Dragon’s Guide To A 100% Renewable Home — Part 3 (Axe Gas)

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Welcome back to our series of guides covering how to power your life with renewable electricity. If you’re new to this series, check out the introduction in Part 1.

Articles in the series:

Part 3: Eliminate Gasoline Use

Walk or electric bike

Bay Area BikeShare electric bikes

City dwellers have the option to go completely without a car. Walk, bike, use public transportation, hail a ride share (hopefully in an EV), and rent a car just for long trips.

Electric bikes are becoming cheap enough to replace regular bikes, and they will get you over hills and across long distances without breaking a sweat. TreeHugger offers a great overview of electric bike options starting around $1,500. I’ve also heard stories of great deals on used e-bikes found on Craigslist.

If you’re a DIY fan, you can mod your existing bike into an e-bike for a few hundred dollars plus some elbow grease. With that goal in mind, I purchased The Ultimate DIY Bike Guide by Micah Toll. I haven’t gotten around to modding our bikes yet, but I do feel the guide was a worthwhile purchase.

Buy an electric motorcycle or scooter

Electric motorcycles have the speed and range of a car for a fraction of the price. Scooters are even cheaper, though are limited to city speeds. Of course they’re both most suited to Mediterranean climates, but even if you only use an e-motorcycle on dry, sunny days, you can still save a lot of gas.

Green Car Reports has details on American e-motorcycle models starting at $15,000 minus 10% federal rebate. E-scooters seem to have fewer options and I could not find a buyer’s guide, but I did find the GenZe for $3000 and Vmoto models currently only sold in Europe starting at £1,799.

Buy an EV

Walking and biking is by far the best choice for kicking greenhouse emissions, but many need to make longer trips regularly. Using 100% renewable electricity to power an EV saves about 19.64 pounds of CO2 per gallon of gasoline avoided. The Federal Highway Administration estimates Americans drive 13,476 miles per year. Compared to a Toyota Camry that gets 30 mpg, an EV saves 13,476 / 30 * 19.64 = 8,822 lbs of CO2 per year.

Climate denialists like to claim that EV manufacturing emits so much CO2 that you’ll never save CO2 by driving one. Making a Toyota Camry emits 22,000 lbs of CO2. Making an EV emits about 15% more CO2 than a similar gas car, so by buying something like a Bolt EV instead of a Camry, you emit about 22,000 * 0.15 = 3,300 lbs extra CO2. If you run that EV on 100% solar power, an average driver will save those extra 3,300 lbs of CO2 emissions in the first 4 months of driving. The EV should last longer (with possible battery replacements) than an ICE, spreading the CO2 emitted during its construction over a longer time period. An EV’s batteries should also be recycled to lower the CO2 needed to make the next battery pack.

I wrote an article about our experience purchasing a used Tesla Model S 60. It’s been over a year now and we’re still quite happy with it. The only thing I can complain about, as a fan of exploring nature, is that it can be tough to find chargers near camping spots. If there are RV hookups, great, but in many parks there are no outlets to be found at all. On the plus side, Model S has so much trunk space that we’ve cut foam pads to fit inside and use the car instead of a tent. So much nicer.

CleanTechnica has been talking about great deals on used LEAFs and other EVs for a while, but if you need more range, the Bolt EV is available now in many regions and throughout America by September. The Tesla Model 3 will hopefully be here not much later — though, if you’re not already on the pre-order list, it will be much longer before you can get one.

The Bolt’s MSRP is $37,495 (minus the $7,500 federal tax rebate if you can use that, plus a state rebate in some states — here are the top ten). If you live in Europe, the new Renault Zoe with a 41 kWh battery is another great option for longer-range drives, and there are plenty of other European options.

It’ll still be a few years before we have a wide selection of EVs that trample ICE in affordability, but for a lot of city commuters, the gas savings even now can pay for an EV. In fact, with a good credit rating, there have been deals where you can get a Spark EV in California on a lease that costs you nothing per month. If we actually paid for the true external costs of ICE vehicles, almost any EV out today would be a bargain.

