Mount an electric bike on an exercise stand, tap into the 3-phase motor with a DC rectifier ($10 on Amazon), wire up some voltage regulators, and pedal. Electricity will come out at the other end.
100W for one hour is a decent workout for me. 10 hours @100W average output yields a single killowatt-hour. The first generated killwatt-hour ran a 10w fan while working out, accounted for losses, and kept my iPhone SE running for two months under normal use.
Demonstrated here with a single 12/24V 8W LED:
An electric bike motor has 3-phase output, which this switch routes to either the DC rectifier (top) or the electric bike controller.A distribution box monitors voltage, amps, wattage, and total generated power. Voltage from the rectifier varies by pedaling speed; two voltage regulators keep the voltage at 12 and 24V respectively. Some batteries use the 24V line for charging, while others use 12V or 12V->5v adapters for USB devices and batteries.
How much power can be generated depends on the “load,” which is usually several batteries connected at once. Two large batteries (about 50W charging together), plus several smaller ones and the lead acid batteries for the LED lights (not pictured) can provide around 100W resistance.
Juniuslabs 