EVs and MPGs
Pure battery electric vehicles (EVs) do not consume gasoline or any other fossil fuel, at least not directly. However, they currently exist in a market where buyers might be cross-shopping between a gasoline model and an EV. In order to allow for comparisons between conventional vehicles and EVs, the Environmental Protection Agency (EPA) has derived a method by which a miles per gallon of gasoline-equivalent (MPGe) is provided for EVs. This figure is displayed prominently on the Monroney window sticker which, by law, must be displayed in all new cars sold in the US.
The use of MPGe to rate electric vehicles can be confusing to consumers as the units of miles per gallon have no obvious relation to energy usage or efficiency in electric vehicles. However, a close look at the derivation of this rating reveals its utility. In order to obtain the MPGe rating, as well as range, an EV is fully charged and allowed to sit overnight before being subjected to a multi-cycle test. A dynamometer, essentially a large treadmill for cars, is used to simulate various road conditions, or drive cycles. Following a standardized procedure, the vehicle is subjected to steady-state, highway and city drive cycles repeatedly until the battery is discharged and the vehicle can no longer continue the test. Throughout the test, the EV’s battery voltage and discharge current are carefully measured and logged.
With the dynamometer test complete, the EV is then recharged. While recharging, the energy consumed by the vehicle from the grid is recorded. Using this information, in conjunction with the data from the dynamometer test, an efficiency value in units of watt-hours per mile can be derived for both city and highway driving. This value is then converted to MPGe by dividing the volumetric energy density of gasoline by the vehicle’s efficiency in Wh/mile. For the purpose of the EPA’s calculations, the energy density of gasoline is taken to be 33,705 Wh/Gal. The resultant is a value with units of miles per gallon of gasoline. Before it is printed on the window sticker, this value is multiplied by an adjustment factor of 0.7, or a value provided by the manufacturer from real-world testing. This is done in order to account for consumers’ driving habits, varying environmental conditions, the use of HVAC, etc. The combined MPGe rating is simply a weighted average of the highway and city ratings, with a 55% weight assigned to city driving and the remaining 45% to highway driving.
It is important to note that MPGe ratings are so-called wall-to-wheel figures. That is, the data provided is based on energy consumed from a wall outlet to charge the car. It does not account for upstream inefficiencies associated with the electrical grid or generation. However, it does account for the losses associated with the vehicle manufacturer’s provided charger.
While MPGe does not offer a perfect comparison between gasoline and electric vehicles, it certainly does provide one valid point of reference. More importantly, since the test procedure is standardized, this rating allows for a fair comparison in efficiency between EV models. At the Solar Initiative, we wish to see the continued adoption of EVs; in order to make informed decisions, it is important for consumers to understand the various facts and figures associated with EVs.