[whswhs]

Draw Power is not the right spell.

In the first place, there's the Generator/Battery distinction.

But in the second place, if you have a treadmill or something else with a power train that you can tap, 1 FP/second equates to 360,000 watts. A sustained human energy output is 100 watts. So you need 3600 humans (or human-equivalent zombies) to get just 1 FP/second.

I worked out a lower-tech, more modest version in GURPS Thaumatology: Fantastic Cities. It did 1 FP/minute or a multiple. That was only 6000 watts. You needed 1 FP/minute to keep the spell going, so to get a net yield of 1 FP/minute you needed 12,000 watts, or about the output of a hundred men with ST 11—say, rowers for a bireme. Or about 20 harnessed mules.