This calculator determines the energy consumption and payback period for standard commercial and ultra-efficient commercial water heaters and compares them with consumption and payback for residential models. By necessity, this screening model makes simplifying assumptions, so use it for initial screening only. For more accurate performance predictions, conduct a more detailed analysis that includes additional factors such as actual usage patterns, hot water loads, and part-load performance of equipment.

Begin by entering the average daily water draw in gallons, local natural gas cost in U.S. dollars per therm, and electricity cost in U.S. dollars per kilowatt-hour. For the residential model enter the energy factor, and for the commercial and high-efficiency models enter the thermal efficiencies (as fractions, not percentages: 0.8, for example, rather than 80). These numbers are available from manufacturers' data sheets. For typical values, use 0.55 for the residential model energy factor, 0.8 for a standard commercial water heater, and 0.94 for an ultra-efficient model. Finally, enter the purchase price for each unit and hit the "calculate" button.

Notes:
a. Assumes that each water heater is properly sized to meet the loads; residential case features two 50-gallon tanks, 120,000-Btu/h burner; commercial case features one 37-gallon tank, 160,000-Btu/h burner; ultra-efficient unit features one 60-gallon tank, 125,000-Btu/h burner.
b. Annual hot water consumption based on operation six hours per day, 365 days per year, 100^oF rise in water temperature.
c. Based on 500-W load, six hours per day, 365 days per year.
NA = Not applicable.