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