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Tom looking cool
Tom looking cool

Efficiency: Equivalent Fuel Consumption

Greenpower cars are pretty efficient compared with normal road going cars.
For Rotary Racer at the National Greenpower final in 2011, which did 120 miles in 4 hours averaging just over 30miles per hour, the basic maths produces something like:

Greenpower car uses 17Amps average over 4 hours at 24V
Greenpower car power 408 Watts, 0.4Kw on average
Energy Usage for Race 1.632 kWh (5.875 MJ)
Electricity Cost per race 1.632 * 0.12 (12p per kWh) = 19 pence
Petrol Energy 34MJ/L = 154MJ/Gallon = 47 kWh per gallon
Cars equivalent petrol usage for race 1.632/47 = 0.034 Gallons
For a car doing 120 miles 120 / 0.034 = 3529 miles per gallon

This ignores efficiency of charging the batteries: about 90% ? (reduces to about 3176 MPG)
Also ignores efficiency of generating the electricity initially if generated from and comparing directly with fossil fuels.

Bloodhound SSC will use about 250Kg of aviation fuel (45MJ/L) and 180Kg of HTPB (45MJ/Kg ?) for a run. This is roughly 19,350 MJ of energy. Rotary Racer, traveling a tad slower (!), could travel 395,234 miles with this amount of energy. This is almost 16 times round the world or almost the distance from the Earth to the Moon and back (1.75x)  !