Two days ago, we ran down all the rated ranges of every plug-in electric car offered for sale in the U.S. market this year.

We did not include ratings for efficiency.

In part, that's because the unit the EPA uses doesn't actually have much to do with the electricity owners use to recharge the battery packs that power their electric cars.

Instead, the EPA has chosen to use an artificially created metric called Miles Per Gallon Equivalent, or MPGe.

Setting aside the issue that when a plug-in car is traveling on battery power, there are no gallons of gasoline involved anywhere in the process, here's how it works.

The efficiency rating measures how far an electric car can travel on the amount of electricity contained in 1 gallon of gasoline (which, for the record, it says is 33.7 kilowatt-hours).

Note that the gallon of gasoline costs \$3.50 to \$4.75--while the equivalent energy content delivered over the electric grid to your car costs from \$1 to \$8, depending on your local electricity rates.

But it's impossible to buy a MPGe, and so the efficiency measure is useless for actually calculating the cost of operating an electric car.

Instead, electric-car drivers buy kilowatt-hours of electricity, most of which goes to power their cars (plus some for overhead like keeping the pack at the optimal temperature, in many cars).

Of course, the non-linear Miles-Per-Gallon measure isn't very helpful in calculating how much it'll cost to go a given distance.

To get the cost of going 100 miles, for instance, you have to divide by gas mileage (whether it's 10 mpg, 25 mpg, or 50 mpg) to get the number of gallons, then multiply that by the cost of gas.

So why not measure an electric car's efficiency in what the owner actually buys?

If you like the miles-per-gallon equivalent, we'd suggest Miles Per Kilowatt-Hour, or MPKwh.

2012 Toyota Prius Plug-In Hybrid - production model

If you live outside the U.S., however, you don't use MPG at all. Instead, you use a measure of consumption: the gasoline used to travel a given distance.

That's usually liters per 100 km or, for non-metric North Americans, gallons per 100 miles.

To parallel that, we'd suggest kilowatt-hours per 100 miles, which can then be multiplied by your electricity cost to get the total cost of running your car for some useful distance.

For a 2012 Nissan Leaf, its average rated efficiency of 99 MPGe translates to 34 kilowatt-hours per 100 miles. Just multiply that by your electric cost.

So if you pay the U.S. average of 12 cents/kWh, the Leaf will cost you \$4.08 to go 100 miles (versus \$16 in a 25-mpg car with gas at \$4/gallon).

Tell us what you think is the most useful way to measure the efficiency and running costs of a plug-in electric car: Does MPGe work for you? Or would you prefer a measure using kilowatt-hours?