If you want to do a little something extra to keep EV use growing, why not toss a couple bucks a month towards helping produce Fully Charged? It’s a video series about electric cars and the future of energy hosted by Robert Llewellyn. He manages to make every episode both informative and funny! Plus, he played Kryten on Red Dwarf, so you can’t get much cooler than that. This is also a great series to share with people who might not be so familiar with EVs. I showed my dad a few episodes and he seemed to get really into it.

Eliminate or offset air travel

circle-slash-airplaneAirline travel has been called “the biggest carbon sin” because it burns so much fuel per person. One round trip across the US puts out about 10% of the CO2 an average American emits per year. We avoid air travel and take our rare vacations within driving distance.

If you can’t avoid air travel, many airlines now offer carbon offset credits for a fee. I’m skeptical about them because it’s so easy to game carbon offset systems, but the underlying idea is sound if it’s done in a legitimate way. I would tend to trust a more standardized system like Renewable Energy Credits (RECs), but even then, 19–33% of the money goes to marketing. The rest of the money helps pay bonuses to renewable generators (including energy from solar panels on your own roof!) but it’s impossible to know how much additional renewable generation is built specifically because of the extra profit available from selling RECs and how much would have been built even without RECs. I’m guessing RECs offset 50% of their value in emissions at most.

CleanTechnica recently wrote about Cool Effects, which is a group that sells CO2 offsets that directly fund projects they’ve vetted as things that would not have happened without the funding their users provide. You can see exactly where your money is going and even choose to support a particular project you like. Under 10% of the money goes to admin costs. This would be my choice for offsetting air travel if we couldn’t avoid a trip.

Chip in a few dollars a month to help support independent cleantech coverage that helps to accelerate the cleantech revolution!

Estimating EV energy use by miles you drive

In order to power an EV by 100% solar energy from your roof, you need to calculate how much energy it uses per month.

Option 1: Buy an EV and drive it for a couple months before you buy a solar system. Check how much your power bill rises to determine the EV’s average energy use per month. Make sure you subtract any other new sources of electricity you might have added to your home, including seasonal changes in electrical heating.

Option 2: If you’re installing solar before buying an EV, the rest of this section describes how to estimate the energy use of any particular EV you may want to purchase or lease. To estimate that, you first need to know how many miles you drive per month. I happened to write the date and odometer down each time I changed the oil in my Prius, so I used that to find average miles driven per month. Many insurance companies ask for odometer readings periodically and you may be able to get that info to use for an estimation. Otherwise, you may need to watch the odometer for a month or two and try to adjust for any unusual trips.

Subtract any long-distance trips included in your odometer readings that would be powered by chargers on the road or at your destination. Try to subtract the exact number of miles you charged while away from home. Even if you wanted to try to generate solar to cover those trips, unless the power actually comes from your solar panels, it won’t count towards your yearly usage with the electric company and many electric companies won’t let you generate much more power than you use. If your particular electric company lets you over-generate and you want to offset your road trips with solar panels, more power to you!

Once you know how many miles you need to power, you need to know how much power your EV uses per mile. Start by taking its total battery capacity in kWh divided by its EPA-rated range in miles. I multiply by 1000 to change kWh/mi to Wh/mi because many EVs display instantaneous Wh/mi in their instruments and it’s usually what you’ll see people using in forums. Wh/mi to an EV is a measure of efficiency like mpg (miles per gallon) is to a gas car, but with Wh/mi, lower numbers are better.


  • Chevy Bolt: 60kWh / 238mi * 1000 = 252 Wh/mi
  • Nissan LEAF (30kWh version): 30kWh / 107mi * 1000 = 280 Wh/mi
  • Tesla Model S 60: 60kWh / 208mi * 1000 = 288 Wh/mi
  • Tesla Model S 85: 85kWh / 265mi * 1000 = 320 Wh/mi

Once you know Wh/mi for your EV, you can multiply that by miles driven per month to get close to the amount of power it will use per month. Say you travel 30 miles per day average in a car that you’ve decided uses 345 Wh/mi. 30 miles times 30 days in a month times 345 Wh/mi = 310,500Wh per month / 1000 = 310.5kWh/mo.

