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Solar Panel Calculator

Size a solar system from daily kWh use, sun hours and panel wattage.

Best for: Early sizing of a rooftop solar system before getting professional quotes.

Complete guide
Country (units):

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How it's calculated & sources
The Solar Panel Calculator uses the standard method for this calculation. It runs entirely in your browser, so your inputs are never uploaded. Last reviewed June 2026.

Free & no sign-up · runs entirely in your browser. Results are estimates for general information, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.

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Complete guide

Quick answers

Short, sourced answers to the questions people (and AI assistants) ask most.

What is Solar Panel?
A solar panel calculator estimates the system size and output needed to cover a given amount of electricity use.
Why does Solar Panel matter?
A planning estimate. Actual output depends heavily on your location's sun hours, roof orientation and tilt, shading and the season.
How is Solar Panel calculated?
Daily generation is roughly the system size in kilowatts times the local peak sun hours times a performance ratio (commonly about 0.75-0.8 for real-world losses). To size a system, divide your daily kilowatt-hour need by the sun hours times that ratio. Formula: System kW = daily kWh need / (peak sun hours x performance ratio).
What are common solar panel mistakes?
Ignoring local peak sun hours, which vary widely by region and season.
When should you use the Solar Panel Calculator?
Early sizing of a rooftop solar system before getting professional quotes.

What is the Solar Panel Calculator?

A solar panel calculator estimates the system size and output needed to cover a given amount of electricity use.

How the Solar Panel Calculator works

Daily generation is roughly the system size in kilowatts times the local peak sun hours times a performance ratio (commonly about 0.75-0.8 for real-world losses). To size a system, divide your daily kilowatt-hour need by the sun hours times that ratio.

System kW = daily kWh need / (peak sun hours x performance ratio)

Solar system size by daily use (4 peak-sun hours, 0.8 derate, 400 W panels)

10 kWh3.13 kW8
15 kWh4.69 kW12
20 kWh6.25 kW16
30 kWh9.38 kW24
40 kWh12.5 kW32

Key terms

Peak sun hours:
Daily hours of full-strength sunlight equivalent at your location.
Performance ratio:
A factor (about 0.75-0.8) for real-world system losses.

Common mistakes to avoid

  • Ignoring local peak sun hours, which vary widely by region and season.
  • Using the nameplate rating without a performance ratio for real losses.
  • Overlooking shading, roof orientation and tilt.
  • Treating one annual average as if output were steady year-round.

Keywords: solar, panel, kw, system.

Sources: NREL — PVWatts solar estimator

Reviewed by the Free Tools Galaxy editorial team · Updated June 2026 · Calculated privately in your browser.

Frequently asked questions

Is the Solar Panel Calculator free to use?+

Yes. Every tool on Free Tools Galaxy is 100% free, runs in your browser and requires no signup.

How accurate is the Solar Panel Calculator?+

Solar Panel Calculator uses the standard solar panel formula in double-precision arithmetic, so the same inputs always produce the same result and you can verify any figure by hand. It is an educational estimate — real-world outcomes depend on your actual rates, rules and assumptions.

Do you store my inputs?+

No. The Solar Panel Calculator runs entirely in your browser. Nothing is uploaded or saved to a server.

Can I use the Solar Panel Calculator on mobile?+

Yes — the interface is fully responsive and works on phones, tablets and desktops.

What are common mistakes to avoid?+

The most frequent mistake is mixing units. Double-check your inputs use a single, consistent unit before clicking Calculate.

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Size a solar system from daily kWh use, sun hours and panel wattage.

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