Only a month ago the benchmark for fuel efficient vehicles was the Toyota Prius hybrid at 51 MPG. Now the rules have changed and GM, using EPA guidelines for Plug-In vehicles, is the undisputed winner in the hybrid miles-per-gallon race with a claimed 230 MPG for the Volt.
Not to be out done in the ever escalating EV PR wars Nissan used the same EPA Plug-In guidelines and got a figure of 367 MPG for their all electric Leaf. The obvious question is � how is an electric car rated with miles-per-gallon when, in an electric vehicle or a hybrid's electric-only mode, the miles per gallon are infinite and have no meaning since no gasoline is being consumed.
Before the major auto makers started all their MPG marketing guff, EVs were rated by watt hours per mile (Wh/mi). For example, the legendary General Motors EV1 was rated at 179 wh/mi, while something with more 'conventional' aerodynamics like the RAV4 EV uses around 250 wh/mi. To convert these figures to equivalent MPG is fairly straight forward but it is only a theoretical comparison because no gasoline is actually being used.
Surprisingly we haven't found a calculator on Google for this, but we're sure there will be one soon. The base figure needed for this calculation is the number of watt hours of energy in a gallon of gasoline, that being 33,705 wh/gallon. Using this figure we can easily calculate that the EV1 got the equivalent of 188 MPG and the RAV4 EV gets 134.8 MPG (the same as the Volt, but more on that later). Surely the Nissan Leaf has to fall within that range somewhere, not 367 MPG as stated?
Where these calculations go off the rails is when a plug in hybrid is combined with a part time duty cycle Internal Combustion Engine (ICE) to extend an EVs range. GM says the EPA will weight plug-in electric vehicles as traveling more city miles than highway miles on only electricity, presumably figuring that people buy electric cars primarily for local driving. GM expects the Volt to consume 25 kilowatt hours per 100 miles of city driving (that's 250 wh/mi in the old numbers). At the U.S. average cost of electricity (approximately 10 cents per kWh), a typical Volt driver would pay about $2.50 for enough electricity to travel 100 miles, or 2.5 cents per mile. (Conversely, a gasoline-powered car that gets 20 MPG, for which the driver pays $3 per gallon, has a per-mile fuel cost of 15 cents or $15.00 per 100 miles.)
Based on the Volts 250 wh/mi energy use in EV mode (the same as a RAV4 EV) it's MPG compared to the energy content of gasoline is 134.82 MPG so HOW do the EPA guidelines allow GM to come up with 230 MPG?
Apparently the EPA used a traffic survey that was done in 2001 to create a composite to take into account the distance traveled in EV mode and the fuel economy after the charge is depleted. This is mated to an aggregate sample of the population and how far they drive in a day. With this data they created a 'utility factor'
This 'utility factor' or 'Driving Pattern Factor' is just one variable in what the DoE/EPA officially call the 'petroleum-equivalency factor' (PEF) which has been on the books since July 2000. The background is that Congress wanted to allow auto manufactures to quote high equivalent 'fuel economy' for electric vehicles as an incentive to help accelerate the commercialization of electric vehicles. While the formula is based on the actual energy consumption of the vehicle it also includes correction factors that take into account 'upstream' factors such as national average electricity generation and transmission efficiencies, the need for national energy conservation in all forms, and an EV driving pattern factor.
The Petroleum Equivalency Factor starts with PEF = Eg * 1/0.15 * AF * DPF
1) Eg = Gasoline-equivalent energy content of electricity factor
2) 1/0.15 = the 'Fuel content' factor
3) AF = Petroleum-fueled accessory factor
4) DPF = Driving pattern factor
The basics are, the DoE has created a formula to balance the up stream efficiencies of both EVs and ICEs. On the EV side the formula includes fossil fueled power plant efficiency at 32.8% and electricity transmission efficiency at 92.4%. On the ICE side Petroleum refining and distribution efficiency is included at 83%. The result is a Gasoline-Equivalent Energy Content of Electricity Factor (Eg) of 12,307 wh/gal. That number is then divided by a 'Fuel content' factor of 0.15 (gasoline vehicle efficiency of 15% at the wheels??) to give 82,046 wh/gal.
To keep a very long story short (you can read the full EPA explanation here)
after further calculations the GM and Nissan EV mode MPG numbers are based on an equivalency number of 81,407 Wh/gal instead of the true 33,705 Wh/gal which doesn't compensate for anything 'upstream'.
Based on this knowledge and presuming the accessory and driving pattern factor don't get included in the Nissan Leaf calculations because it's a 100% EV we can use a simple desktop calculator to figure out how Nissan arrived at 367 MPG. Divide 81,407 by 367 gives the Leaf an energy consumption of approx 221 Wh/mi in the old money. The uncompensated figure for the Nissan Leaf becomes 152.5 MPG, less than half the EPA adjusted number. Feel free to visit the EPA site and calculate the full duty cycle for the GM Volt figure yourself.
So to answer the question why are EVs rated in MPG, at the moment it's a government instituted incentive that allows auto makers who start making plug-in EVs to claim huge MPG bragging rights compared to non Plug-In EV auto makers. As EVs grow to become a significant percentage of the road fleet we may see a return to energy rating formats like Wh/mi.
For now watt hour/mile will remain a 'technical' term that tells you how much it costs to run an EV while the purely 'marketing' MPG term tells you how much you might save compared to the old ICE clunker you're thinking of trading in.