But wait, you can get more accurate than this. The EPA range estimates try to measure real-world range by taking a weighted average of city and highway driving in cold, mild, and hot conditions. Unfortunately, EPA’s tests tend to underestimate the energy use of actual drivers, especially if cars are driven over the speed limit. Charging is also not 100% efficient (especially if you insist on wireless charging). Therefore, I recommend you multiply your energy estimate by 1.20 to add 20% to it and maybe an additional 10% if you live in a cold area. A better estimate is rather difficult to derive scientifically, but if you want to give it a shot, or if you’re simply curious, read this indented section:

Speed has a huge effect on energy use.

It seems counter-intuitive, but as you can see in the graph to the right, the Model S uses the least energy per mile by driving 20–25mph and more energy driving slower. Add in power losses from stopping and starting at intersections and it’s possible to get less range with city driving than with highway driving at the speed limit. The Model S 60 gets only a little extra range on the highway: 210.7 highway miles vs 205.7 city miles.

On the other hand, the 30kWh LEAF gets less range on the highway: 95 highway miles vs 116 city miles. I suspect this reversal of city/highway efficiency is due to Model S’s wind drag being about as low as you can find in the auto industry.

Few people in my area drive the speed limit, so I tend to drive our Model S 60 at 73 mph on the highway, which I’ve measured uses around 0.345kWh/mi. My measured result fits well with the Model S 85 line on the graph if you move the line down a little to account for Model S 60 being a lighter vehicle. 0.345kWh/mi is about 20% higher than the EPA rating. Few of EPA’s driving tests are done over 60 mph (even their one 80 mph test only stays at 80 briefly), and it takes 25% more energy to hold 73 mph than to hold 60 mph (based on the graph), so 20% higher makes sense.

In the city, the Model S should get close to the EPA estimate of 60kWh / 205.7 mi = 0.291kWh/mi, but in my calculations, I used 0.345kWh/mi for everything because I drive a lot more highway miles and I’d rather overestimate energy use than underestimate it. Even if you swear to never drive over 60 mph, I still suggest adding 10% to the EPA rating because speed has such a big impact and so little of their testing is done at 60 mph or more.

Cold weather energy use 

Rob M. has a chart of Wh/mi used during winter driving approximately 100 miles per day in New England. Throughout January, he averaged 357 Wh/mi or 11% higher than the 320 Wh/mi EPA rating of his Model S 85. He also mentions he can do his commute using only 294 Wh/mi in the summer which is 8.2% lower than the EPA rating. Based on the graph, he’s probably driving around 62 mph to achieve that, and he beats the EPA rating because most of his trip is one long highway stretch which is more efficient than city driving in Model S.

Hot weather energy use 

Many people say they notice no range reduction in hot weather. This may have something to do with batteries working more efficiently when warm. Someone in this thread posted a picture showing 365 Wh/mi driving 80 mph for 252 miles in temperatures over 100°F. The graph shows he should be using 400 Wh/mi driving 80mph, yet he used a lot less. It’s possible he was driving downhill or had wind at his back, but it’s more evidence that hot weather makes little difference or may even help, at least in a Model S.


I would take from all this that your EV’s energy use is most affected by driving above the speed limit and by excessive cold. Remember that the worst of cold weather only sticks around for 3–4 months. I can’t give you an easy equation to take all these factors into account, but I think you should have enough info to make an educated guess based on your climate and driving habits. If you’d rather keep things simple, use the EPA rating + 20% or even + 30% to make absolutely sure your solar system provides more energy than you use to drive.

Mountain driving takes extra energy

If you regularly drive up and down mountains, you must account for the extra energy that uses.

Use the following formula to estimate the extra energy needed to raise your EV’s elevation:

W * H * 0.00000047077 = kWh

W is the weight of the car in pounds, H is the height up the mountain in feet, and 0.00000047077 converts foot pounds to kWh plus 25%. That extra 25% represents the inefficiency of converting battery energy to lifting the car and is derived from the rule of thumb that EVs convert 80% of their stored energy to motion (vs 20% on a gasoline vehicle).

