Gas hot water heaters can work without electricity when they use a pilot light for ignition. Efficiency matters, as gas heaters are about 85% efficient, whereas electric heaters are effectively 38% efficient when considering power plant conversion and transmission losses, making gas heaters more fuel-efficient.
Many gas hot water heaters can operate without electricity, especially conventional tank-style heaters that do not require power for their basic function of heating water. Instead, they rely on a pilot light and can ignite the gas burner using a thermocouple when hot water is needed. However, modern hot water heaters may have electronic ignition systems or other features like power venting that do require electricity, meaning they would not work during a power outage.
Efficiency is a key factor when comparing gas and electric water heaters. Gas-fired units tend to be about 85% efficient in transferring the heat of the combustion into the water. Contrastingly, electric water heaters are close to 100% efficient at transferring the electrical energy into heat inside the water, primarily because the heating coil is immersed directly in it. The source of electricity greatly affects this comparison, as conventional power plants have a thermal to electrical energy conversion efficiency of about 30-40%, with an additional 95% transmission efficiency to the house. On-demand electric water heaters specifically only heat water as it is needed, contributing to their efficiency.
When comparing the two, if the electricity is generated from natural gas at a power plant with a 40% efficiency and then transmitted at 95% efficiency, the total fossil fuel energy usage is higher for electric heaters. A gas water heater with 85% efficiency compared to an electrical system's effective 38% efficiency (40% x 95%) shows that the gas heater is more fuel-efficient, with the power plant requiring about 2.2 times more gas to produce the same amount of hot water.