Watt-hours per mile is common, and easy enough for a reasonable consumer to calculate the expense of given the cost of a kWh in their neighborhood.
EPA MPG estimates for electric vehicles are unfortunately purposely distorted for the purposes of giving electric vehicles higher MPG [ http://www.sae.org/mags/AEI/6559 ]. For instance, a gallon of gasoline contains about 33 kWh of energy, but an normal car wastes about 2/3 of this, netting only 11 kWh per gallon.
Logically, an electric car should arrive at an MPG by calculating the number of miles that it would travel on this same amount of energy. Instead, the EPA has told all-electric companies that they can do this computation using 82 kWh (almost 7.5 times more than the logical value), giving all-electric cars a huge benefit (Nissan's upcoming electric has claimed 367 MPG).
Plug-in electric cars use a different value, which is somewhere between the two, giving Chevy some magic number in between. Just to confuse matters, the EPA sticker on the electric Mini quotes kWh/100 mi, giving it "33" city and "36" highway, numbers that resemble current gasoline car mileages.
[EDIT: Also, it has been said that the gasoline engine in the Volt will provide around 50 MPG. Whenever you travel farther than 40 miles (assuming a full battery), this car is no more gasoline efficient than the Prius.]
Agreed. It'll have to be a measure of energy consumption per mile. Consumers will also have to develop an understanding of the relative cost of different energy sources.
Maybe we rate cars in Miles/Joule and rate energy sources in Joules/$. Makes the math pretty simple.
You might need to add some cargo/people capacity in there. A tiny electric car will have a hard time beating a diesel MAC truck for efficiency when it's hauling a few tons :)
There is no intuitive way to convey what you are proposing. If you tried to move 4 tons of cargo, a big rig would use less energy than a Civic. If you are trying to get yourself across town, a Civic will use less energy than a big rig. It would be complicated and expensive to calculate the full spectrum of efficiencies for a vehicle under different cargo loads. Personally, I would use the data, but I suspect the average person would find multiple efficiency graphs to be overwhelming.
MP$, MPKWH?
MP$ is what we really care about but it has to be adjusted for price fluctuations of power and gasoline, pretty impractical.
Maybe list MPG City, MPG Highway, MPKWH Battery on the sticker.