Electric vehicles might eventually be the future but..

Finally someone summarizes this point about not being misleading on electric vehicle efficiency:

  • When we say that electric motors are 97% efficient, we mean that the actual physical work produced per unit of time is 97% of the electrical power used by the motor, which equals the current flowing to the motor times its voltage.
  • When we say that the internal combustion engine is 45% efficient, we mean that the physical work we get out of the engine is 45% of the heat liberated from burning its fuel.

…¬†Taking these numbers, let’s convert the 97% efficiency number for electric motors to an efficiency number all the way back to the fuel so it is apples to apples with internal combustion. ¬†We take 97% times 90% transmission efficiency times 50% electricity production efficiency equals 43.6%.


The problem with all this is that most of the barriers to using electricity in more applications are not related to motor efficiency. For vehicles, the problem is in energy storage density. Many different approaches to powering automobiles were tried in the early days, including electric and steam powered cars. The main reason, I think, that gasoline won out was due to energy storage density. 15 gallons of gasoline weighs 90 pounds and takes up 2 cubic feet. This will carry a 40 mpg car 600 miles. The Tesla Model S 85kwh battery pack weighs 1200 pounds and will carry the car 265 miles (from this article the cells themselves occupy about 4 cubic feet if packed perfectly but in this video the whole pack looks much larger). We can see that even with what Musk claims is twice the energy density of other batteries, the Tesla gets 0.22 miles per pound of fuel/battery while the regular car can get 6.7. More than an order of magnitude, that is simply an enormous difference, and explains the continued existence of internal combustion engines much better than electric motor inefficiencies.

He’s not bashing electric cars he goes out of his way to point out where stats used might in reality be more in favor of electric cars, but this should be mandatory reading for having a discussion about electric cars, battery storage, and Tesla as such a high profile company.

Even Eric Schmidt (co-founder of Google, obvious genius) and the mostly excellent Economist trip over this. There are plenty of reasons to like electric cars and even all things being equal distributing air pollution out of inner cities is an advantage of electric cars but there’s a lot of common sense missing generally about how electric cars compare to combustion engines and doing reality checks on what subsidy programs are achieving.

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