Is an electric hot water heater 100 percent efficient?

Answer 1

No, but close to it.

In regards to just the heating of the water, Tank-type, and tank-less, electric water heaters are about 99% efficient* because the heating elements are immersed in water, and little energy is wasted between heat source and water.

However, all tank-type water heaters loose heat from their reservoirs (tanks). This heat loss lowers the overall efficiency.

Most electric heaters fall between 90-95% total. This is the number that is usually reported by the manufacturers.

* Efficiency is not the same as cost savings. For example electric heaters have higher efficiency, but cost more to operate than same-size gas water heater. Also tankless gas heaters, gas water heaters with blowers, and gas heaters with high recovery may claim higher efficiency, but they consume more fuel per hour of operation.

Additional Information:

Efficiency of heating elements does not consider inefficiencies in production, transmission, refining, and delivery of electricity. Estimates say coal-fired power plants waste half the energy of coal during electric generation. However hydroelectric power plants are very efficient.

Tank-type gas water heaters are 69% efficient, since hot combustion gas escapes out the flue after hydrogen content of fuel is burned. This number does not consider inefficiencies during production, transmission, refining, and delivery of gas.

Tank-less gas water heaters are 79% efficient, but tank-less burn more gas per gallon of hot water than tank-type heaters. Hot combustion gasses are released out the flue and are not actively recycled without slowing combustion which turns off unit. Newer type tank-less with blower recycles heat into a tank operate at 96% efficiency, and are made for oversize consumption of both energy and hot water.

High efficiency tank-type gas heaters operate at 96% efficiency by using blowers to circulate hot combustion gas through coiled tube located inside tank. Same amount of combustion gas is released since only heat is recycled. The use of electric blower affects overall cost.

Environmental efficiency: Electric water heater does not release CO2 at each home, instead CO2 is localized at power plant. Gas water heaters release CO2 at each home which factor in environmental efficiency.

Efficiency of all gas water heaters falls and cannot be fully restored if untreated hard water deposits sediment over heat transfer surface. Generally, gas heaters exposed to hard water require softener which affects overall efficiency.

Efficiency of tank-type electric water heater remains at 99% until sediment reaches element, causing element to burn out. Full 99% efficiency is restored by cleaning out tank and replacing element. Generally, tank-type electric heater does not require water softener.

Downsizing water heater, reducing consumption, selecting water heater with fewer parts, reading the manual, and draining tank each 6 months to avoid sediment are key to saving energy.

Answer 2

Lousy and it should say so on the Energy Star label.