some cases the flooring needs to be reinforced
2. Maintenance, it is an air conditioner with a filter that needs replaced
3. Access, it can not be placed in a position next to a wall where the flow of air is impeded.
4. Design life; while the HVAC side will last quite a while, 15 years perhaps like a heat pump, the actual durability is only as good as the life of the steel tank, Then, you have an expensive replacement in 6 to 10 years vs solar with a SS tank which does not rust or a DX drainback system which uses a simple steel glass lined tank that is inexpensive
5. You need to add a condensate line like an A/C system. That may not always be convenient to route depending on where the tank is located in the home
6. It works best in hot climates. In northern areas, with cold water temperatures, the unit works mostly like a standard electric water heater since there is not enough latent heat energy in the surrounding air to transfer to water in the tank. this is the same reason that heat pumps are not commonly used in northern climates for space heating, the COP is too low to deliver any heat.
7. Who do you call for service? Plumbers can not service an HVAC system.

There has been much talk lately about a PV powered electric water heater but again, why take a technology that can generate a product which is so versatile ( electricity) and use it to warm up water?
For those of you that read this comment, thanks for your time and please keep the push on for solar water heating.

John L Alger, P.E.
Manager, Engineering and Commercial Projects
AET Solar, LLC

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  • johannjohann

    Back in the 70’s many home owners installed hot water systems to save money and many home owner got flooded with leaks.
    Such systems improved put there is still a potential for leaks.
    Controlling temperatures with water systems is more problematic.
    But water systems are more efficient.
    Installing pipes is more expensive.
    Then you have the weight of water to deal with.
    You can use it as hot water only.

    I think it is easier to install PV panels run wires and control power with a switch or thermostat.

  • zazu

    Didn’t know that this is a controversial subject.
    But I think that eveyone should do his own math and see what’s more usefull for is own case and what’s more cost-effective.
    Also I don’t tink anyone mentionned that in hot climates you’ll use Solar Heaters only on a small period and the system would be not used in summer, while you can use the installed extra-PV panels for both heating water in winter and for AC in summer.
    On the other hand, the achill’s hill for PV is energy storage, batteries are more expensive, while I think tank heat storage is less expensive for heating 1L, I still didn’t the math on this, I hope someone did and link it, it could be interesting, instead of factless arguing.

  • naaupono1

    A “hybrid” system consisting of a PV powered home with a solar water heating system backed up with an air heat pump water heater would do well in warm/hot weather locations.

  • logeshs

    Solar is really a very best choice when compared to gas or electric solar water heaters. It never emits pollution or gas like other resources and reduces risks of Global warming.

  • Julian.

    I believe the author has a point. In my country, for the cost of a solar thermal system, I could buy between 1 and 2 kw of PV and run it straight into a resistance heater (load matched). I wouldn’t have any pumps, extra pipework, leaks etc so it would actually be simpler and less intrusive. Also, although the efficiency is generally lower, it will maintain the same efficiency regardless of water temperature whereas a solar thermal loses efficiency as the water temperature increases. A waste of electricity ? not really, as I would have to use electricity (purchased) to heat the water anyway.

  • The choice is up to you but i prefer solar water heaters as it saves more than PV. It depends on your requirements. If you want to buy a water heater then you should refer following link:

  • Ann Shirley

    This is a very insightful post with a lot of valuable information. I work at a school that just had HVAC systems installed. I believe they are operated with thermal energy. I like them because they are more energy efficient and quieter than the pv ones. Since PV seems to offer newer technology I’m sure they’ll be advancing over the years and I’m interested to see how they modify them.

  • Olivia Heartelly

    Both solar powered water heating systems and photovoltaic systems as of recently have a great commercial background. Proficiency of energy increase of a sunlight based warm systems is in the vicinity of 80%, while photovoltaic is dependent upon 20% these days. PV is ordinarily better than sun powered warm on the grounds that it transforms a higher-quality energy power instead of low-grade heat.

