I answer a lot of questions on Quora. Many of those questions are about automotive things that may be of interest to CarNewsCafe readers. Like this one.
My Answer: The standard is MPGe (also written MPG-e), or miles per gallon equivalent. This is a measure of the vehicle’s electric use versus the already common MPG numbers used for internal combustion engines (gasoline- or diesel-powered). This metric is standard for all vehicles as set by the EPA for fuel efficiency standardization. It will be found on every vehicle’s window sticker (“Monroney”).
The other analog, for the electric car driver, is the “expected range” meter in nearly all plug-in vehicles now. This is a (more or less) real-world expectation for how far the vehicle will go with its current charge state given the driving habits recorded thus far. Most expected range meters are based on a miles per kilowatt hour (MkWh) measurement that works in a way similar to the MPG measurement in most combustion vehicles.
Different manufacturers use different metrics for creating this number, but for the most part it begins with the best possible for the vehicle and then drops down according to factors affecting the vehicle’s efficiency (hills, throttle use, speed, etc). For example, in a BMW i3, the expected range dropped from 140 miles to about 80 after I’d been on the 80mph freeway for about four minutes as the vehicle realized that my speed would greatly reduce range. This number continued to drop as I went and then settled and rose again slightly once I left the freeway for surface roads at lower speeds.