The energy efficiency of a water heater is measured in energy Factor or (ef). The larger the number of ef, the more efficient the water heater is.

For even greater efficiency consider buying a tankless water heater. Tankless water heaters only heat water when it is needed, instead of keeping a reserve of water at a constant temperature.

This paper introduces a new type of solar water-heater, which works with direct-circulate compounded form. It can raise heat efficiency.