Switching to a Plug-In Hybrid—With Our Own Solar Power

Our 12 kW PV system going in on the roof of our restored 1812 barn.Photo Credit: Alex Wilson

Among the energy-related features of our new house in Dummerston, Vermont, is one parked in the garage.

We are hoping to power a plug-in hybrid car using the electricity generated on our barn. We have 12 kilowatts (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 the 12 kW 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 ten-year-old Honda Civic Hybrid (the first year that the Civic was offered in a hybrid version) 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 soon with at least the Honda.

Our hope is to trade it in on a Chevy Volt, a Toyota Plug-in Prius, 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 waited and invested in new batteries on the Honda when the original hybrid battery system was failing a year ago at about 170,000 miles.

My brother-in-law loves his Chevy Volt. Through conversations with a number of car experts I’ve generally gotten the sense that General Motors leapfrogged Toyota with its own 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.

Solar charging station at the GridStar demonstration home in Philadelphia.Photo Credit: Alex Wilson

I’d really love it if VW introduced an affordable plug-in diesel hybrid. Prior to our two current cars, I drove VW diesels for years and loved them—first a 1983 Diesel Rabbit (which was a little too noisy and smoky) and then a 1996 TDI Passat wagon that we loved. We drove the Rabbit well into 200,000 miles with never having done anything major to the engine, transmission, or clutch. And we sold the Passat with 140,000 miles after running it for a couple years on biodiesel.

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, Massachusetts, 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 models 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 thought 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. 

Our solar array this past winter.Photo Credit: Alex Wilson

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 to generate enough electricity for that amount of driving.

Despite the fact that our 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 our wood stove, I’m hoping that we’ll be using less energy than the HERS model predicts (once we have our low-e storm windows up), and we’ll have enough left over for powering our around-town driving.

The proof will be in the pudding.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.