Anybody who has driven an ICE powered vehicle understands one of the two commonly used fuel efficiency ratings: miles per gallon (mpg) or litres/100km. Here in the US, buyers of cars are familiar with the standardized stickers that include three different mpg values: highway, city and an average. Modern cars usually display real time fuel efficiency as miles per gallon too. But what are the equivalents for an EV driver?
Miles Per Gallon Equivalent
The MPGe rating is what car buyers in the US will see first for alternative fuel vehicles, including EVs. The intention is to provide a number that allows comparison between vehicles of different types, but the equivalence is based on the amount of energy in a gallon of gasoline, 33.7kW. Furthermore, it is based on the transfer of energy stored in the battery to distance, rather than the energy from the wall (the difference is negligible for gasoline, but not when charging batteries).
While this might be a scientifically accurate method for comparing efficiency of cars, it is less useful at comparing the operating costs. A gallon of gasoline in California costs around $3 at the time of writing, but 33.7kWh of electricity is around $6 here (and around $4 using the national average price per kWh). At California prices, a car rated at 100 MPGe costs about the same as a gasoline one rated at 50 MPG. Of course, get your electricity from solar panels, or free public chargers, and you change that math again.
Miles Per kWh and Wh Per Mile
Inside an EV, as you’d expect, there are usually lots of different display options to monitor energy consumption. Our B Class has two rolling averages, instantaneous consumption, last fifteen minutes historic, and real time energy flow (showing when we are using battery and when regenerative braking is charging it). The unit used in the car is miles per kWh (mpkWh).
Some EVs use Wh per mile (or kilometer), which is similar in concept to the litres/100km used in many places for ICE vehicle fuel efficiency. While the mpkWh numbers are typically less than 5, Wh/mile will be in the low hundreds (Four miles per kWh is 250 Wh per mile.)
Further complicating this, some manufacturers are using kWh/100 miles (or km), which yields numbers in the tens (250 Wh/mile is 25 kWh/100 miles). The advantage to using kWh is clear: we buy electricity by the kWh, so this is the unit people see on their bills. It also matches the unit used to specify battery capacity in EVs. Switching to 100 miles (or km) makes the numbers more approachable for some, and more closely aligns with the litres/100km metric, but might make it harder for some to estimate usage for actual trips.
As alluded to above, when charging an EV the kWh used from the wall socket doesn’t exactly match the kWh available in the battery. There are some losses in the process, mostly in the form of heat. For EVs, charging efficiency is often estimated at 85%, meaning for every kWh shown on your meter at home, the battery will gain 0.85kWh of usable charge. Put another way, charging our B Class’ 28kWh battery from empty would need about 33kWh from the wall.
All of that, no matter which unit you prefer, is consistent. In the real world though it appears it is not as simple. The first wrinkle appears if you take the battery capacity and multiply by the mpkWh number to estimate range. Our B Class battery is rated at 28kWh (without using the range extend option), and will typically start with an estimate of 2.5 mpkWh after a reset. That’s only 70 miles or range in a car that is meant to do almost 90 miles on a single charge. So where’s the difference?
Firstly, the 2.5 mpkWh number is apparently lower than reality. It looks as though Mercedes is factoring the charging efficiency into the number shown in the instrument cluster (though not into the historic bar graph). That gets us to 84 miles range. I first noticed this when trying to reconcile the graph with the average in the instrument cluster display. The average was shown as 2.3 mpkWh:
The graph showed an average of 3.0 mpkWh over the 14 minutes of the trip (worst case, 2.5 mpkWh if the bars round – they do not represent fractional values):
Multiplying the 2.3 mpkWh by the magic 1.2 number I found online gives an average of 2.8 mpkWh, which is more believable when matched to the graph. Hence my conclusion that the graph is not “correcting” the mpkWh.
Secondly, and this one depends a lot on driving style, regenerative braking will add a little to the range. Range estimates in EVs are usually conservative. For most of our trips, the estimated range drops by a few miles less than the distance driven. I prefer to watch the battery percentage, though my wife prefers the range estimate (it is also more prominent in the B Class dash since it is permanently displayed in the top display area).
You can see in the graph above, the small green bars that drop downwards indicating the percentage of time when regenerative braking was charging the battery during each minute.
Another thing that ICE vehicle drivers think less about is ambient temperature. For the past few weeks it has been cooler here (highs in the low 50s Fahrenheit) and we have been struggling to get above 2.5, and often seeing numbers below 2. This weekend though, with temperatures back in the high 60s, I easily reached 3.2 mpkWh.