Energy factor

The energy factor is determined using a stylized hot water use pattern. Hot water is drawn in six equal draws totaling 64.3 gallons, and a standby period of 18 hours follows.

The energy factor for residential water heaters is determined using the results from the 24-hour simulated use test. During the test 64.3±1.0 gallons of water are drawn from the water heater in six equally spaced draws that begin one hour apart. The hot water flow rate for each draw is 3.0±0.25 gallons per minute. After the beginning of the last draw a standby period of 18 hours follows. During the test, the test conditions must be operated at a specified value and accuracy. Heat pump water heaters (HPWHs) have different values specified for ambient air temperature and relative humidity.[6]

From the standard test procedure, the energy factor is defined as

${\displaystyle EF=\sum \limits _{i=1}^{6}{\frac {M_{i}C_{pi}\left(135^{\circ }F-58^{\circ }F\right)}{Q_{dm}}},}$

where ${\displaystyle Q_{dm}}$ is the modified daily water heating energy consumption (Btu), ${\displaystyle M_{i}}$ is the mass withdrawn from the i^th draw (lb), and ${\displaystyle C_{pi}}$ is the specific heat of the water of the i^th draw Btu/lb°F.

Minimum federal energy conservation standards are defined by fuel, type, and rated storage volume. All standards are calculated as a function of the rated storage volume V in gallons. The current conservation standards are less efficient than the standards that go into effect in 2015.[5][7][8]