One Month in an EV

It’s not our first month in the EV (in fact, we’re coming up on the one year anniversary of our purchase of the Mercedes B Class EV we use as much as possible). But I wanted to get data for a full month to see how much electricity we use, how far we drive and what sort of fuel economy we get from using the EV.

Up front, I will say that the B Class is not the most efficient EV on the market. It is not as light as, say, a Nissan Leaf or the BMW i3. It is also very much a compliance vehicle intended only for use around a city (it does not support rapid charging, even though the electronics and battery system, which were made for Mercedes by Tesla, are believed to). In terms of quoted efficiency, it comes in around the same mark as the Tesla vehicles, and a long way behind the Nissan Leaf.

I tracked the mileage using the car’s trip feature. We started with 100% in the battery on June 1, 2018, and the image on the right was taken earlier today, June 29, 2018, also with the battery at 100%.

During the month, we have added 132.71 kWh at home (costing around $25 at the current rate we pay), and we also had a couple of free top-up charges from the local Volta Charging point here in Alameda. Unfortunately, there is no way to know how much energy was added there.

Mismatched Numbers

The car believes that during that time we averaged 2.9 miles per kWh, and I am given to understand that the number displayed here takes into account charger loss, so it should match the energy taken to charge the car rather than just the energy stored in the battery. That would imply we used around 113 kWh in the month.

Unfortunately, that doesn’t match the number reported by our JuiceBox Pro. That says we used almost 133 kWh, and that also does not include the free electricity from Volta. At 133 kWh, the efficiency would be just under 2.5 mpkWh. If I guestimate the Volta sessions as 17 kWh total, bringing the month to 150 kWh, then the efficiency drops to around 2.2 mpkWh.

Cost Per Mile

Our cost per mile is easier to calculate, and, using 20¢/kWh (which is a rough estimate of how much we pay at present) it works out that in June we spent about 8¢/mile. California has higher electricity rates than most states, averaging 16¢/kWh. The range starts at Washington state, averaging just under 10¢/kWh, through to Hawaii where the average is over 30¢/kWh.

June was also slightly higher at home than our normal usage because we had less free charges away from home. In May we added over 20 kWh using free Chargepoint network connections, as well as a few top-ups at Volta locations.

Comparing to our ICE SUV though, we have saved a lot in fuel costs. Around town, the SUV typically comes in at around 15 mpg (the manufacturer amusingly claims 23 mpg in “city” driving – we have never seen that). At 15 mpg, we would have used almost 22 gallons of gasoline, which costs around $4/gallon here at the moment (California is also one of the most expensive places to buy gasoline). That means over $80 in fuel for the SUV, or a cost per mile of around 25¢. If, by some miracle, we managed to average 23 mpg, it would still be almost $60 of gas (17¢/mile).

That is just fuel costs too. EVs are lighter on brake wear, because they use re-generative braking aggressively to recover charge reducing the wear on the friction brakes. There’s also no need for regular engine oil changes.

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