I think generally, the comparision between "electric hp" and "gasoline hp" is how the rating is determined. In many cases, gasoline engines are rated at their maximum output. Electric motors are rated based on continuous loading, often with a service factor that derates the output. The peak hp output of an electric motor is higher than the rating, but can only sustain that higher output for a limited time before it over heats. The gasoline engine typically can't produce any additional power over its rating.
What this all comes down to, is that when you have a spike in load, the electric motor can often bear down and run through the load spike. The gasoline motor doesn't have reserve and will just stall. This means the electric motor can be sized to the nominal load, while the gasoline motor needs to be sized to the larger peak load.
As far as pumps are concerned, hp is hp, doesn't matter if their electric or gasoline. The power source may effect the service factor of the pump, but that depends on a lot of factors, and probably only maters to large industrial concerns.
Adam
