Is there a market for a stationary bike/generator so people can reduce their electric bill while losing weight?

People are already selling these online and I don't think anyone has got rich yet. Typical cost £500 and up. Anyone with DIY skills can make one much cheaper but it's not easy to understand the electrics if you don't have experience. DIY models can use cheap E-scooter motors to generate 50 to 200 watts depending on your fitness. Don't expect to power anything more than a small laptop unless you are a keen cyclist. Reducing your electric bill with one is impractical, but it's a fun way of charging a battery in a remote location with no power..

Sorry but even a very fit athlete can only generate a very, very small amount of electric power and only for a short time. It is a good idea but not feasible.

A trained cyclists can only produce 400 watts for about one hour.

a very small one

Doing a high power workout is probably around 250 watts. That would take 4 hours to produce 12 cents of electricity.

yep. especially for off grid cabins where kids can crank to produce electricity for watching television...