  • Saranya

    Solar water heater is one of the best to boiling the water.
    It’s a cost effective. It is very useful for the human beings

  • Matthew Wright

    This article is spot on, but has missed the main point. If we get off gas which is a dirty fossil fuel. (ie rip up the gas networks and stop all this Fracking behaviour). Then we’ll need to move to electric hot water heating at least as our backup. We can’t afford to add 3.6kV loads to the grid at every abode across the globe. It’s a really expensive excercise in grid sizing. The simplest and cheapest way to go is with a $1000-$1500 heat pump hot water unit which optionally can have evacuated tubes (with a small controller and pump) fitted to it to extend the life of the pump/motor).

    Evac tubes will not deliver anywhere near the required heat every day (and flat panel systems are even worse in winter and should be banned as they’ve clearly been superseded). People have longer showers in winter (when evac tubes are delivering less) and shorter ones in summer (when there is ample solar thermal heat available). So if we backup with 3.6kV then we need about $10-20,000 worth of grid to cater for that decision. If we use a heat pump we only need $3,000-$6000 worth of grid. A 66% saving. It is just a no brainer.

    • QB

      Flat plate collectors should be banned? Flat plate collectors have been proven to last longer than vacuum tube collectors, double glazed flat plate collectors can produce quite a bit of heating in the winter rivalling evacuated tube collectors. Plus they are cheaper and are much less suseptible to be covered and blocked by snow. This is why flat plate solar thermal collectors still have the greatest market share in ALL parts of the world. Please please please stop looking at stories from the 70s and look at modern solar thermal systems. Electric water heating with PV still puts stress on the grid unless you use battery backups. Plus it is quite irresposible to use electricity for heating when it is such a versatile energy carrier.

      • Matthew Wright

        Stress on the grid. Electric heating with a heat pump has the same solar fraction ie 75% but the impacts on the grid are not all just in the height of winter as is the case with a solar thermal hot water service which will hit the grid with a large 3.6kVA capacity requirement for a few hours. Combine that with millions of customers and you’ve got expensive grid provisioning problems. It’s like building a 3 lane highway when you only need 1 lane (heat pump)

        Evacuated tubes are doing fine in China and all new installations are of this kind. Evacuated tube heat pipes cost $3 a tube ex china so they’re also cheap as chips.

        • QB

          Combined solar thermal with a heat pump is probably a better combination if you are going to look at it from that perspective. That’s a road that has been very well travelled by Northern Europe to abate grid stress in the winter.

          “Evacuated tube heat pipes cost $3 a tube ex china so they’re also cheap as chips.”

          Yes I am aware of those since I am a professional in the solar industry. Show me some of those cheap tubes that maintain vacuum for more than 2 to 3 years. I am talking about well designed and manufactured evacuated tube collectors from the likes of kingspan etc that have a 20 year warranty.

      • Bob_Wallace

        ” it is quite irresposible to use electricity for heating when it is such a versatile energy carrier.”

        There’s nothing irresponsible about using electricity to heat water as long as that electricity has a low carbon footprint.

        It comes down to economics, and one size probably doesn’t fit all. If you live in Antelope Valley with 360 days of sunshine then your best fit solution is likely to be different than someone living in Fairbanks.

        • QB

          I personally consider it irresponsible because electricity is a high quality energy carrier that can be used for other purposes. PV panels also inherently have a higher embodied energy and a higher carbon footprint on manufacture. However, I do agree with you that it is not one size fits all. However, modern solar thermal systems can be sized to provide most needs almost anywhere without all the silly drawbacks stated in the above article.

          • Bob_Wallace

            Where do you live? I’m guessing some place very sunny.

          • QB

            I actually reside in SWEDEN! Not very sunny at all. Today was around 9 degrees C and my double glazed flat plate solar thermal collectors were producing 60 C water with ease. This trend will continue on sunny days until the days get very short from November to February. After which there will be plenty warm water again. My back up will be a medium sized district heating system run on wood pellets which are primarily waste from the timber industry or combustible municiple waste. Oh and I have PV that run the pumps for my collectors.

          • Bob_Wallace

            How many people do you think would install a pellet stove as backup for a solar water heater?

            Do you not see that as part of the cost?

