Which is more energy efficient re: heating water
Which is the more energy efficient – to fill my pot of water for mac and cheese with cold water and heat it to a boil on the stove or to fill my pot of water with hot water (from the faucet but heated with the water heater) and then boil it?
It’s probably a wash.
Clearly, if you had to choose one method or the other (only using the stove, or keeping water warm in a water heater purely for the production of Mac and Cheese), the stove method is more efficient, because you aren’t keeping 40 gallons of water hot 24 hours a day. But, since you probably use your hot water for other things, the additional energy used to heat up the quart of cold water in the water heater is going to be roughly the same as the energy used to heat a quart of cold water on the stove.
In any event, it’s a small amount of energy.