much more efficient Department of Energy almo﻿st 50% of the heating costs energy costs energy factor calculate the Energy Factor Operating costs calculate the annual operating cost Department of Energy Energy Factor Department of Energy Energy STAR qualified US Department of Energy Department of Energy Energy.gov energy factor Department of Energy water tank’s size estimation guide EF (Energy Factor) consider the energy factor of the water heater Department of Energy there is negligible maintenance cost of running there is negligible maintenance cost of running efficiency of the water heater energy factor (EF) and first-hour rating (FHR) Department of Energy US Department of Energy