          • QB

            Please don’t misunderstand me. Of course this is a part of the cost, though comparatively marginal since the pellet boiler is a community owned installation specially installed for our district. However, this is what works for me and what is cost effective for me. Some people have heat pumps, though most are ground source heat pumps rather than air source since at the coldest times of the year air source heat pumps are so poor that one needs to use an electrical or pellet fired back up. Not to mention electricity prices tend to sky rocket around the coldest periods. This is what works for me here.

            This takes me back to the purpose of my original post. The article above basically says; solar thermal systems don’t work and so PV and heat pumps are the way to go. What I am saying is that’s not true. Modern solar thermal systems do work and they are suitable for some people not everyone. There’s no silver bullet or one size fits all. Here in Sweden we have rubbish feed-in tariffs so if you don’t utilise all your solar electricity as it is produced sending it to the grid is an economical loss. So those of us like me who have a very low daytime usage are basically screwed and would have to resign to a rather small PV system. Secondly, our main energy demand is for space and water heating. So with a low temperature heating system installed (i.e floor heating in my bathroom and bedroom). Solar heat is perfect for me during spring, summer and autumn. When winter hops around I get some heat from the district system (which is metered) so I take as much as I need and leave the rest for the rest of the community 🙂

            I would like to reiterate that this is what is economically viable for me, not necessarily for everyone!

  • wattleberry

    After so much attention having been concentrated on PV and wind renewables it’s a relief to see some serious evaluation of the lower tech thermal heaters which. along with improving insulation, have been largely overlooked for too long-the ‘honeymoon period ? ‘. Their most obvious contribution is to ameliorate the effect of the dark hours, again reducing the dependence on the developing and inevitably highly sophisticated battery storage requirements.

    Many thanks. Zachary, for re-opening this debate.

  • QB

    As was said above, most of the problems associated with solar thermal systems were based on highly publicised events of the 70s and 80s, when solar thermal systems were (largely) being done by backyard tinkerers and novices. Focusing on trial and error and empiricism rather than thorough expermentation and engineering. With the advent of high powered simulation tools and ICT for monitoring, solar thermal installations have come a long way. If PV and heat pump systems have made leaps in terms of performance, cost and reliability why would anyone think the same hasn’t occured in the solar thermal industry?

    The difference is that PV has gotten a big boom in the last half decade with falling prices so everyone is running around singing its well deserved praises but that doesn’t mean that solar thermal systems haven’t made any break-throughs either.

    As for the system destroying itself. Utter rubbish! with the right materials and a well designed drainback system there are no such issues.

    Fouling of heat exchangers by particulates are avoidable with user replaceable filters.

    Many many many heat transfer fluids with additives to prevent corrosion, sedimentation, degradation and freezing have been developed and are used in thousands of applications not only solar thermal.

    100% solar fraction is attainable by venting excess heat to the air if necessary during summary months, this is nothing new and has existed for a long time. However, just like with PV the cost effectiveness of a 100% solar fraction installation is highly dependent on the application and user needs.

    There have been major developments in the usage of phase change material infused tanks which (are set to) offer much higher storage densities (i.e. smaller storage tanks).

    As for maintenance and other issues pump technology and robustness, filters, monitoring equipment, materials and other major components commonly used for, not only solar thermal systems, but in many other industries, have been developing extremely rapidly so it is borderline ridiculous to conclude that solar thermal systems don’t work and we should hop over to PV for our hot water needs.

    PV has it’s place on the market but shouldn’t be using its relatively recent fame to bash other (more mature) technologies. There are constant developments in all branches of technology.

    PV has much higher embodied energy (from manufacturing) than most solar thermal systems and it is thermodynamically more efficient to use a low quality energy source (solar energy) to provide a low quality heating than to convert low quality energy (solar) to high quality energy (electricity) to then convert it to low quality energy (heating). Thus PV and solar thermal energy should be working together to meet our sustainable energy goals.

    Additionally there is also a soon to market development that will address the (apparent) disadvantages of solar thermal hot water installations with respect to heat pump systems by integrating both into a single collector for hot water and cooling (and optionally electricity also). (

    • Ivor O’Connor

      “being done by backyard tinkerers and novices”.

      Some of us enjoy tinkering. I learned a lot at this site and highly recommend others to tinker as well.

