A combination of interface (how do you get the water heater to use the excess heat?), quality (a/c waste heat isn't that hot), and matching generation with dema

A combination of interface (how do you get the water heater to use the excess heat?), quality (a/c waste heat isn't that hot), and matching generation with demand. There are power plants that use excess heat for heating. The term is cogeneration or combined heat and power.