How large would your photovoltaic array have to be, if you wanted to power your electric vehicle with sunlight?

The first step is to calculate the mileage of your electric car. With a gasoline car, mileage is measured in miles per gallon. With an electric car, mileage is measured in miles per kilowatt-hour. If you read our recent post “Electric Car Cost Per Mile” you will see we calculate a light-duty electric car can achieve about 3.0 miles per kilowatt hour. A more in-depth set of calculations can be found in our feature “The Battery Powered Car.”

The charge/discharge efficiencies of batteries are high, usually over 90%, and the efficiency of the on-board vehicle batteries is taken into account in our calculations of 2.9 miles per kilowatt hour. But if your car is being powered by photovoltaics, presumably you will be off driving to work and back during most days when you want to collect electric power. Therefore you will need a battery system at home to store the photovoltaic electricity before discharging your home batteries in order to charge your car batteries at night. This will add 10% to the size of the photovoltaic array required.

Using a 1,000 square foot photovoltaic array as an example, assume 10 watts per square foot of array in full sun. Assume 8 hours of full sun or full sun equivalent (two hours of rising or setting sun, or sun obscured by clouds equals one hour of full sun) per day. This means that 1,000 square feet x 10 watts per square foot x 8 hours = 80 kilowatt hours of stored electricity per day. If you multiply by 2.9 miles per kilowatt hour and deduct 10% for the charge/discharge efficiency of your home storage system, then you have a 209 mile range.

This is a most encouraging fact. The average commute is well under 100 miles, meaning a homeowner owning a battery powered car would only need 500 square feet of photovoltaic panels to drive to work for free. At ten dollars per watt installed, this means that for $50,000, you would never buy gas again.

Clearly anyone owning a battery powered car is better off today buying power from the grid at night-time rates of, say, 10 cents or less per kilowatt hour. But photovoltaics are going to get cheaper. When they do, there are no practical engineering obstacles to seeing cars increasingly powered by the sun.