My wife and I built a zero energy home in 2015. The energy model we conducted as part of that process said that we would need to generate about 6200 kWhs per year to provide the energy our home was projected to use. So, we installed a solar electric system with that in mind. As it turned out, due to our well planned energy-saving measures and conservation practices, we actually produced more energy than we used.

Our utility interconnection (net-metering) agreement allows us to receive a credit for each kWh that we generate in excess of our monthly usage. We can carry this credit forward each month, to cover the cost of the energy we use in the winter when our production is lower. But every April any remaining credit is lost to us and we start over at zero on our energy bill balance sheet. As utility policies go, that’s not too bad. We essentially gain the retail value of our excess monthly generation and have the opportunity to build a credit over the summer to use the following winter.

Since we were producing more energy than we used, we were leaving a lot of electricity on the table. We used only 60% of the energy we generated in 2016 and 80% in 2018. The retail value of that electricity is about 10¢ per kWh, so we were donating a couple hundred dollars per year to the grid. Then we realized that we could capture the value of our excess energy production by using it for transportation instead.

So at the end of 2017, we purchased a used Nissan Leaf. It was two years old and had just 7,000 miles on the odometer. We have now owned it for almost two years and use it exclusively in a 30-50 mile radius. We always charge at home and pay nothing for fuel, and as with all electric vehicles our maintenance costs are extremely low. Actually so far, our maintenance cost has been zero. So our solar panels power both our zero energy home and all our local transportation while significantly lowering our cost of ownership for both home and car.

Solar-powered Transportation

By installing sufficient solar collectors to power both your home and an electric vehicle, almost anyone planning a zero energy home can reach the energy potential of a positive energy home. While my household backed into the positive energy zone after our home was completed, many people want to know how they can plan ahead to power their home and car from their solar panels. Most energy modeling will provide fairly accurate guidance as to the number of panels needed to fully power a net zero energy home. So the critical decision becomes how many additional solar panels does it take to power an electric car?

It’s not rocket science, but estimating the amount of energy requires some calculations.

Miles You Drive

If you’re a daily commuter, this should be pretty easy to estimate. Most early electric vehicles, like our Nissan Leaf, have a range of about 90 miles per charge. These are “around town” cars, and many people would probably drive them between 5,000 and 10,000 miles per year. This is rapidly changing with the availability of longer range EVs, such as the 238 mile range Chevy Bolt, already on the market, a variety of new longer range EVs coming in 2020, and and many more coming on the market between 2020 and 2025. Five long-range full-electric pickup trucks and 13 electric SUVs will also soon be available.

Efficiency of Your Vehicle

Currently, most EVs are small to midsize sedans so the efficiency among them is fairly similar. Our 2015 Leaf travels 4.3 miles on one kWh. My friend’s solar-powered Tesla Model 3 gets 4.4 and his Chevy Bolt is rated at 3.5 miles per kWh. Of course, that varies with your driving style, the terrain, and season, as well as with air conditioner and heater use. During cold winters or hot summers the mileage may drop by up to 25% due to temperature extremes. On the other hand, during the moderate shoulder seasons and in mild climates, most EVs get considerably better miles per kWh. For example, the Chevy Bolt can drive up to 6 miles on one kWh during mild weather. Incidentally, fuel economy in fossil fuel cars also drops significantly in extreme weather, but few people seem to notice.

Annual output of one solar panel in your climate

The easiest way to learn how much electricity one solar panel will produce annually is to ask a local solar contractor. If you already own a solar electric system, you probably have access to the actual production data through a website, user manual, or mobile app. You can also look up typical system performance information for your area through

Simple equation

Once you know how much electricity a single panel will produce each year, the next step is to divide the number of miles you intend to drive per year by the miles per kWh of the vehicle. That gives you the kWhs needed for driving. Then simply divide your kWhs needed for transportation by the expected yearly output of your solar panels.

Here’s how it works out for us. In the first two years of EV ownership, we have averaged 4,000 miles per year. Our Nissan Leaf gets an average of 4.3 miles per kWh. Each of the solar panels we installed generates 360 kWhs annually. Here is the calculation:

4000 miles per year ÷ 4.3 kWh/mile = 930 kWhs needed to power our vehicle per year

930 kWhs ÷ 360 kWh/panel = 2.5 panels needed to provide 930 kWhs per year

Obviously, you would want to round up to the next full panel in this situation. The little bit of extra output will give some room to increase your solar-powered driving distance as well as accommodate an acceptable margin of error. Also, solar output will decline by about 0.8% each year as panels age and car batteries will lose a bit of efficiency over time, just as internal combustion vehicles decline in efficiency as they age.

What if you don’t have solar panels for your electric vehicle? It’s still cheaper and less polluting to drive an EV than a fossil fuel burner, even if you purchase electricity from the grid. To compare for yourself, see the USDOE’s Cost Comparison tool. So don’t let that stop you from transitioning away from internal combustion vehicles. But if you’re planning to install solar panels, consider adding sufficient panels for your daily transportation needs, and take a giant step towards a more economical zero energy, zero carbon life.