[gburlison]

Just because the lawnmower engine is rated at 3.8 Hp, does not mean it developing 3.8 Hp in operation. Without knowing the torque curve of the engine, the speed at which it develops rated power, and the torque at its normal operating speed, I think that there is no simple way to compare them. I would look at commercially available DC electric lawnmowers and see what the manufacturer used. Compare similar sized DC electric and gasoline lawnmowers to develop a rough idea of what would be equivalent to your 3.8 Hp engine.