[markweatherill]

the Prius isn't a hybrid at all. All non-plugin hybrids get there energy from petrol. OK, they can then store some of it is a battery for use later but all the energy the car uses comes from petrol. So to me, current hybrids are not really dual energy source vehicles.

I think of a 'hybrid' in terms of the number of motive sources on the vehicle and not the number of outside energy sources it uses. It's the combining of motive sources, in effect increasing the amount of useful energy available from petrol, that does the magic.