Yeah, 20 amps seemed high to me also, but I went and actually checked the breaker, which I had thought was 15 amps, and was surprised when I found the 20 amp breaker. My system covers about 2500 sq ft, which may account for a larger system and thus a more powerful fan. I suspect the breaker is larger to account for the power surge of the fan kicking in, but that just a guess.

In any case, best to see what is actually called for in actual application.

+1 on checking the continuous and surge power ratings. When my sump pump kicks on, there is a big surge and you can hear the generator react to the sudden load.