Where Water-Heating Energy Really Goes

Water heating--which accounts for about 16% of U.S. residential energy consumption--is one of the most complex end uses to evaluate, both individually and nationally. Each home's use varies greatly, depending on technical, behavioral, and regional factors, as well as on the types of appliances present that use the hot water.

Studies of water heating often report the number of gallons per day of hot water use per household, averaged over all households with different appliances. This number is useful for determining total water-heating energy use, but it provides no guidance as to what impact specific conservation measures might have on that usage. It is also of limited usefulness in assessing the energy use for water heating in a particular household.

Now, however, researchers at Lawrence Berkeley Laboratory have disaggregated total hot water use to reflect the various components of hot water loads (see Figures 1-3). The analysis, developed for the U.S. Department of Energy, (DOE) breaks down total hot and cold water use (gallons per day) into their component parts: showers, baths, faucets (flow-dominated and volume-dominated), toilets, landscaping, dishwashers, and clothes washers. The report deals separately with standby losses associated with keeping the water in the tank hot and ready for use.

Figure 1. Percentage breakdown of total U.S. residential water by end-use in 1993.

Figure 2. Percentage breakdown of U.S. residential hot water energy use by end-use in 1993: electric water heaters.

Figure 3. Percentage breakdown of U.S. residential hot water energy use by end-use in 1993: natural gas water heaters.

From an energy auditor's standpoint, this report gives crucial information on typical hot water consumption of major appliances. This allows an auditor to make a reasonably accurate estimate of hot water energy use.

The researchers also found that residences consumed, on average, about 60 gallons per day, just a little less than that assumed in DOE laboratory test procedures. This means the energy costs shown on yellow EnergyGuide labels give, on the average, a fairly accurate estimate of energy use.

The researchers also used the end use breakdown and data on equipment characteristics to assess the impacts of various efficiency standards on hot water use and water heater energy consumption. People designing programs that affect the energy use of electric water heaters now have a baseline to calculate savings against, and will know what the effect of national standards will be.

See "The Effect of Efficiency Standards on Water Use and Water-Heating Energy Use in the U.S.: A Detailed End-Use Treatment," LBL-35475, J.G. Koomey, C. Dunham, and J.D. Lutz, Energy Analysis Program, Energy and Environment Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 , May 1994. Tel: (510)486-5001; Fax: (510)486-5454.