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The household's daily draw; the DOE benchmark is 64.3 US gallons (243 litres), an average household of three.
The Department of Energy suggests sizing against a benchmark load, or on the number of bedrooms, rather than on this year's use, because the people in a house and their habits change over a system's life. Its benchmark is the water heater test procedure's 64.3 gallons a day. If you have summer fuel bills from a gas heater, the page's other route is to take about 60% of the summer gas use when the same meter also serves cooking and a dryer.
The mains temperature; the test procedure uses 58 °F (14.4 °C).
Incoming water is colder in winter, which raises the energy every gallon takes. The benchmark is a yearly figure; a colder supply makes both the load and the saving larger in proportion.
The stored temperature; the test procedure uses 135 °F (57.2 °C).
The temperature the heater holds, not the temperature at the tap after a mixing valve. Where a store is held hotter for legionella control, use that figure: it is the energy the backup heater actually has to supply.
The fuel the solar system's backup heater uses; it decides which heater efficiency applies.
The backup heater's efficiency is part of what a solar system's rating measures, so use a rating listed for the fuel you will actually use. Gas is counted in therms for a US reader, the unit US gas is sold in, and in kilowatt-hours for a metric one.
The Uniform Energy Factor on the heater's label; the DOE's examples use 0.64.
The UEF is the hot water a heater delivers per unit of fuel in the federal test. The Department of Energy's worked examples use 0.64 for a household gas heater. The higher the existing heater's UEF, the less there is for solar to save.
From the system's certification listing; 2 or 3 is the most common.
The SEF is the energy the system delivers divided by the gas or electricity it uses, pumps and controls included. The Solar Rating and Certification Corporation lists it for certified systems, with the solar fraction beside it. The DOE gives the range as 1.0 to 11.
Per therm on a US gas bill, per kWh on a metric one — the unit named in the fuel choice above.
Use the price on your own bill, including the delivery charge that rises with use. Prices vary by region and by month, which is why this page publishes none.
Your installed quote, after any incentive; leave blank for no payback figure.
The simple payback divides this by the money saved a year. Enter the difference between two quotes instead to see how long the dearer, higher-rated system takes to earn back its extra cost — the comparison the DOE page works through.
Backup energy saved per year
177 therms
Figures that depend on a rate wait for yours — this page does not assume one.
- Water heating energy per day
- 0.42 therms
- Backup energy per year without solar
- 238.34 therms
- Backup energy per year with solar
- 61.02 therms
They open the calculator with your figures already in it
Solar Water Heater Savings Calculator: 177 therms — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- US Department of Energy, Energy Saver, Estimating the Cost and Energy Efficiency of a Solar Water Heater: daily water heating energy = gallons of hot water per day × 8.35 lb/gal × 1 Btu/lb·°F × (hot − cold water temperature); annual solar energy savings = daily energy × 365 × (1/UEF − 1/SEF); benchmark 64.3 gallons a day from 58 °F to 135 °F
- The same page: the solar energy factor is the energy delivered by the system divided by the electrical or gas energy put into it, ranging from 1.0 to 11 with 2 or 3 the most common; the solar fraction is typically 0.5 to 0.75; certified systems' SEF and solar fraction are listed by the Solar Rating and Certification Corporation
- The same page: a conventional heater's efficiency is its Uniform Energy Factor (10 CFR Part 430, Subpart B, Appendix E); the page's examples use 0.64 for household gas and take electric resistance as 1.0; one therm is 100,000 Btu
Inputs used
- Hot water used per day
- 65 gal
- Cold water temperature
- 58 °F
- Hot water temperature
- 135 °F
- Backup heater fuel
- Natural gas (energy in therms)
- Existing gas heater's UEF
- 0.64
- Existing electric heater's UEF
- 1
- Solar energy factor (SEF) of the system
- 2.5
- Gas price per therm or kWh
- Electricity price per kWh
- Installed cost of the solar system (optional)
Intermediate steps
- Water heating energy per day
- 0.42 therms
- Backup energy per year without solar
- 238.34 therms
- Backup energy per year with solar
- 61.02 therms
What this calculation does not cover
- A solar energy factor is a rating under standard test conditions, not a measurement of your house. Your climate, your roof and how your household draws its hot water through the day all move the real figure, in either direction.
- The saving is counted against the existing heater's rated efficiency. A heater whose real efficiency has fallen with age, scale or a failing element saves more than this; a heat pump water heater, far less.
- Maintenance is not deducted. The DOE page puts it at about half of one per cent of the installed cost a year, with most years costing nothing and an occasional one — antifreeze replacement, a pump — costing more.
- The payback is simple: no interest, no incentives, no change in fuel price and no replacement of the heater the solar system would have spared.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-25 · v1.0.0
Regulatory standards & verification citations3
- US Department of Energy, Energy Saver, Estimating the Cost and Energy Efficiency of a Solar Water Heater: daily water heating energy = gallons of hot water per day × 8.35 lb/gal × 1 Btu/lb·°F × (hot − cold water temperature); annual solar energy savings = daily energy × 365 × (1/UEF − 1/SEF); benchmark 64.3 gallons a day from 58 °F to 135 °F
- The same page: the solar energy factor is the energy delivered by the system divided by the electrical or gas energy put into it, ranging from 1.0 to 11 with 2 or 3 the most common; the solar fraction is typically 0.5 to 0.75; certified systems' SEF and solar fraction are listed by the Solar Rating and Certification Corporation
- The same page: a conventional heater's efficiency is its Uniform Energy Factor (10 CFR Part 430, Subpart B, Appendix E); the page's examples use 0.64 for household gas and take electric resistance as 1.0; one therm is 100,000 Btu
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