How accurate is this solar panel calculator?

It provides an estimate using system size, peak sun hours, performance ratio, electricity price, self-consumption and export tariff. Actual performance depends on roof angle, orientation, shading, equipment, weather and local tariffs.

What is the default performance ratio?

The calculator uses an 80% performance ratio by default to represent common real-world losses such as inverter losses, heat, dirt, wiring and panel mismatch.

Does this replace a solar installer quote?

No. It is an educational estimate and should not be treated as a final installation quote, financial advice or engineering design.

Free solar savings estimator

Estimate solar output, savings, payback time and 25-year ROI.

Use transparent assumptions to calculate how much energy your solar panels could generate, how many panels you may need, and whether the investment could make financial sense.

Start calculating See the formulas

80%Default performance ratio

25 yrsLifetime model

0.5%Default annual degradation

Solar panel calculator

Adjust the figures below. Use your local currency mentally if you are outside the eurozone; the math is currency-neutral.

Monthly electricity use kWh Electricity price €/kWh Solar system size kW Peak sun hours / day hours Panel wattage W Installed cost € Feed-in / export tariff €/kWh Performance ratio 0-1

Self-consumption65%

Annual panel degradation0.5%

Your estimate

Results update instantly from your inputs.

Daily output0 kWh

Monthly output0 kWh

Annual output0 kWh

Panels needed0

Bill savings€0

Export earnings€0

Payback period0 yrs

25-year savings€0

25-year ROI0%

25-year cumulative savings

Net of the installation cost, with yearly panel degradation applied.

How the estimate works

The calculator starts with your solar system size and average peak sun hours, then applies a real-world performance ratio. It splits generated energy between self-used solar power and exported solar power. Savings are based on the electricity you avoid buying plus any export tariff earnings.

Annual production = system kW × sun hours × 365 × performance ratio

Panels needed = system kW × 1000 ÷ panel wattage

Annual benefit = self-used kWh × electricity price + exported kWh × feed-in tariff

Payback = installation cost ÷ annual benefit

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Solar calculator FAQ

How many solar panels do I need?

Divide your target system size in watts by the wattage of each panel. For example, a 5kW system with 430W panels needs about 12 panels.

What is a good solar payback period?

It depends on local installation costs, electricity prices, incentives and self-consumption. Many users compare payback against the 25-year expected life of the system.

Does a battery improve the return?

A battery can improve self-consumption but also adds cost. It is most useful where export rates are low, evening electricity rates are high, or backup power has value.

Why does roof orientation matter?

Solar panels generate more when they face strong sun exposure with limited shading. Roof angle, azimuth, nearby buildings, trees and climate all affect real production.