We could put pedal powered generators into every prison cell. Each prisoner would be expected to generate 1.2 kWhr of power per day (100 W/hr * 12 hr). They could eat and sleep the other twelve hours. This not only would generate a lot of power, but would provide valuable exercise for the prisoners and it would be an excellent deterrent for future incarceration.
That’s thinking outside of the box and solving more than one problem to boot!