To Randy Taylor and others, here is a good predictor on estimating any small gasoline engine generator / engine match-up. Remember 85%, a very important number.
Small generators (alternators) average about 85% efficiency, that is mechanical to electrical conversion. Knowing that 746 watts = 1 BHP (brake horsepower) and the efficiency, we have:
Generator HP = Rated Watts/634
Example: A 2500 watt generator requires 2500/634 = 3.94 horsepower to drive.
Now, small air-cooled engines are derated to 85% of their full BHP rating for continuous, 3600 rpm applications, as stated by the manufacturer. Thus for a 5 BHP engine used on a generator set, max power available would be as follows:
Engine BHP continuous = .85 X 5 = 4.25
In this example, a 5 hp advertised rated engine is slightly more than adequate for driving a 2500 watt generator. This is a good match.
For determining watts output knowing the rated power of the engine, here's another example.
Engine is K301 rated 12 BHP @ 3600 rpm.
Continuous duty rating would be .85 X 12 = 10.2 BHP. This will be the actual mechanical power input into the generator. Solving the first equation for watts output, we have:
Rated Watts = 634 X 10.2 = 6500
Does this help?