Before we can have some of the more difficult calculations we need to know a few things. According to Marks’ Standard Handbook for Mechanical Engineers 11th Edition a normal trained person can produce around 80 Watt per hour (~0.1 horsepower/hour) but a well trained person can produce as much as 400 Watt per hour. A function for useful power output, if we assume 20 % of the total output is useful, is depending on age and time of the activity. For a 25 year old man the functions is 0.1log t, a 35 year old has the function 0.09log t and a 60 year old man had the function 0.08log t. Where t is in minutes and the output is in horsepower, and 1 hp equals 746 W.
So, on to the mathematic. If we start in a home, a mobile charger use 5 W/h which means a normal trained person would only have to spend 3 minutes and 45 seconds to produce the electricity the mobile charger use for a whole hour. The maximal output for a normal trained person is 1,92 kW per day which means not a lot of ordinary things in our home can be power by it. If we now move on the where all of the focus is, we focus on gyms and work-out machines. A single bike with a generator could produce about 220 kWh per year, while a full gym could power somewhere around 4400 kWh per year. (Source) A small house use around 20 000 kWh which mean it couldn’t be operate by just work out machines. We can even se to the example: A gym has 2 spinning lessons every evening with 20 different bikes that can produce electricity. It means 220 kwh multiple 20 bikes multiple 2 lessons, it equals 8800 kWh per year and it still doesn’t cover the energy need of a whole gym.
The conclusion is, even if a gym could operate from bikes, vario machines, cross trainers etc. The technology isn’t here yet. Right now we’ll have to watch gyms only partly produce their own electricity, but it’s still a step to a greener environment. Hopefully we’ll see a future with gyms using their own produced electricity.