Measuring Efficiency in a Battery-Powered Car

It’s time for MPG to RIP.

For more than a half-century, consumers have associated vehicle efficiency with the federal government's Miles Per Gallon rating. Automakers are legally bound to use MPG on window stickers and in their advertising. While it's easy to understand that 28 is a big and better number than 15 for MPG, plug-in vehicle efficiency is another matter altogether.

Some critics say that ditching MPG is past due. Before we even start talking about electricity, consider that a more accurate method of understanding the relative efficiency of vehicles would be to look at fuel consumed per distance driven—say per hundred miles. For example, switching from a car that gets 12 miles per gallon to one that goes 20 miles per gallon decreases fuel consumption by 3.3 gallons, while moving from 20 to 30 miles per gallon saves just 1.7 gallons.

The big numbers on the window sticker for BMW’s Mini are 33 in the city and 36 on the highway. But those are posted in kilowatt hours per 100 miles. The smaller text explains that the equivalent is 102 mpg city and 94 mpg on the highway. The EPA is still figuring out how to label efficiency in an plug-in car.

Using this “per mile” framework in the electric age could put two new measurements in front of consumers: the cost of driving per mile and CO2 emissions per mile. On both fronts, plug-in vehicles are the clear winner here. According to data from the European Commission, even when considering all energy losses including transmission and distribution, driving on electric power generates just 45 percent of the CO2 of a gasoline-powered vehicle.

Your (Electric) Mileage May Vary

The CO2 and cost per mile calculations are based on the mix of coal, natural gas, nuclear and renewable energy used to generate electricity. Specifying wind power—which in many places is produced in excess at night—or solar power for your vehicle charging can drop the CO2 emissions down to near zero.

Plug-in vehicles produce fewer emissions largely because electric motors are more energy efficient than gasoline or diesel engines. Because much of the energy consumed by burning gasoline is lost as heat, gasoline engine require up to 80 percent more energy to propel a car the same distance.

For those who are more concerned with greenbacks than greening the plant, plug-in vehicles will cost less to operate. When driving on electric power, plug-in vehicles are estimated to cost about 3 cents per mile to charge, or just 25 percent of what it costs to fuel a typical sedan when gas costs $3 per gallon. Your monthly transportation bill could drop from around $120 per month to just $30 per month if you drive all-electric. So here plug-in vehicles are a clear economic winner, right? Not so fast.

This rosy calculation doesn't incorporate the additional cost of the plug-in vehicle, which is heavily influenced by the cost of the lithium ion batteries. If you factor in the cost of the batteries—assuming you can sell them after their useful life in the car—the total cost per mile of plug-in vehicles rises to a more comparable 10 to 13 cents per mile. If gas stays above $3, then plug-in vehicles could be cheaper to drive in the long run. However, the cost of lithium ion batteries is expected to drop by 15 percent or more per year, so future plug-in vehicles could be much cheaper to drive, especially if gas tops $4 a gallon.