Sunday, November 11, 2012

The Mystery of Miles per Gallon Electric MPGe

What does gasoline, Kilowatts, and British thermal units have in common?   They all are used to determine miles per gallon electric (MPGe)--whatever that means.

MPGe was devised by the EPA to explain the level of efficiency electric cars have to the motoring public.   We have become accustomed to miles per gallon (MPG) being the universal standard of efficiency as listed on the window of every new car. But the efficiency rating of EV's doesn't naturally translate into MPG because there are no gallons of anything being consumed per mile.

Once you drive an EV, you get used to the term of miles per kilowatt hour (Mls/kWh).  For example, in my ActiveE I commonly achieve 3.2 Mls/kWh.  Whereas someone else may achieve 4.0 Mls/kWh, which tells me they're a far more efficient driver.

But Mls/kWh doesn't have a universal meaning outside the EV world, so MPGe was invented instead.  I find MPGe to be a confusing standard, but here goes with an explanation: it starts with a gallon of gasoline, which when burned generates 115,000 British thermal units of heat.  If using electricity, it would take 34 kWh to produce the same amount of heat.  So one gallon of gas equals 34 kWh in terms of the energy generated.  The EPA then determines how far an EV can travel on 34 kWh under differing conditions and that distance is reported as MPGe.

The problem with this conversion is that not every EV has a 34 kWh battery.  The ActiveE has a 32 kWh battery and an MPGe of 102, but its range isn't always 102 miles on a single charge (although the BMW is probably closest to the 34 kWh mark).  Other EV's (aside from Tesla) range from a low of 16 kWh in the Volt to around 24 kWh in the Leaf.  So the MPGe reported for those cars is not the distance they can travel on a single charge, but rather the distance they could travel over multiple charges until using 34 kWh.  The Chevy volt for example is reported at 98 MPGe, but it can only travel about 35 to 38 miles on pure electric on a single charge.  The Honda Fit EV has an MPGe of 118, but has a range of 80 miles or less on pure electric.

Also, while a gallon of gas may cost less than a "gallon" (or 34 kWh) of electricity--it doesn't last nearly as long.  A gallon of gas in Southern California may cost $4.00 to $4.50 on any given day, whereas a typical kWh when purchased through Southern California Edison costs between 16 to 25 cents, making a "gallon" of electricity (34 kWh) cost $5.44 to $8.50.  It sounds higher, but not when you consider how far each gallon will take you.

Since EV's are far more efficient with the energy they use, a "gallon" of electricity goes a long way.  In a gas-powered car, a single gallon will allow you to travel about 25 miles.  In contrast, a "gallon" of electricity will take you 100 miles or more.  So $5.44 in electricity is roughly the same as four gallons of gas, or $16.00 in gas costs.  Why aren't they the same?  Because MPGe only measures the amount of energy generated by gas versus electricity.  It does not measure the cost of each energy source when put into practical use driving down the road.

Is MPGe a helpful measure?  Not entirely once you know the right questions to ask about EV's.  But it does look impressive to have 100 or more MPGe on the window of every new EV and hopefully it helps the gas-driving public understand just how powerfully efficient EV's can be.


  1. One thing to consider is that while the ActiveE has a 32 kWh battery, only 28kWh's are usable with the remainder left as a buffer.

    Also, the EPA rating takes into consideration charging losses because it's "wall to wheels" not Battery to wheels so while the AE needs 28kWh's to fully recharge, it really uses about 31 kWh's to do so because the energy lost while charging is ~3kWh.

    When the EPA range test is calculated (the AE is rated at 94MPC) they simply fully charge the battery without regard to how much energy is lost during the charging process because they are only concerned in how far it can go, not how much energy is uses. That is for the MPGe to reflect.

    1. Thanks for the insight, Tom. Its very interesting, yet very confusing stuff. Now that I have driven EV for 6 months I prefer just knowing the Mls/kWh to compare efficiency from one EV to the next. MPGe doesn't mean much to me, but there it is on the car window.