Hey Mr. Green,

If one has limited funds, what would you recommend purchasing first: residential solar panels or an electric vehicle? 

—Debra in Porterfield, Wisconsin

There is no easy answer to your question because utility rates vary enormously, from under 10 cents a kilowatt-hour (kWh) in several states up to almost 34 cents/kWh in Hawaii. The most efficient hybrids, in comparison, use about 28 cents/kWh per mile, or 3,773 kWh per year, to travel 13,476 miles (that happens to be the distance traveled by an average US car in a year).

In Wisconsin, a kWh for residential power is now 14 cents at the peak rate. In your area, you can install a six-kilowatt solar system for about $12,000 after your tax rebate. This should generate about 7,020 kWh per year, or about $1,000 worth of power. 

Your total cost for power to run an electric car is about $539 a year (or $403 if you take advantage of off-peak rates). A gas-powered car that gets 35 miles a gallon would cost you $909 for gas, so the EV would save you $370 to $506 a year on gasoline. If you drive a hulking SUV that gets a lousy 20 mpg, your gas costs shoot up to $1,590. 

Keep in mind that the cost of car ownership is high, with a small EV losing about $7,500 a year in value, for the simple reason that cars depreciate. By contrast, your solar unit should last for 25 years or more, only losing a small amount of power each year.