What is solar fraction for a solar water heater?

Solar fraction is the portion of the conventional hot-water heating load displaced by the solar thermal system. A 60% solar fraction means the backup heater supplies the remaining 40%, before separately accounting for pump electricity.

How do I estimate solar water heater savings?

Estimate the useful annual hot-water load, multiply it by the expected solar fraction, convert that displaced heat into avoided backup-fuel units using the heater efficiency, then subtract solar-system pump electricity and extra maintenance costs.

What solar fraction should I use?

Use the solar fraction from an SRCC-certified system rating, installer simulation, or site-specific model. DOE notes that economically optimized systems may supply roughly 60% to 90% of the load, but climate, collector area, orientation, storage, and water-use timing matter.

Does a solar water heater eliminate the backup heater?

Usually not. Solar output varies with weather and season, so a conventional electric, gas, or propane heater normally supplies the unmet load.

Should I subtract maintenance when calculating payback?

Yes. Enter the expected annual difference between solar-system maintenance and the baseline heater. Include anticipated inspections, heat-transfer-fluid service, pumps, controls, or other costs when applicable.

Are rebates or tax credits included automatically?

No. Incentive rules change and depend on location, equipment, dates, and eligibility. The calculator assumes zero incentives until you enter an amount you have verified.

Is this the same as a solar photovoltaic payback calculator?

No. This calculator models solar thermal equipment that heats water. Solar photovoltaic panels produce electricity and require a different production, self-consumption, export, and tariff model.

Are my inputs uploaded or stored?

No. Calculations, copying, and CSV creation occur locally in your browser, and this tool does not send or store your input values.

Solar Water Heater Savings & Payback Calculator

Estimate solar thermal hot-water energy, bill savings, simple payback, projected break-even, and lifetime value. This US-focused screening tool runs locally and does not upload your inputs.

System and household

1. Hot-water load

Daily hot-water use
Estimate useful heat from volume and temperature rise Annual fuel use
Use water-heating-only energy from a bill, meter, or label

Daily mixed hot water Volume unit Temperature rise Temperature unit

Water-heating days per year

2. Backup heater and solar performance

Backup water-heater fuel Current heater efficiency Use UEF or a consistent effective efficiency for screening. Marginal fuel rate ($/kWh) Use the variable rate avoided—not fixed account charges.

Expected solar fraction (%) Prefer an SRCC-certified rating or site-specific simulation. Solar pump electricity (kWh/year) Enter 0 for a passive system with no pump. Electricity rate for pump ($/kWh)

3. Project cost and analysis

Currency Installed solar system cost Other upfront work

Verified incentives No rebate or tax credit is assumed automatically. Conventional replacement cost avoided Extra annual maintenance Annual difference versus keeping the baseline heater.

Analysis period (years) Energy-price change (%/year) Discount rate (%/year)

Results update as inputs change.

Results

Core modelUseful hot-water heat × solar fraction

Backup fuelsElectricity, natural gas, or propane

Sources checkedDOE, NREL, and EIA; August 3, 2026

PrivacyNo input upload or storage

How the solar water-heating calculation works

Estimate useful annual hot-water heat from daily volume and temperature rise, or derive it from water-heating-only annual fuel use.
Multiply that useful load by the expected solar fraction to estimate heat supplied by the solar thermal system.
Divide displaced useful heat by the backup heater efficiency and convert it into kWh, therms, or propane gallons.
Value the avoided fuel at its marginal rate, then subtract pump electricity and extra maintenance.
Compare yearly cash flow with net upfront cost for simple payback, projected break-even, lifetime savings, and NPV.

