The energy saved in the domestic water heater (DWH) is calculated by multiplying the heat gain rate by the annual operating hours and dividing by the water heater efficiency. The division by efficiency accounts for the fuel energy that would have been required to provide the same amount of useful heat. This represents the primary energy savings from using waste steam instead of conventional water heating.

\begin{equation}\label{eq:water-heating-steam-energy-saved-dwh} E_{saved,DWH} = \frac{\dot{Q}_{gain} \cdot t_{op}}{\eta_{heater}}\end{equation}

Symbols

\(E_{saved,DWH}\)	Annual energy saved in domestic water heater \([\unit{ \kilo\joule\per\year}]\)
\(\dot{Q}_{gain}\)	Heat gain rate \([\unit{ \kilo\joule\per\hour}]\)
\(t_{op}\)	Operating hours per year \([\unit{ \hour\per\year}]\)
\(\eta_{heater}\)	Water heater efficiency (fraction) \([\unit{ \unitless}]\)