People are a certain size, air has a certain density, and people want to go 
a certain speed, this fixes the constant power a car needs to be able to 
produce to around 25 kW.  Higher peak values might be needed to get 
acceptable acceleration and hill-climbing performance.

People want their car to go a certain distance between being "refueled" 
whatever that might mean, that fixes the amount of energy a car needs to 
carry onboard to about 1000 MJ.

Now look up the various ways of storing energy:

http://en.wikipedia.org/wiki/Energy_density

Note the "energy density" column, which is how much mass you need to store 
that energy.  For our 1000 MJ we would need to carry around in our car:

7 kg of liquid hydrogen
22 kg of petrol/diesel
2 tons of flywheel
3 tons of Li-Ion batteries
3000 tons of mechanical spring power

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