For example:

  • W = the curb weight (weight when empty) of Model S 60 which is 4,323lb + 410lb of passengers = 4,733lb. Just web search “<name of your EV> curb weight” to find this info.
  • H = the elevation difference between our house and the base of our mountain = 3500 ft. Find this with a GPS or an address elevation calculator.
  • Plug W and H into the formula and we get 4,733lb * 3500ft * 0.00000047077 = 7.798kWh. I’ve recorded numerous trips up our mountain and they all used very close to this amount of extra energy beyond the horizontal distance traveled.

So, for every round trip down and up our mountain, we use an extra 7.798kWh to get back up. You might be thinking we generate that same amount from regenerative braking going down the mountain. Unfortunately, the amount we get back varies based on the amount of energy the battery already has. Above 90% charge, regenerative braking puts almost no energy into the battery because it overheats if you charge it too quickly when it’s that full. Therefore, I try to keep the car at 80% charge before going down the mountain. In that case, traveling 14 miles and down 3500 feet regenerates about 3.9kWh, which is about half the extra energy it takes to get up the mountain. At 70% charge, we regenerate more, but I rarely leave with such low charge. So, to estimate energy use on a round trip down the mountain and back up, I use the formula: (total miles * 0.345kWh/mi) + 7.798kWh – 3.9kWh.

EV energy use while parked

The Model S uses around 18 kWh/month while parked in a temperate climate with energy saver mode active. This energy is used to monitor the car’s systems and maintain its batteries in a safe temperature range where they won’t be damaged. In very hot or very cold climates, the car will use more energy to keep the batteries in that safe temperature range.

Many EVs, such as the LEAF, have no temperature regulation to keep batteries cool, but do have heaters for extreme cold. From what I’ve read, the LEAF uses around 6.42kWh/mo while parked in a temperate climate, but may use more in a cold winter to keep batteries at a temperature where they won’t be damaged.

Given that Tesla uses extra power in both hot and cold temperatures, I made a wild guess that ours might use 50% more power averaged over the year, so I added 18kWh/mo * 1.5 = 24kWh/mo to our monthly energy estimate. This is probably an overestimation.

Accuracy of EV energy use estimation

Using the techniques described above, our estimated monthly EV energy use was 304 kWh/mo. Electricity bills in May through Aug 2016 rose 220 kWh, 318 kWh, 304 kWh, and 430 kWh compared to the same months in 2015. The main change between the two years was the EV, so I averaged those results to get 318 kWh/mo. So, our estimate was pretty accurate but you do have to be careful to get all the details right.

Since December 20th, 2016, I’ve had The Energy Detective monitoring how much energy our Tesla charger is actually using. Averaging the readings over the 57 days it’s been recording shows it’s used only 203 kWh per 30-day period. That’s about 33% less than the 318 kWh/mo average — though, it does come close to the May 2016 usage. This highlights that driving habits can change a lot from month to month so use as many months as you can in your estimate. Overestimating what you might need is better than underestimating.

Oil Sucks

As a final F.U. from the oil industry, the day we tried to get rid of the last of our used motor oil from the previous car, we spilled it on our deck. Here it is 3 months and 3 rain storms later:

I doubt the stain will ever come out. That feels symbolic of our whole civilization’s relationship with oil.

In America, people who protest oil pipelines get shot with rubber bullets and stun grenades. In Africa, such people get shot dead. These are companies that I, for one, can not tolerate supporting when there is a clean alternative.

Stay tuned for Part 4 when we’ll talk about using solar electricity for home heating and cooling to chuck fossil fuel into the dustbin of history.

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Chris Dragon

I grew up programming on an Apple IIe and now run a software company. Despite a hatred of family camping trips, I've always had a deep concern for the creatures we share this planet with. In respect of those creatures, I spent a number of years working at an environmental non-profit, but these days I just try to live with sustainability and encourage others to do the same. I've even come to tolerate camping trips!

Chris Dragon has 14 posts and counting. See all posts by Chris Dragon