MPGe discussion

[Mirageman38]

Warning this is off topic and long. Don't blame me for boredom.

So if you look up cars with the best MPG lately all you see are electrics with fancy triple digit MPGe ratings. I wanted to delve into how this was calculated and how a large, heavy car, with massive tires, and mediocre aerodynamics could be getting such great MPGe ratings.

MPGe is the abbreviation for “miles per gallon of gasoline-equivalent.” It’s an energy efficiency metric that was introduced by the Environmental Protection Agency (EPA) in 2010 to compare the amount of energy consumed by alternative fuel vehicles to that of traditional gas-powered cars. If a vehicle uses non-liquid fuels that aren’t burned and gets its power from electricity or compressed natural gas, it’s rated in MPGe.

According to the EPA, burning one gallon of gas produces 115,000 BTUs (British thermal units). To generate the same amount of heat by way of electricity, it takes 33.7 kilowatt-hours (kWh). Kilowatt-hours is the standard energy unit for electricity.

In simplified terms, if an electric vehicle can travel 100 miles on 33.7 kWh of electricity, the EPA rates it at 100 MPGe.

This definition felt a bit unsettling for me as for the same reason cars don't do 115k BTU worth of mechanical work on a gallon of gas, neither should electric vehicles. An article on Reuters I found pins the average modern new gas engine to a 40% thermal efficiency while Nissan is working on a 50% thermal efficiency engine and some very inefficient vehicles clocking in at 20%.

This begs the question how thermally efficient is commercial scale electric? As it turns out, not very efficient. In 2020 in the US 36.47 quadrillion BTU of electric was generated. Among losses to this are conversion losses, plant electric use, transmission loss, and delivery losses. Coal power has among the lowest thermal efficiency, while renewables are significantly better. Despite this less than 20% of the US grid is renewable and only 12.96 quadrillion BTUs of electric was used in 2020. This equates to about a 64.5% loss or only about 35.5% efficiency. This seems to suggest a modern internal combustion engine can have better efficiency than wall power.

Then the next aspect is charging inverters. EV chargers comes in Level 1 (slowest) to Tesla supercharger. According to the IEEE level 1 charging efficiency for electric cars is only averaging 85.7%. Level 2 and original superchargers could be up to 90% efficient. Supposedly Supercharger V3 could be 95% efficient but I haven't seen any real world data. So of the 35.5% efficiency another 5-15% of this number is lost in the battery charging process. This brings down efficiency overall to between 30%-33.7%.


According to the EPA the Tesla model 3 long range is rated at 134 MPGe. With the previous set of efficiency calculated this puts the average MPG down to between 40-45MPG depending on slow charging or ultra efficient supercharger V3. This number is a long ways off from the publish MPGe figures. With this math a Chevy volt could get as little as 35MPG vs its published 118 MPGe rating or a Nissan leaf at only 33.6MPG.

While the raw thermal efficiency may not be as good, this does little to address the two other factors, of costs for fueling and environmental cleanliness. As it stands with costs I went into detail on my electric car rant how the math doesn't add up, so what is left is further investigating the environmental impacts. I will save that part for another day. Let me know if I mathed wrong or forgot something.


Update: After thinking about this some more, some energy is lost in the refining of gas at around 10%. This number seems to be guessed at often but not really known. Also while some electric is used to refine gas most fuel refineries use isn't electric itself. Additionally there are some transportation losses delivering the gas to a station that haven't been calculated in.