You are conflating range (time or distance) vs efficiency. First, % battery loss per time would be an unnecessarily complex metric, you would just put the time. That does not tell you how efficient the car is as battery size is not held steady. In terms of a gas car, a Hummer could have a longer time until you need to fill up, but it is obviously not as efficient as a Prius. MPGe while not necessarily the best metric, does provide an efficency metric. It also has a rough correlation to the MPG metric used for years in the US.