in a previous post:
"a good rule of thumb to follow is 750 watts per horsepower. "
Actually, this is not the case.
Yes, it is true that there are about 745 watts per 1 horse power. However, alternators aren't 100% efficient, so, the generator industry uses a 500 watts per horse power rule of thumb.
So, a 3 h.p. Briggs would be rated as adequate for a 1500