kWh UsedWhen you fill up a normal ICE car you know exactly what your costs are for the fuel. With an electric vehicle it is not that simple. There is a charging efficiency factor that comes into play which means that that energy that your car may be telling you that it used could be quite a bit lower than your actual usage.

A while back I wrote about installing an EKM Digital Submeter on my NEMA 14-50 outlet to measure actual power usage of the Model S so I could compare that to the reported power used. I had just had the meter installed and collected some initial data about the charging efficiency based on a very short interval. Now I have more data and the loss is larger than I originally expected.

Model S charging efficiency is worse than you may think.

Test Setup

I charge at home 99% of the time. So far in 3 months, 7,500+ miles i’ve only used a SuperCharger once and a HPWC (at the Tesla store) twice. At home I have a NEMA 14-50 outlet installed by a licensed electrician. I’m using the factory supplied mobile connector as the connector/cable between the outlet and the car.

Added to that outlet is an EKM digital sub meter to measure actual draw from the outlet. That meter is accurate to within 1% and does not add any measurable load of its own.


On the “anniversary date” of taking delivery of my Model S I record a bunch of pertinent information and then reset the Trip A setting. Before driving the next day I record the reading on the EKM meter. That way i’ve got the mileage and the Tesla reported power usage over the period driven and the the actual kWh used to get back to the original charge state (90% for me).

This process will let me see a bunch of information I want to track over time:

  • Monthly miles driven
  • Monthly kWh used as reported by the Model S
  • Monthly kWh used as reported by the EKM meter
  • Monthly Average energy used

I plan on using this information to look at how average energy used changes as the months/temperature changes and perhaps as the Model S gets more miles on it. Unrelated specifically to the car, but useful in other analysis is my current “all in” cost per kWh from my electric company and the current local gas price i’d be paying if I had purchased another Acura. Those will both change over time too.

While I don’t drive consistently on any given day (test drives, special trips and the like), the numbers will average out and my driving style is not likely to change much after over 30 years of driving (yeah i’m getting old but the Model S makes me feel young again!). I also drive pretty consistent patterns of commuting with a lot of miles to the same places which helps average out the special trips to locations with different terrain/conditions.

Basically, while the conditions aren’t perfectly stable over time, the averages and data from this real world testing will be pretty accurate.

The Data

The last period (6/21 – 7/21) was my first full period with both the car and the EKM meter. A month of driving and charging, especially with the miles and kWh’s involved is a decent period over which to look at the results versus the 2 days from my prior blog post.

Here’s the data:
Actual Energy June-Jul

In the above table you can see that the Model S reported 728 kWh used during the period but the meter reported 894 kWh used. This means my charging efficiency is only about 82% and electric usage (and cost) is 23% higher than I may have expected based on the readings the Model S provides. For that month this is an extra $26 of charging cost which is a small number but a decent percentage of the total. The good news is that even using this larger kWh number, the savings versus driving my old ICE car for energy alone comes in at $334 — i’m saving $334/month in gas driving my Model S!

My electric cost is 23% higher than I may have expected based on the readings the Model S provides, but i’m saving $334/month in gas driving my Model S!


External research indicates that an average charging efficiency loss in the industry is in the 10-12% range. In the forums of the other EV vendors there are people doing a similar analysis and getting charging efficiencies close to 90% (10% loss) on both the Nissan Leaf and the Chevy Volt. Consumer reports also did a study on the Nissan Leaf and reported 85% efficiency although that test was short term and possibly flawed from that.

Average charging efficiency loss in the industry is in the 10-12% range

Over this one month period of over 2,400 miles i’m seeing an 23% loss using the standard home charging setup that Tesla recommends. Many people quote an 85% charge efficiency for Tesla, and Tesla’s own charging calculator appears to assume a 91% charging efficiency which is quite different than the 82% actual charge efficiency i’ve measured and  significantly worse than the average industry charging efficiency.

It would be great to see a Model S owner do a similar test with a HPWC setup at home to see if that HPWC is somehow more efficient (it likely is) and gives results closer to what Tesla is providing. I’d love to do the test but i’m not quite ready to shell out $1,200 plus electrician costs to get that data — assuming a cost of about $3,000 all in it would take me over 20 years to break even assuming the HPWC improves my efficiency by 10%.

From the results above, my conclusion is that the Model S charging efficiency using the standard home setup is 5-10% worse than other EVs on the market.

Model S charging efficiency is 5-10% worse than other EVs on the market.

Does Tesla have better battery technology, or is this just a battle of size? More to come on that front.