Well it will be if your 100 HP engine only has a 50 - 70 amp alternator. 70 x 12 = 840 watts and that is assuming it has absolutely no extra electrical systems loads on it.
Realistically most modern vehicles are lucky to have an extra 20 - 40 amps peak reserve available with a stock alternator once system loads are factored in.
If you need a 1000 watts or so of sine wave power and you plan to run your car to get it you may want to consider going with a small belt driven AC generator that runs off your engine instead.
Just some more things to think about.