A shortcut for this is that a horsepower is about 750 watts (745.7 or so), and that you can extract one horsepower from a fit human for a very limited amount of

A shortcut for this is that a horsepower is about 750 watts (745.7 or so), and that you can extract one horsepower from a fit human for a very limited amount of time before they stop humaning. You can sustain about half this, or a little less, with the correct human. These super-approximate numbers come in useful for, e.g., human-powered airplane design. (The equivalent horsepower numbers for a horse are about 15 hp peak, 2 hp sustained. Naming is hard.) So, if you're proposing a system that extracts energy from N humans at a time, and you're hoping to get more than N hp out of it... you're probably off by a bit. If you're hoping to do this without the humans noticing, you're probably off by an order of magnitude.