The overall efficiency of the electric car depends most signficantly on the source of electricity. If you electric car is recharged from your own windmill, the efficiency can be as much as 80%, but if power station losses and grid losses are also included that may drop to as much as 28%. Still better than the petrol car though.
|Electric-Powered GM EV1||Gasoline-Powered Acura .2TL|
|Start with||1 million BTUs||Start with||>1 million BTUs|
|Energy left after generation (39% efficiency)||390,000 BTUs||Energy left after refining (92% efficiency)||920,000 BTUs|
|Energy left after charging losses (88% efficiency)||>343,000 BTUs||Energy left after transport (95% efficiency)||874,000 BTUs|
|BTUs per kilowatt-hour||3412 BTUs||BTUs per gallon of gasoline||115,400 BTUs|
|Electricity available||100.6 kWhr||Gallons available||7.6 gallons|
|Energy efficiency||0.19 kWhr/mile||Fuel economy>||24 mpg|
|Miles per million BTUs||529.5 miles||Miles per million BTUs||182.5 miles|
|Equivalent mpg||69 mpg||Equivalent mpg||24 mpg|
From http://www.electroauto.com/info/pollmyth.shtml a bit out of date
This would suggest, even with current technology, that you are greener to buy an electric car and charge it off the grid then drive an equivalent petrol car.
Here is a more entertaining comparison of electric vs. petrol. Mythbusters converted a petrol go-kart to electric and it performed almost as well as the petrol one, which considering it was nearly twice as heavy with the batteries, wasn't bad going.