Can we power our car with the sun?


I’ve written about a lot of the features we included in our new house in Dummerston to reduce its energy use and environmental footprint, but there’s another one -- a big one -- that doesn’t really relate to the house.

We are hoping to power a plug-in hybrid car using the electricity generated on our barn. We have 12 lilowatts (kW) of photovoltaic (PV) modules installed on the barn (there is another 6 kW in the group-net-metered system that belongs to a neighbor), and we’re hoping that will be enough to not only power our all-electric house on a net-zero-energy basis, but also power our car for around-town use.

Trading up to a plug-in hybrid

My wife and I have two cars: a nine-year-old Subaru Forester with 128,000 miles on it and a 10-year-old Honda Civic Hybrid (the first year that car was made) with 180,000 miles. Aside from being embarrassed by how many miles we drive -- less now that our daughters are out of college -- I’m aware that end-of-life decisions might be coming up with at least the Honda.

Our hope is to trade it in on a Chevy Volt, a Toyota Prius plug-in hybrid, or a plug-in hybrid made by some other manufacturer. I first began thinking about a plug-in hybrid before they were commercially available, and I’m glad we invested in new batteries on the Honda when the original hybrid battery system was failing at about 170,000 miles.

A brother-in-law with a Chevy Volt loves it, and in discussions with a number of car experts I’ve generally gotten the sense that General Motors leapfrogged Toyota with plug-in technology -- but I’m still doing research on this. I’m hoping that by the time we really need to replace the Honda there will be even more choices.

How much solar electricity would I need for around-town driving?

Back when I first started thinking about powering a car with the sun, I asked my friend Steven Strong, of Solar Design Associates in Harvard, Mass., how many extra kilowatts of capacity I would need on my PV system to provide for driving. Steven had converted his standard Prius to a plug-in version by adding additional battery capacity and the necessary controls (this was before the Chevy Volt or Prius plug-in model were available).

Back in mid-2011, Steven told me that his plug-in conversion Prius required 265-275 Watt-hours (Wh) per mile in eastern Massachusetts where it’s reasonably flat and winter temperatures are more moderate than in Vermont. He estimated that 300 Wh/mile would be a more realistic estimate here. He also said that the Prius conversion isn’t an optimal electric vehicle and that the next-generation, factory-engineered EVs and plug-in hybrids should provide better performance.

I just looked online and saw some claims as low as 200 Wh/mile for a Volt, but most are in the 250 to 300 Wh/mile range. If I go with Steven’s estimate of 300 Wh/mile (0.30 kWh/mile) and estimate our commuting and around-town driving to be ten trips per week at 18 miles round-trip, or 9,360 miles/yr, then our annual electricity usage for that commuting and around-town driving would be 2,800 kWh.

If I assume 1,200 kWh of output per kW of rated capacity for a PV system (typical for Vermont), that works out to 2.3 kW of additional PV for provide for that amount of driving.

Despite the fact that my HERS score (Home Energy Rating System) showed that my house will need the full output of a 12 kW PV system plus a little bit of heat from the wood stove, I think we’ll be using less energy than the HERS model predicted, and we’ll have enough left over for powering our around-town driving.

The proof will be in the pudding.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (, both based in Brattleboro. Send comments or suggestions for future columns to


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