      • QB

        I agree with you 100%. However my reference is to those who were/are unqualified and inexperienced in system design installing systems for others who may have different needs or less tinkering capacity in their backyards.

        I myself enjoy a good tinker 🙂

  • I faced the same choice 4 years ago: PV or solar thermal.

    I made the calculation for a system of the same collector area and found they would both save about 150 m3 of natural gas per year (indirectly in case of PV, assuming 100% electricity generated by natural gas). But for the solar water heater there was a big uncertainty because the data on real-world energy yield is less reliable. It depends very much on how much hot water you use and how big the storage vessel is. In my case, the only way to make solar thermal work was to couple it to my central heating system too. I simply wasn’t using enough warm water.

    I estimated the costs of both systems to be more or less equal. After looking at all the work I had to put in to make solar thermal work and integrate it with my central heating boiler, installing a large, heavy 500 l storage vessel in the attic, piping, etc,I just gave up and went for PV. I am more convinced by the day that I made the right choice.

    Rebuttal of the rebuttal posted at the end:

    1. A solar thermal system in general needs a larger storage vessel than a heat pump water heater. So it is an advantage of PV vs thermal instead of a disadvantage.

    2. Modern heat pumps need very little maintenance, replacing an air filter every now and then is trivial.

    3. What about airducts? What happened to that technology?

    4. Baseless FUD

    5. Can be an issue, but piping is part of the solar thermal system too, so what is exactly the disadvantage?

    6. In cold climates solar thermal systems have a negligible yield at all in winter when you need it most (because the water is the coldest). All solar systems work best in warm, sunny areas: same problem for both technologies.

    7. Ridiculous FUD. The company that installs the heat pump will do the maintenance, if needed at all.

    If these are the so-called drawbacks of PV, then the rebuttal ends up reinforcing the case for PV

  • Ivor O’Connor

    Most people on a farm should probably make their own solar water heater.

  • Altair IV

    Most homes here in Japan have on-demand water heating systems. No tank, just a small box that heats the incoming water flow as you need it. You get unlimited hot water, you only use as much energy as needed for the purpose at hand, and they take up very little space. The only minor drawbacks are a slightly longer wait for the flow to get hot, and a higher up-front cost.

    The one in my apartment is gas powered (and only about the size of a large VCR, mounted on the wall outside my balcony), but there are certainly also electric induction-heating models available, which would integrate just fine into a PV-powered electrical home.

    • ThomasGerke

      We got an on-demand electric water heater in our appartment, but I find it very undesireable and they are properly one of the worst ideas when considering an increase of solar energy use.

      They draw huge amounts of power. Ours draws 11-21 kW which is massive and is thus by far the biggest consumer in our household. It certainly could not be powered by a typical residential roof top solar system (2-6 kW) and most people don’t shower at noon, so without heat storage it doesn’t really make sense at all.

      Just imagine 5 million people coincidentally taking a shower at the same time… that would require 50-100 GW.

      Water & space heating in combination with efficient heat-storage(tank) and heat-pumps is the way to go. Having that extra shift-able load on the grid is very important.

      • “Just imagine 5 million people coincidentally taking a shower at the same time… that would require 50-100 GW.”

        Just imagine 5 million people coincidentally switching on the electric kettle at the same time.

        Just imagine 5 million people coincidentally plugging in their electric car at the same time.

        Just imagine clouds obscuring 5 million solar panels at the same time.

        Sorry Thomas, I am used to you making more sensible comments, not parroting naysayer fud. Take a course in basic statistics and you’ll be able to calculate that it takes the lifetime of the universe or something as ridiculous like that for that situation to occur just once.

        • ThomasGerke

          If every household would use electric on demand water heaters, that could easily happen. In Germany or Japan millions of people work shifts and need to be at work at the exact same time.

          In February 2012 too many french households pushed the thermostat of their primitive electric heating Systems to far up during a cold spill, causing a week long electricity supply crisis. Suddenly 64 million French consumed up to 100 GW while the 82 million Germans consumed just 72 GW at the same time (with approx. 12 GW coming from wind, solar & biomass). This February the typical french peak load was 15-25 GW lower than during the 2012 cold spill.

