How is the yearly value calculated?

System size times peak sun hours times 365 gives annual production. The self-used share is multiplied by your electricity price, the rest by your export price, and the two are added.

What self-use share should I enter?

Without a battery, most households land between 25% and 50%. A battery, a heat pump, or daytime car charging pushes it higher. If your inverter app reports it, use your own figure.

Does it include payments for exported surplus?

Yes. Exported energy is valued at the export price you enter, separately from self-use. Enter 0 if your agreement pays nothing for surplus.

How is payback calculated?

The system cost divided by the yearly value. A 9,000 system worth 1,314 a year pays back in about 6.8 years. Interest, maintenance, and degradation are not included.

Why is real output often lower?

Shading, panel angle, temperature, dirt, and gradual degradation all reduce real production below the ideal figure that sun hours alone suggest.

Can I share or save a calculation?

Yes. Share copies a link that reopens the calculator with the same figures, and download or print keeps the inputs and results as plain text.

Solar panel savings calculator

Enter your system size in kilowatts, your average peak sun hours, how much of the production you use in the home, the price you pay for electricity, the price you are paid for exported surplus, and the system cost. The calculator estimates annual production, values the self-used and exported halves separately, and shows how long the system takes to pay for itself.

Written by Kenneth Schrøder, Founder of WiseCalcs

Updated September 7, 2026·Sources verified September 6, 2026

Solar pays off on two different prices at once: the retail price you avoid on the energy you use yourself, and the lower export price on everything you send to the grid — this calculator values both and gives the payback that follows.

Yearly saving and income

$745

Self-use: $526Export: $219

Payback period: 18.8 years
Annual production: 7,300 kWh
Used at home: 2,920 kWh
Value of self-use: $526
Exported: 4,380 kWh
Income from export: $219
Average value per kWh: $0.10

What this assumes

Energy you use yourself saves buying electricity at your own price. Everything else is sold to the grid at the separate export price.
Production: 7,300 kWh a year, from 4 peak sun hours × 365 days. Real output varies with shading, angle and weather.
Simple payback: system cost divided by the yearly value. No interest, price rises, maintenance or panel degradation.

Yearly value by self-use share

Left: everything exported. Right: everything used at home.

The headline is the yearly value of the system: self-use savings plus export income. The bar underneath shows how that value splits between the two, and the small chart shows how much the whole thing moves if you use more or less of your own production.

How does it work?

Self-used and exported energy are valued separately because they are worth different amounts: self-use avoids the retail price, while export earns whatever your agreement pays. Valuing all production at the retail price overstates the saving unless you genuinely use everything you generate. Payback ignores interest, maintenance, and panel degradation.

Solar savings formula

S = kW \times h \times 365 \times \big(s\,p_{\text{use}} + (1-s)\,p_{\text{sell}}\big)

kW: System size in kilowatts.
h: Average peak sun hours per day.
s: Share of production used in the home.
p_use: Retail electricity price a self-used kWh replaces.
p_sell: Price paid for a kWh exported to the grid.
S: Yearly value of the system's production.

A 5 kW system at 4 peak sun hours produces about 7,300 kWh a year. Using 40% of it at home replaces 2,920 kWh at 0.30, worth 876, and the exported 4,380 kWh at 0.10 brings in 438 — 1,314 a year in total. That pays back a 9,000 system in about 6.8 years, and values an average produced kWh at 0.18.

Method & sources

Annual production is system size times peak sun hours times 365. That production is split by the share you use in the home: the self-used part is valued at the retail price it replaces, and the rest at the price your agreement pays for exported surplus. Adding the two gives the yearly value, which divides into the system cost to give payback and into production to give the average value of a produced kilowatt-hour.

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

Homeowner's guide to going solar — U.S. Department of Energy, verified 2026-09-06
Photovoltaic Geographical Information System (PVGIS) — European Commission, Joint Research Centre, verified 2026-09-06

How we calculate

Annual production = system size × peak sun hours × 365.
Production splits into a self-used share and an exported remainder, valued at two separate prices you enter.
The self-use share is a yearly average; it is not modelled hour by hour against your actual load.
Payback divides system cost by the yearly value of self-use savings plus export income.
Real output varies with shading, angle, weather, and panel degradation, none of which are modelled.

