energy-solutionsheader image
1 Helpful?

Can We Power Our Car With the Sun?

We oversized our PV system so that can use solar energy to power around-town driving with a plug-in hybrid car

Posted on May 22 2014 by Alex Wilson

I’ve written about a lot of the features we included in our new house in Dummerston, Vermont, 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 kilowatts (kW) of photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) modules installed on the barn roof (there is also a 6-kW group-net-metered PV 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 to 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 (manufactured 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, at least with 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.

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 past 200,000 miles without ever having to make any major repairs to the engine, transmission, or clutch. And we sold the Passat with 140,000 miles after running it for a couple of 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/year, 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 to generate enough electricity for that amount of driving.

Despite the fact that our HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. 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-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. storm windows up), and we’ll have enough electricity 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.

Tags: , , , , ,

Image Credits:

  1. Alex Wilson

May 22, 2014 9:17 AM ET

Edited May 22, 2014 11:12 AM ET.

solar powered car
by stephen sheehy

0 Helpful?

Alex-For your numbers to work, i. e. 2.3 kw of pv, won't you need to limit driving to when the sun isn't producing power? I guess there's no point in a convertible. I assume you don't have batteries that could store power and then charge the car batteries. Swappable batteries would be a big plus. Maybe someday, we'll be able to use pv to make hydrogen (from water? ) compress and store it, then use it to run fuel cells to power our car and our house.
It occurs to me that you must have a net metering deal, so your utility is your battery, at least for now.

May 22, 2014 1:46 PM ET

The fun EV to own...
by Dana Dorsett

0 Helpful?

... would be the Chevy Spark-EV, which has performance & handling characteristics comparable to BMW's EV without the price tag. The nominal range is about 80 miles, but during a VT cold snap I'd derate that to 50, which is still plenty for your town commuting. It's currently only available in CA & OR, though I have pretty solid 3rd party reports of bootlegged Spark-EVs with WA plates on them, and a fuzzier allegation of a recent sighting in MA.

It's a fairly small car, with a ~21kwh battery. The totally dumb math on watt-hours per mile using range & battery capacity comes in at about 250, but flatlanders in OR do quite a bit better than that.

In VT the summertime output of the PV is about 2x that in winter. When looking at the sun-to-long-term storage issue solar-to-methane is about 40% efficient for small scale (~60% for large scale systems), and methane is an easier gas to manage, compress, & store than hydrogen for small scale systems.

Both hydrogen and methane from electricity is being considered fairly seriously on a commercial basis going forward in Germany, which has large and growing renewable power assets, as well as pre-existing natural gas distribution & storage systems. It only makes financial sense to make methane or hydrogen when the intermittent renewables are delivering as surplus relative to the daily grid load, and the marginal cost of that power is near zero (or even negative), but those scenarios are occurring at ever greater frequency & duration in highly saturated wind & PV markets.

So far Germany hasn't run into the same levels of surplus output that have occurred in Spain on windy days, but about a week ago renewables were covering about 3/4 of the total load for the entire country, with significant local surpluses. It's only a matter of time before that becomes commonplace, as year on year drops in the cost of both wind & solar keep on coming, and more & more capacity gets installed.

May 22, 2014 2:55 PM ET

Dana-how does solar to
by stephen sheehy

0 Helpful?

Dana-how does solar to methane work? Where does the methane come from? Can small scale methane users avoid dumping it into the atmosphere, where it is a highly problematic greenhouse gas?

Pretty amazing info on German renewables.

May 22, 2014 4:53 PM ET

Solar to methane conversion
by kye ford

0 Helpful?

Stephen, easy equation solar to methane. You get some of that good fresh VT manure, throw in the trunk of your Prius, park in the sun for a couple days and to methane.

May 22, 2014 5:21 PM ET

Edited May 22, 2014 5:29 PM ET.

And then there is....
by John Crocker

0 Helpful?

the Nissan Leaf.

My wife and I have been driving our new 2013 Nissan Leaf for two weeks. She drives about 12000 miles/year on country roads, so a $200/mo Leaf lease for 36k miles, 36 mos makes great sense. Gas savings pay more than half of the lease.

We fuel it with 100% PA wind power (our house is heavily shaded, no PV). The EV says it is getting 3.6 mi/kWh, or 278 Wh/mi, driven aggressively. I figure 12000 miles/3.6 = 3,300 kWh/year. Not that much.

The leaf battery is 24 kWh, 2x that of the Volt, and the cost is ~$10k less (no ICE or ICE drivetrain). We are getting 80+ mile range the way we drive, and logged more that 400 miles in the first 9 days.

The really exciting thing...the Gen 2 Leaf is supposed to have 2x the battery capacity, 2x the range, cost only $5k more, and go for sale in 2015, model year 2016. That will be a happenin' vehicle. Imagine a pure EV with a 160 mile range, for $27k MSRP (after tax rebate)!

May 22, 2014 6:30 PM ET

Edited May 22, 2014 6:36 PM ET.

Reply to Stephen
by Donald Endsley

0 Helpful?

Solar Methane works by electrolysis and methanation. Electrolysis, which as we know, produces hydrogen and oxygen. The hydrogen produced is then combined with carbon dioxide in a process called methanation, which creates methane. Basically CO2 + 3 H2 with the correct heat and/or pressure and/or in the presence of a catalyst combines to create CH4 (methane) and H2O. The US navy is developing a similar process to create jet fuel out of sea water, except they are looking at using nuclear energy to power the process.