          Simultaneity is a big issue in grid optimization and on-demand electric water heater without storage are simply not a good idea. They are cheap to buy, but expensive to operate and a pain in the ass for the grid.

          I am certain that there are already universities & institutes thinking about the future strains on the grid caused by electric vehicle fast charging during the vacation period. But we know that millions of people are actually traveling almost at the same time during a few days of the year. (Christmas, Eastern,…)

          However I think that it’s a manageable challenge – I am not a naysayer at all. 🙂

          • Bob_Wallace

            I think that all that is being said here is that we are in the process of moving off fossil fuels, for all sorts of use, and to electricity.

            That will require that we improve our grid’s ability to handle larger peak loads at times. And, because EVs will largely charge at night, we’ll have to swap out smaller transformers as they won’t be able to cool down at night as was past practice.

          • Total energy use does not change by using on demand heating. Hot water is hot water, that’s what counts.

            Instead of taking a 10 minute shower @20 kW, that same shower from a 1 kW water heater means it has to run for more than three hours to re-heat the water in the tank. The aggregate effect over millions of households is the same.

            Every hot water boiler loses heat and that has to be replenished with regular intervals. A storage type system uses more energy than an on demand system.

            In the case of a heat pump there is another advantage to on demand heating. For reasons of hygiene (legionella), the water in a hot water tank must be kept at least 70° C iirc. In an on demand system, the heat pump only has to heat to a comfortable temperature (~30-35° C). Higher temperature differential makes a heat pump less efficient.

          • Bob_Wallace

            “Higher temperature differential makes a heat pump less efficient.”

            More? Since tank water has to be heated to a hotter temp, requiring more energy.

            Demand can create higher peak loads. Three hour vs. ten minute heating cycle means higher draws.

          • Matthew Wright

            That is why we should use heat pumps. I have a Chinese made Macon heat pump (They are $800-$1200 on Alibaba) and the maximum impact on the grid that it has is 1.5kVA (runs at 900-1300Watts) Compare that to a 15kW or 20kW 3 phase on-demand heater -it’s just a crazy idea to have to build your grid sizing around that. We pay much more for grid capacity in Australia than for kWh sent over it. In Japan Eco-cute on-demand CO2 heat pumps exist that are interesting but it is hard to get details on them.

            As for everyone having a shower at the same time. Well the window for morning showers is 2-3 hours and evening showers 4-5 hours. So having 25 hours available for heating water and having a much lower demand on the grid is definitely the ideal. Six people can happily share Macon heat pump for showers etc and it only uses 3-5kWh per day in a temperate climate (ie average low is 5C and high of 47C)

          • Marcn

            Hi, saw your 1 yr old comment on Macon. Curious how it is holding up. thinking about using their whole house ASHP on a new house and concerned about reliability, service, etc… any insights you can provide would be greatly appreciated.

            Marc Nehamkin

          • Matthew Wright

            Personally I would buy Japanese if I was doing it again and that’s what I’ve been recommending to friends and the units run really well. It works ok but a Sanden heatpump hot water service is far superior.

        • Ivor O’Connor

          Those on demand heaters often take 60 amp circuit breakers…

          • johannjohann

            And very BIG wires, another expense when installing them

    • I have city heating, but most other new places in the Netherlands use high-output central heating boilers powered by natural gas, currently 96% efficient.

  • My heat pump water heater doesn’t use the back-up element unless it drops below 40F in my basement which is like 8 days per year in the northeast…. it runs in heat pump mode about 3 hours per day at 500 watts/hr…. I’m amazed the masses don’t get or use energy star electric heat pump water heaters, they save the average Amercian $350 per year and coast about $800 more than a typical electric water heater……


  • Others

    There are millions of Solar Water Heaters in China. Over the entire day, the water gets heated and is used for next day. If we install PV panels, it does not generate electricity in the morning for the hot water needs for bathing.

    Sometime back, I attended a Solar Seminar in a public library. The lady there said that, just 4 hours of Sunlight is enough to heat enough amount of hot water for next day usage.

    Besides converting heat to heat is a direct usage, where as converting electricity to heat involves energy loss. We can use Sun’s power as light, as heat, as electricity.

