Decoding Electric Car MPG

The fuel economy window sticker on the 2014 Ford Focus Electric proclaims 105 "miles per gallon equivalent" in large type. This mpg-equivalent (MPGe) figure is how the Environmental Protection Agency (EPA) converts the power used by an electric vehicle into a term that's familiar to most Americans: mpg. The Focus Electric's 105 MPGe number is ultimately more useful for comparing energy consumption than fuel costs, but we'll decode the agency's electric-car mpg stats and explain what it means to you and your pocketbook.

The agency's new mpg sticker for an electric vehicle (EV) has four key elements: kilowatt-hours per 100 miles (kWh/100m); mpg-equivalent (MPGe); annual electricity cost; and cruising range. We'll tackle each below.

Efficiency Rating

According to the EPA, when the 2014 Ford Focus Electric is driven on its combined (city/highway) mpg cycle, it requires 32 kilowatt-hours of electricity to travel 100 miles (32kWh/100m). Similar to miles per gallon (mpg), kWh/100m is a measure of the Focus's efficiency and the most important operating-cost figure for EV owners. Unlike mpg, the lower the kWh/100m number, the better. Adapting to this smaller-is-better figure is likely the roughest math most of us will face in relation to EVs.

The calculation of energy unit per miles (instead of miles per unit of energy) is common in Europe, where it is expressed in the metric system as liters per 100 kilometers. Americans, however, do not generally measure fuel efficiency with this 100-unit yardstick. There are plenty of online tools to convert mpg to gallons per 100 miles, and two easy-to-grasp examples of the logic of this measure can be seen by comparing the 2013 Toyota Prius and 2013 Bugatti Veyron.

With an EPA combined fuel-efficiency rating of 50 mpg, the Prius burns 2 gallons to travel 100 miles (2 g/100m). The thirstier Bugatti's 10-mpg rating means it takes 10 gallons to travel the same distance (10 g/100m). When converted from mpg to gallons per 100 miles, you can see how the Prius' lower figure of 2 g/100m denotes better fuel efficiency than the Bugatti's 10 g/100m. Go electric and you'll also see how 32 kWh/100m represents better fuel economy than 34 kWh/100m.

Kilowatt-Hours to MPG-Equivalent

So how does the EPA get from 32 kWh/100m to 105 MPGe? To create the mpg equivalent, the EPA uses an established energy standard of 115,000 BTUs (British thermal units) per gallon of gasoline. Grossly oversimplified, this means that if you ignited 1 U.S. gallon of unleaded gasoline, it would generate that much heat. To create the same amount of heat, you would need 33.7 kilowatt-hours of electricity.

So if the 2014 Ford Focus EV could travel 100 miles on 33.7 kWh of electricity (the energy equivalent of 1 gallon of gasoline), it would receive an mpg equivalency of 100 MPGe. But the Focus EV actually requires slightly less than the 33.7 kWh to travel 100 miles (in this case, 32 kWh), so it received an mpg equivalency rating of 105 MPGe.

The EPA label provides both city and highway numbers as well as the overall average fuel economy. For EVs, which tend to do better in stop-and-go driving, where regenerative braking can help contribute to the battery's charge, city fuel efficiency is typically better than highway. That's the reverse of most gasoline vehicles. The Focus EV receives a 110 MPGe rating for city driving but scores only 99 MPGe on the highway.

What does all this have to do with fuel economy? Not much.

The MPGe rating is really only useful for comparing the relative energy consumption of gasoline (or hybrid) cars with that of electric cars. The Focus EV uses the energy equivalent of 1 gallon of gasoline for each 105 miles of travel, compared to a hybrid Prius, which would use roughly 2 gallons of gasoline for every 100 miles it travels. Far more pertinent for electric car owners focused on cost is the kilowatt-hours-per-100-miles rating (kWh/100m), which shows you how efficient the vehicle is at converting electricity into miles traveled. The kWh/100m rating is the new EV mpg, and from a pocketbook standpoint, lower is better.

Annual Fuel Cost

An EV's EPA sticker also lists an annual electric cost figure ($600 for the Focus EV). The Edmunds article called "The True Cost of Powering an Electric Car" will show you that the cost of electricity (and thus EV fill-ups) varies significantly around the U.S.

For yearly electric cost, the EPA uses a national average of 12.5 cents per kWh, and the number can change each year. It assumes an annual driving average of 15,000 miles. If the Focus EV travels 1 mile on 0.32 kWh (32 kWh/100m), multiply 0.32 kWh by 15,000 and you've got 4,800 kWh. At 12.5 cents per kilowatt-hour, this equals $600.

Also note that though the efficiency rating of the Focus EV is 32 kWh/100m, this doesn't mean it will travel 100 miles on a full charge, only that it would take that much power to go that far. This brings us to range.

Range Rating

An estimated driving-range number also is a key part of the EPA sticker for EVs. In the case of the Focus EV, this is about 83 miles on a full charge of the car's 23 kWh battery pack.

The EPA label complicates things, though, by basing its calculations on the premise that because of various charging options, the typical EV will have only 90 percent of a full charge. Thus the range stated on the Focus EV's label is 76 miles.

Much like mpg, EV range can vary widely depending on how you drive. For the EPA range test, EVs are driven continuously on the combined city-highway cycle until the wheels stop.

PHEVs Have "Hybrid" Label

Plug-in hybrids, which combine attributes of the all-electric car and the conventional hybrid, have a more complex fuel economy label that attempts to cope with their dual-fuel status.

PHEVs, as plug-ins are often called, have rechargeable battery packs of varying size, depending on the vehicle's weight and the automaker's determination of how many miles of all-electric range the car should have. The 2013 Toyota Prius PHEV, for instance, offers just 11 miles of all-electric drive and has a 4.4 kWh battery. The 2014 Honda Accord PHEV delivers up to 13 miles in all-electric mode and carries a 6.7 kWh battery. The 2013 Ford C-Max Energi plug-in hybrid is rated at 21 miles of all-electric range and has a 7.6 kWh battery.

Labels for PHEVS show their all-electric fuel economy in the MPGe format: The Prius is rated at 95 MPGe, the Accord at 115 MPGe and the C-Max Energi at 100 MPGe. Their fuel economy while operating in traditional hybrid mode (which happens once the grid-charged battery is drained) is denoted in the traditional city-highway-combined mpg format: 50 mpg for the Prius plug-in; 46 mpg for the Accord PHEV and 43 mpg for the C-Max Energi.

Because of overall fuel economy on trips that require combinations of the all-electric and conventional hybrid modes, the label doesn't attempt to give a single fuel economy average for each PHEV, but it does show total combined all-electric and gasoline range.

Mileage Will Vary

When driving any car that uses electric drive, whether it's full-time, as with the Ford Focus EV, or part-time, as with the Honda Accord PHEV, fuel economy will vary depending on factors such as cargo load, terrain, climate and how much of a leadfoot you are.

We're all pretty comfortable with understanding our vehicle's mpg on gasoline: for the electric-drive part of the ride, the number to focus on is the kilowatt-hours per 100 miles (kWh/100m).

Start wrapping your head around a new fuel economy measure where smaller is better.