Daily Water Heater Energy Use

To estimate average daily hot water thermal-energy consumption, we use the following equation (Lutz, et al., 1996). Tin is calculated based on the weather data for the weather station to which the house was assigned, described more fully in the Climate Modeling section.

         Equation 5

      where

            Qin = water heating energy consumption (MBtu/day)

            usewh = hot water use per day (gallons) from Equation 4

            dens = density of water (8.293752 lb/gal)

            Cp = specific heat of water (1.000743 Btu/lb-°F)

            Ttank= water heater thermostat setpoint (°F)

            Tin = inlet water temperature (°F)

            RE = recovery efficiency of water heater

UA = standby heat loss coefficient of water heater (Btu/hr-°F) from Eq. 6

Tamb = annual average air temperature around water heater (°F)

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