    Here are some news on Solar Thermal Power

    I guess Cleantechnica is hijacked by PV companies that they started opposing all other energy forms including Solar Thermal and Wind. Sad

    • Bob_Wallace

      “I guess Cleantechnica is hijacked by PV companies”


      • Others

        So why this hatred against Solar Thermal which is one of the wonderful technologies which could provide electricity at night, hot water for next day and ofcourse this hot water can also be used for space heating.

        • Bob_Wallace

          What hate do you imagine you see?

          Does a critical analysis constitute hate in your world?

          • Others

            Critical Analysis. Huh

            Read the 14th Page in Renewables 2013 Global Status Report.

            Solar Hot Water Capacity at the end of 2012 was 255 GW Th.

            Because its cheap, so many Chinese Households have installed this Solar Hot Water System. Certainly its cheaper than PV in developing countries.

            Both the Solar Hot Water and Solar Thermal power system will continue to increase along with Solar PV.

          • Bob_Wallace

            “Because its cheap, so many Chinese Households have installed this Solar
            Hot Water System. Certainly its cheaper than PV in developing

            And it has problems. Did you bother to read about them above?

            BTW have you ever seen one of these systems up close?

          • Ivor O’Connor

            I went back to the article and read the problems section in detail. I have seen these systems up close. This is a hit piece on solar hot water.

          • Bob_Wallace

            The article starts –

            “Solar thermal water heating is a temperamental thing. Water weighs a lot, it expands when it freezes, and it can cause scaling damage to pipes when it boils. Solar thermal systems are wonderfully efficient, and some systems work just fine for decades, but even these need regular inspection. When a solar thermal system fails, however, it sets about destroying itself….”

            I have first hand experience with solar water heating and all of this is true.

            Does that mean we shouldn’t use solar thermal water heating and work on making systems better? No.

            Should we consider other approaches? Yes.

          • Ivor O’Connor

            I like the right approach and the knowledge to select the right approach. I’ve seen the 70s and the damage done… I’ve also seen the experts in this new millennium and know they are quite capable of doing just as much damage.

          • SunDog

            I have a grid-tie PV system in Ontario. It is not the trouble free panacea painted above.

            I have 12 kW of PV panels installed, they produce 9.5 kW of peak output (w/ clear “solar south” exposure). I have replaced 7 of 24 inverters in the first 2 years of operation. Additionally, the cost of my property insurance had tripled when I installed the system.

        • i love solar thermal. but i’m open to the possibility of PV being better. (see comment above for reason for publishing this article.)

          • Ivor O’Connor

            It is these blanket statements I object to like “i’m open to the possibility of PV being better [than solar thermal]”. The whole article read in a similar manner. I would correct every statement like the above with qualifiers. Qualifiers like “I’m open to the possibility of PV being better than solar thermal in certain scenarios”. Just like I would never say wind is possibly better than PV without qualifications. Or say the contrary.

            However I would say RE is clearly better than nuclear and fossil fuels in almost all situations because of economics. Either subsidized or non subsidized.

          • Good call.

    • Ivor O’Connor

      I agree. The piece above reads much like the nuclear industry writing about PV. It totally misses the mark and instead takes you down a rabbit hole no sane person would follow.

    • ha, sorry, i just found it to be an interesting article. someone sent it along to me and i thought it was worth a share.

      • Ivor O’Connor

        Ahh. It sure has sparked a lot of interest.

  • spec9

    PV is much better than solar thermal. In fact one guy I’ve spoke with decided it was smarter to just install more PV and get an electric water heater than to install a solar thermal water heater since the PV is so cheap and the PV system is much less worries.

  • JamesWimberley

    I found this very hard to follow and am no wiser than when I started.
    What about hybrid panels that produce both electricity and hot water? The combination cools the solar cells a bit, improving efficiency. But you still need a complete hot water circuit.

    • carlos

      In fact there are what they call hybrid solar panels for sale. Like you said it will cool off the cells down, making it more efficient…

  • Matt

    If you use geothermal instead of air then the issue of warm air goes away. But up front costs are higher.

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