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kWh usedOne of the things that many electric vehicle owners don’t know is that the energy the cars report they use isn’t the same as what they draw from their power source. For many EVs, about 10-12% of drawn electricity is lost during the conversion process from AC to DC charge. This means that beautiful screen and counter you may have on your EV dash showing kWh’s used cannot be converted directly into dollars spent for transportation just using your power company’s rate per kWh.

The energy that EVs report they use is less than what you’re paying for.

Problems calculating actual energy costs

For the Tesla Model S, there are several counters and charts available. The “Since Last Charge” section is a great automatic counter that counts distance, energy used and average energy per mile since the last time you unplugged. With judicious use of the trip meters you can measure energy used over specific time periods, but it’s difficult to match those periods up to your power company’s billing cycle.

“Since Last Charge” section is a great automatic counter

It is also difficult to look at energy bills from before you started driving an EV to those after driving the EV to find the difference and calculate the added costs of driving an EV. This requires good records keeping, consistent use of electric for everything but the EV over the compared period, as well as other factors like weather. With a great deal of uncertainty you can calculate the rough amount of added cost from driving the EV on a monthly basis, look at the miles driven during that month and do the math. For example, year over year I used an extra 641 kWh in May of 2014 versus May of 2013. The main difference was taking delivery of my Model S in April so I drove an EV for all of May. I’ve driven an average of 2,350 miles per month (30 day average) since I took delivery so that gets me to 272 Wh/mile. My Tesla is telling me i’m using more than 272 Wh/mile and it’s no surprise that this rough way of calculating is off.

If you use reported energy used for that “Since last charge” report above you’d think I consumed 10.6kWh and at an electric cost of $0.1670/kWh that it cost me $1.77 to drive those 35 miles. But this doesn’t account for any power conversion losses. Your actual costs for driving those miles are higher and it will vary by type of EV and how you’re charging your EV. A standard 110V wall plug in the US is reported to be much less efficient than a NEMA 14-50 connector.

Your actual costs for driving those miles are higher and it will vary by type of EV and how you’re charging your EV

Charging calculators like the one on Tesla’s site can take into account some of these factors. For example for 35 miles of range added it reports that you need 11.6kWh of energy. From the picture above I only used 10.6 kWh to drive 35 miles. This implies that the Tesla calculation includes an assumption of 91% efficiency on the charge but there are other factors that affect this kind of calculation like how fast/aggressively I was driving. Unfortunately Tesla does not disclose their assumptions for their calculators.

It’s not simple to calculate how much actual energy you’re using to charge your EV.

The Solution

EKM Meter installedThe solution for this is to put a kWh meter on the outlet where you charge your EV. There are many types of meters out there from basic kWh counters to advanced meters that can broadcast actual use over a wifi network, plot graphs, etc. After some research I selected a meter from EKM Metering. My electrician did some independent research and came up with the same one so I felt pretty good about my choice. To monitor my NEMA 14-50 outlet (the one Tesla recommends for most people who charge at home), I bought the Basic 100A kWh meter and an enclosure for it. Delivered, the parts were $142.

Installing the meter requires messing with 240V/50A circuits and re-cabling your outlet since power has to flow through the meter to the outlet. While I may have been able to do this myself, I chose to have an electrician do it. I’m glad I did. It took him 3 hours to do it and I was keeping an eye on things throughout. Part of the time sink was the improperly sized enclosure EKM Metering provides — there’s just not enough room in there to route thick electric cables easily. After some blood, sweat and modifications to the enclosure (not really expected when you pay $40 for a piece of plastic), he got it done. He charged me a rate of $50/hour which is reasonable in New England for a licensed electrician.

My total cost to buy and have a meter installed was $292.

The meter is the white box on the right above my NEMA 14-50 outlet.


EKM ReadingOnce installed you have a kWh meter that just counts kWh used. There’s all sorts of fancy electrical math around how it does the counting but important part is its really accurate and doesn’t impact the results much as it does the measurement. This counter is like an odometer. It counts up forever. There’s no reset, no “trip counter” equivalent. Its just a counter that counts up when power is being drawn. You know power is being drawn by the flashing red light on the meter — 1 flash = 1.25Wh used.

With this meter you can measure actual energy used over any time period but you have to record some things. I’ve only had the meter a few days but here are the the measurements and results so far:

kWh Results

The items in orange are calculations. So i’m seeing about 85% charging efficiency on my Model S which is quite a bit less than the 91% Tesla seems to be using in their online calculator. I think the young EV industry could use more education and disclosures in this area.

I’m seeing about 85% charging efficiency on my Model S

I’ll be doing longer term measurements with the meter. Accuracy over short durations like this can have a lot of variation. I also want to look at things like power draw differences at night over long periods with allowing the car to go to sleep or not. It will also be interesting to see if these numbers improve or degrade over time with use, software updates and the like.


The kWh meter is a nice addition to a charging setup and gives you a true picture of your energy usage for driving your EV. For many it is an added and unnecessary cost: at $0.167/kWh and 325 Wh/mile the meter plus installation is the equivalent of 5,380 miles of EV driving.

Simply add about 15% to reported energy used by your EV and you’ll be closer to the actual energy used and will have a better picture of your true costs per EV mile driven. Drive clean and drive smart!

Simply add about 15% to reported energy used by your EV and you’ll be close.