These Exercise Machines Turn Your Sweat Into Electricity - IEEE Spectrum
Still, might this be a reasonable way for a gym to offset at least some of its electricity use? Let's assume that the average piece of exercise equipment is in use 5 hours a day, 365 days a year. If each patron generates 100 watts while using it, that machine creates some 183 kilowatt-hours of electricity a year. Commercial power costs about 10 cents per kilowatt-hour on average in the United States, so the electricity produced in a year from one machine is worth about US $18 dollars.