To calculate the standby heat loss coefficient, we use the equation for heat loss from the DOE Energy Factor test procedure for water heaters, (US DOE 1993) as shown in Equation 6.

    Equation 6

      where

            UA = standby heat loss coefficient (Btu/hr-°F)

            EF = Energy factor of water heater

            RE = recovery efficiency of water heater

            Pon = rated input power of water heater (Btu/hr)

                  Q_out = Energy content of water drawn from water heater during 24 hour test (41093.7 Btu/day)