So I found this interesting question posed on Reddit today -- something I've wondered myself -- could you use all the people working out at a busy gym to generate power? To feed something back into the electrical grid, perhaps? I mean, think about all the people chugging away on an exercise bike right now, across the world -- there must be thousands and thousands. Well, it turns out the answer is: you could, but it wouldn't do much. Last October, a group of 500 cyclists gathered in Manchester, England to set a new world record for the amount of power generated by pedaling on bicycles for 24 hours. (Yes, apparently there was an old world record for this; the frivolity of the Guinness Book never ceases to amaze me.)
So 500 cyclists pedaled for 24 hours on static bikes that generate electricity, and at first blush the amount of electricity they generated sounds pretty impressive: 72,414 watt hours. Of course, most households pay by the kilowatt, but saying they generated 72 kilowatt hours sounds less impressive. It was less impressive still when I dug out my most recent electric bill and discovered that last month my household of two people and one cat used 817 kilowatt hours -- meaning that 500 cyclists pedaling for 24 hours straight could only power my two-bedroom apartment for about three days.
So really, harnessing the power of people using weightlifting machines and pedaling exercise bikes almost certainly wouldn't even be enough to keep the lights in their gym on. That didn't stop Texas State University from retrofitting 30 of its elliptical bikes with power generators, though -- because even if the amount of power generated isn't huge, why not do it?