GM has announced the Chevy Volt, getting 230 miles per gallon, and Nissan has announced an electric car they claim gets 367 mpg. I’m not sure how they arrive at those numbers, but let’s try to estimate a more meaningful mpg value.

The Nissan goes 100 miles on a 24 kilowatt-hour battery charge. That means 4.2 miles per kwh. A friend of mine owns a Tesla Roadster, and he says he consistantly gets 240 wH/mile, which also translates to 4.2 miles per kilowatt-hour. That energy comes from power plants, which could theoretically burn gasoline. A gallon of gasoline yields about 36.6 kilowatt hours of energy when burned, but the engine in conventional cars only used about 20% of that energy. Power plants are about 45%efficient, because they use turbine engines. Furthermore, there is about a 7 percent average loss of power in the transformers and power lines used to deliver electricity to your home.

There is also charge-discharge efficiency of batteries and the efficiency of electric motors, but that is already factored into our direct measurement of 4.2 khw/mile. So assuming 4.2 kwh/mile, and 36.6 khw/gallon, and 42% efficiency, I get 64 miles per gallon. Let’s say 60 to 70, because this is not a precise calculation.

That’s good, but not vastly better than a hybrid. What is really interesting about the electric car is that electricity can come from sustainable sources like wind and solar energy. And in the short term, it can come from coal and natural gas, which America has in great abundance.

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