When you turn on a sink faucet, the flow of water turns on the heater. Whether you use gas or electricity, the powerful gas burners or electrical elements will turn on, quickly heating the water as it passes through the unit. The water heats up right away, and the length of time it takes to get hot water depends on how far it has to travel from the water heater to where it is being used. When you turn off the hot water, the burners or elements stop working. The only thing that uses energy is the hot water that is being used.