Auto Manifesto

February 18, 2008

The Cost of Electricity vs. Gasoline

Just looked at one of my recent electric bills here in Virginia and my average annual cost for a kilowatt hour of electricity is 6.3 cents. Let’s put this in perspective and see how it compares with gasoline.

According to Wikipedia, gasoline has 32 to 34.8 MJ/L. So let’s assume 33 MJ/L. A gallon is 3.78 L so that’s 124.74 MJ/gal. A mega-Joule is a million Joules. So that’s 124,740,000 Joules in a gallon. A Watt is a Joule/second. So a Watt-hour then is equivalent to 3,600 (seconds/hour) times a Joule = 3,600 Joules.

A gallon of gasoline is equivalent to 124,740,000/3,600 = 34,650 Watt hours = 34.65 kWh. So let’s see, my cost to obtain a gallon of gasoline in electricity is 34.65 x 6.3 cents = $2.18295 or about $2.19. That’s not bad.

But it looks even better when I consider tank-to-wheel efficiency. A gasoline engine is maybe 30% efficient at best? Whereas when you compound the losses from a battery to an electric motor, it’s probably more than 60% efficient (say 70% battery efficiency, 85% motor efficiency). So if gasoline is at $3/gallon you’d have to buy $7 of it to get the equivalent work of $2.19 in electricity (70%/30% = 2.33; $3/gallon x 2.33 = $6.999).

Granted I don’t know if the electric meter in a house accounts for transmission losses from the power plant to the house, but I’m going to guess it doesn’t. So the electricity that comes out of your outlet is essentially what’s measured and what you’re getting charged for.

Not considering the environmental side of coal-fired power plants (or any other source) electricity seems to be very competitive with gasoline in terms of fuel cost.

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