May 22, 2014 7:48 PM ET

reply to Donald
by stephen sheehy

0 Helpful?

Doesn't that process require additional energy input and thus isn't it less efficient than just using the hydrogen as fuel?
I wonder if the Navy will experience any cost overruns in the development of its seawater to jet fuel program?

May 23, 2014 9:18 AM ET

Edited May 23, 2014 9:20 AM ET.

We're doing it
by Dean Smith

0 Helpful?

We live in Everett, WA, not exactly in the sun belt. The Pacific Northwest is the worst place in the country for solar. We have a 5 Kw PV array on the roof of our 110-year-old house and it has consistently produced MORE energy annually than we need for the house. Our annual electric bill is negative. Last summer we bought a 2013 Chevy Volt and charge it every night. We essentially use the grid as a battery--feeding energy in during the day and taking it out at night. Our electric bill is still negative and the roof provides about half the energy consumed by the car. We do make some trips longer than the 40 miles supplied by the car battery. Our system can be viewed at

May 26, 2014 6:33 PM ET

Reply to Stephen Sheehy
by Dennis Heidner

0 Helpful?

"I wonder if the Navy will experience any cost overruns in the development of its seawater to jet fuel program?"

Yes, experimental programs are likely to have cost overruns. Would the process be cheaper than the current jetfuel costs? No. But that isn't entirely the point. If we have active CO2 capture/sequestering programs in the US (eventually) and a methanation plant is included on an air craft carrier which may have extra electrical energy production from the reactors. You can use that excess energy and tanks of compressed CO2 loaded from supply ships or ports... and eventually "Augment" the jet fuel as needed. That could increase the fleet mobility and in the case of hostile or oil embargos make it easier to keep the planes in the air. Will it happen in the next four or five years... Nope. In the next twenty - probably.

For methanation to work, you need a cheap - excess supply of electricity and a cheap supply of CO2.. On board nuclear reactors can provide the thermal heat and electricity that enables a more efficient water electrolysis process. Currently jet fuel is cheap... long term that could change.

May 27, 2014 10:22 AM ET

Reply to Stephen Sheehy
by Dana Dorsett

0 Helpful?

"Doesn't that process require additional energy input and thus isn't it less efficient than just using the hydrogen as fuel?"

Yes, methanization makes the conversion to a fuel-cell type fuel less efficient, but months-long storage of hydrogen as well as pipelined distribution of hydrogen is far more problematic (=expensive) than methane. Most metals commonly used for pipes & tanks become brittle when exposed to pure hydrogen under pressure, and the pressures required for storing hydrogen at high energy density are much higher than methane (which can be stored as a liquid at reasonable cost/temperature/pressure, if need be.)

The infrastructure for storing, distributing & using methane (the primary component of natural gas) already exists, and it's also a common feedstock for fuel cells. While its possible to mix some amount of hydrogen into natural gas or methane without creating problems for the infrastructure, storing distributing & using hydrogen in large volumes requires a huge investment in new infrastructure. This makes hydrogen an expensive way to utilize July sunshine in a fuel cell in January compared to methane.

This is all currently being hashed out by policy wonks in Germany (there is some amount discussion on the web if you can read German- some in English as well.) Clearly some amount of the excess power during high-wind-high-solar periods will be used for hydrogen fuel applications, but when the source electricity is available at zero (or negative) cost the efficiency hit for methane conversion will often be worth it compared to the huge capital cost of developing a large hydrogen-fuel infrastructure comparable to that currently used for natural gas.

May 28, 2014 4:05 PM ET

Consider Ford C-MAX Energi
by Scott Raney

0 Helpful?

We do this, although with an 8KW PV system we're a little short (time for a second system, because we were just net positive on electricity before we got that car). Lifetime average is just over 100MPG on gas because all trips shorter than about 15 miles are all-electric. I just drove the car to Grand Junction and back (250+ miles over the continental divide) and averaged just under 50MPG driving about 5 MPH over the speed limit. Which is why we didn't consider the Leaf or any other all-electric vehicle (I could have made it in the top-end Tesla, but not anything less).
Drive the Volt too, but my wife just preferred the C-Max. We also drove the Fusion, but the Energi model of that has almost no trunk and so we had to rule it out.
We leased instead of bought (first time for that) because it made handling the tax credit much easier and because I figure they estimated the residual value too high: These vehicles ought to be able to function as mobile generators when the power goes out (resilient design) and I expect when that feature and/or more compact battery technology becomes available that the value of the current generation of these cars will drop like a rock.

Jun 5, 2014 11:06 PM ET

Edited Jun 5, 2014 11:25 PM ET.

We're Doing It, Too
by Carl B

0 Helpful?

We are currently building our house and have planned to be able to do exactly what you describe, Alex. While our current Ford Focus (13 years old) and Toyota Prius (8 years old) don't have plug-in capability, I'm dreaming of replacing the Focus with the Ford Focus Electric with 76-mile, all-electric range. It'll get me to work and back easily even in CT cold winter weather. If I were replacing the Prius, I'd get the Ford C-Max Energi as it is the same body style (4-door hatchback) and roominess.

We are having GMISolar install a 13.8kW PV array with SunPower panels that tracks the sun via 2-axis tracking (see the attached photo). The tracking will get us an extra 25% or so energy by maintaining the angle of incidence at 90-degrees all day. Also, by not having the panels on the roof, they stay cooler (which makes them more efficient and long-lasting) and don't interfere with roofing needs (new shingles, leak repairs, etc.).

Hopefully the energy efficiency things we are building into the house (thanks to what I've learned from two years of studying everything I can here and Dr. Joe's BSC website/books) will maybe mean we have enough power to run two cars............

And the frosting on the cake: all US companies


Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!