According to data provided on this page, a Tesla Roadster requires about 177 Watt-hours to travel one mile. Assuming that on average, the efficiency of electricity generation in the US is 30%, the Tesla requires 2.1 MJ of primary energy to travel one mile (surely there is a dependence on velocity, but that information is not provided).
My Honda Element gets 24 miles to the gallon, so it requires 0.0417 gallons of gasoline per mile. One gallon of gasoline contains 131.9 MJ, so my car consumes 5.5 MJ of primary energy to travel one mile.
A conventional vehicle would have to get 62 miles per gallon to consume the same amount of primary energy per mile as the Tesla Roadster.
I drive about 17,000 miles per year. At 24 miles per gallon and fuel costs of $1.80 per gallon, I spend about $1,275 per year for gasoline.
An electric vehicle that requires 177 Wh per mile would use 3009 kWh to travel 17,000 miles. At 7.5 cents per kWh, that amount of electricity would cost $226 — about 17% of the cost of a gasoline-powered vehicle. I could save over $1000 per year.
However, a new Tesla costs $98,000 — which means the simple payback would be just shy of the century mark.