Limitations

The self-use share is the hardest input to guess and the one that moves the result most. Without a battery a typical household lands somewhere between a quarter and a half; with a battery or a heat pump it is higher.
Net metering, time-of-use export prices, and schemes that pay differently in summer and winter are not modelled — enter an average that reflects what you actually receive.
Grants, tax credits, and VAT treatment change the effective system cost and are not included; fold them into the cost field if you want them counted.

Rounding

Money is shown as whole units, the average value per kWh to two decimals, production as whole kWh, and payback to one decimal. Full precision is used internally.

What this calculator does

Solar savings come from two sources that are worth very different amounts. Energy you use while the panels are producing replaces electricity you would have bought at the retail price. Everything else goes to the grid and earns whatever your agreement pays, which is often a fraction of that. This calculator estimates annual production, splits it by the share you use yourself, values each part at its own price, and divides the system cost by the total to find the payback period.

How to use it

Enter your system size in kilowatts and your average peak sun hours.
Drag the slider to the share of production you expect to use in the home.
Enter the price you pay per kWh and the price you are paid for exported surplus.
Enter the installed system cost, including anything you must pay to get it running.
Read the yearly value, the split between self-use and export, and the payback period.

Panels produce most at midday, when many homes use least. Without a battery, a typical household consumes somewhere between a quarter and a half of what its panels make; the rest is exported. Because self-used energy is usually worth several times the export price, moving that share is often worth more than adding panels. Running the dishwasher at noon, timing a hot water cylinder, or charging a car during the day all push it up.

A worked example

A 5 kW system at 4 peak sun hours produces about 7,300 kWh a year. Use 40% of that at home and you replace 2,920 kWh at 0.30, worth 876. The remaining 4,380 kWh exported at 0.10 brings in 438. The year is worth 1,314 in total, which pays back a 9,000 system in about 6.8 years and values an average produced kilowatt-hour at 0.18 — well below the retail price, and that gap is the whole point of the split.

Finding your sun hours

Peak sun hours are the daily equivalent of full-strength sunlight, not the number of daylight hours. They are typically 2.5–3 in northern Europe, 3–4.5 in temperate regions, and above 4.5 in sunny climates. A public solar irradiation database or an installer's quote for your address will give a figure closer to your roof than any rule of thumb.

Common mistakes

Using daylight hours instead of peak sun hours, which roughly doubles the production estimate.
Valuing all production at the retail price. That is only right if you genuinely consume everything you generate.
Entering an export price from a scheme you are not on, or forgetting that some agreements pay nothing for surplus.
Ignoring panel degradation, maintenance, and the possibility that both prices change over twenty years.

When it's useful

Weighing a quote, comparing system sizes, testing whether a battery's promise of higher self-use is worth its price, or checking whether the payback period fits how long you plan to stay in the home. Because the two prices are separate inputs, you can also see quickly how much of your case depends on an export rate that could be renegotiated.

FAQ

How is the yearly value calculated?: System size times peak sun hours times 365 gives annual production. The self-used share is multiplied by your electricity price, the rest by your export price, and the two are added.
What self-use share should I enter?: Without a battery, most households land between 25% and 50%. A battery, a heat pump, or daytime car charging pushes it higher. If your inverter app reports it, use your own figure.
Does it include payments for exported surplus?: Yes. Exported energy is valued at the export price you enter, separately from self-use. Enter 0 if your agreement pays nothing for surplus.
How is payback calculated?: The system cost divided by the yearly value. A 9,000 system worth 1,314 a year pays back in about 6.8 years. Interest, maintenance, and degradation are not included.
Why is real output often lower?: Shading, panel angle, temperature, dirt, and gradual degradation all reduce real production below the ideal figure that sun hours alone suggest.
Can I share or save a calculation?: Yes. Share copies a link that reopens the calculator with the same figures, and download or print keeps the inputs and results as plain text.

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