Useful heat (kWh thermal) = US gal × 8.34 lb/gal × temperature rise °F ÷ 3,412.142 Btu/kWh Solar heat delivered = useful hot-water heat × solar fraction Avoided purchased energy = solar heat delivered ÷ backup-heater efficiency First-year cash savings = avoided fuel cost − pump electricity cost − extra annual maintenance Net upfront cost = installed cost + other upfront work − verified incentives − conventional replacement cost avoided Simple payback = net upfront cost ÷ first-year cash savings

A solar fraction is an annual load fraction, not collector efficiency. The model holds solar fraction, pump electricity, and maintenance constant through the analysis while escalating energy prices by the entered rate.

Assumptions, references, and limits

Solar fraction: NREL’s HPXML data dictionary defines it as the portion of the conventional hot-water load displaced by solar. Use an SRCC-certified system rating or site-specific design model when available.
Planning range: DOE’s Solar Hot Water System Calculator notes that an economically optimized system may supply roughly 60%–90% of the load depending on solar resource and economics. The 60% starting value is editable and is not a site forecast.
Energy conversions: EIA’s energy units reference gives 100,000 Btu per therm, 91,452 Btu per propane gallon, and 3,412 Btu per kWh. This calculator uses 29.3001 kWh/therm and 26.8028 kWh/US gal propane.
Hot-water load: the volume method uses 8.34 pounds per US gallon and the entered temperature rise. It estimates delivered sensible heat only; it does not separately model tank standby, distribution, mixing-valve, or pipe losses.
Starting values: 50 US gal/day, 70°F rise, 0.92 electric efficiency, $0.17/kWh, 100 pump kWh/year, 20 years, 2% energy-price change, and 3% discount rate are editable screening assumptions—not quotes, forecasts, or national averages.
Incentives: no incentive is assumed. Verify current eligibility, qualifying equipment, caps, tax treatment, and installation dates with the relevant program administrator or qualified tax professional before entering a value.

Financial and engineering limit: this is a screening estimate, not a system design, contractor quote, tax determination, or guarantee. Collector type and area, climate, shading, orientation, tilt, storage, freeze/overheat protection, plumbing, water quality, load timing, backup controls, permits, roof condition, maintenance, equipment life, financing, and rate changes can materially alter results. Have a qualified installer size the system and confirm code, structural, scald-protection, and freeze-protection requirements.

Solar water heater savings FAQ

What is solar fraction?

It is the portion of the conventional hot-water heating load displaced by the solar system. A 60% solar fraction leaves 40% for the backup heater, before pump electricity is accounted for separately.

What solar fraction should I enter?

Use the annual solar fraction from an SRCC-certified system rating, installer simulation, or site-specific model. Climate, collector area, orientation, storage, and water-use timing all matter; the 60% default is only a planning value.

Can I use my utility bill?

Yes, if you can isolate annual water-heating energy from other uses. Choose Annual fuel use and enter only the kWh, therms, or propane gallons used for water heating—not total household fuel use.

Why does the calculator ask for backup-heater efficiency?

One unit of useful solar heat avoids more purchased fuel when the old heater is inefficient. Dividing displaced useful heat by baseline efficiency converts the solar contribution into avoided billed fuel units.

Does solar hot water eliminate the backup heater?

Usually not. Solar availability changes by hour and season, so electric, gas, or propane backup normally covers the unmet annual load and short-term demand.

What belongs in installed cost?

Include collectors, storage, heat exchanger, pumps and controls, mounting, plumbing, labor, design, permits, taxes, and required roof or utility work. Enter other upfront work separately if it is excluded from the quote.

Should maintenance be included?

Yes. Enter the expected annual difference versus the baseline system. Active systems may need pump or control work, while some climates and designs require periodic heat-transfer-fluid or corrosion-protection service.

Are incentives included automatically?

No. Enter only incentives you have verified for the equipment, installation date, location, and your eligibility.

Is this the same as solar PV payback?

No. Solar thermal equipment heats water. Solar PV produces electricity and needs a model for electrical production, self-consumption, export credits, and tariffs. Use the related Solar PV Payback calculator for that case.

Are my inputs tracked?

No. The calculator does not upload or store input values. Copying and CSV download are created locally in your browser.