Water heaters heat the water in homes and other structures. Screw-type immersion elements and over-temperature guards allow automatic shutdown in electric water heaters. In locations where electricity is more expensive than liquid propane (LP) or natural gas, gas water heaters are commonly used. Hot water is available on demand with tankless water heaters. They are more efficient than water heaters with a storage tank since they are not constantly heating and reheating water in a tank.