By describing the cost to charge an electric vehicle as "squishy," Martin Eberhard begins a post on his Tesla Founders Blog on the right foot. Seriously. That's because it's a very difficult question to answer and it totally depends on where you live, what company your power is coming from and when you charge it. There are other factors too, such as what, if any, alternative sources you have and how much power you usually use on an average day. To help explain all of these little details, Eberhard has made a spreadsheet based on Northern California's rates from Pacific Gas and Electric.
After doing all the math, Martin figures that it costs him 3.6 cents per mile to run his Roadster. That's more than he initially figured but still just a small fraction of what it would cost to run on gas. For instance, at $4.00 a gallon, a car that gets over 40 miles per gallon would still cost double per mile to operate. Of course, YMMV. To help figure it all out, Martin suggests that you create your own spreadsheet and share it with the rest of the class.
[Source: Tesla Founders Blog]