Originally Posted by MechEngVT
Why don't you make do with the smaller generator but improve its abilities with some batteries as well?
7.5 kW is about 10 HP, so it wouldn't be enough unless your car's really light. Even then, acceleration & hill climbing would be horrible. But if you have some batteries to power your main drive, then you can run the generator to add charge, and extend range. This is basically what GM is doing with the Volt.
If you wanted to be even more efficient, put enough batteries in the car to cover your local driving, then put the generator on a (streamlined) trailer that you hook up for longer trips.