FreeToolStation

Solar Panel Calculator

System size, yearly output, roof area and payback.

Home & Energy Calculators Free, no sign-up Runs in your browser

About 2.5 in northern Europe, 4 to 5 in southern Europe, 5 to 6 in much of South Asia.

Losses from the inverter, wiring, heat and dirt. 75 to 85 is typical.

Optional, for payback.

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System size
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Panels
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Yearly output
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Share of your use
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Payback period
MeasureValue
A real quote depends on your roof pitch and orientation, shading, local irradiance and what your supplier pays for exported power. This sizes the system and shows the arithmetic behind an estimate; it does not replace a site survey.

About the Solar Panel Calculator

Solar Panel Calculator sizes a photovoltaic system three ways: from your electricity use, from a system size you have in mind, or from the roof area available. All three converge on the same arithmetic -- system size times peak sun hours times efficiency gives daily output.

Peak sun hours is the input that varies most and matters most. It is not daylight hours: it is the equivalent number of hours at full rated irradiance, and it runs from about 2.5 a day in northern Europe to 5 or 6 across much of South Asia and the southern United States. The same 4 kW array produces roughly twice as much in Karachi as in Edinburgh.

System efficiency, often called the derate factor, covers everything between the panel rating and the meter: inverter losses, wiring, heat, dust and panel degradation. Seventy-five to eighty-five per cent is realistic, and using 100 per cent -- as a naive calculation does -- overstates output by a quarter.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

Looking for something this does not cover? FreeToolsZone.org is more free browser-based tools, organised by job, and it is worth a look if solar panel calculator is only part of what you are working on.

How to use the Solar Panel Calculator

  1. Choose start fromThe fields below change to match what you pick, so only the figures that matter to your case are asked for.
  2. Enter electricity use (kwh a year), system size (kw) and usable roof area (m²)Decimals are fine, and thousands separators are accepted and ignored.
  3. Read the system size, panels and yearly outputThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
  4. Copy or check the resultCopy the summary with the copy button, or open the table to check it row by row.

Features

  • Shows system size, panels, yearly output, share of your use and payback period together, so you are not running it twice
  • A results table you can read across
  • Updates as you type, with no button to press and no page reload
  • Copy the result in one click
  • Free, no sign-up, and it runs entirely in your browser

Who uses this tool

The solar panel calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

  • Sizing a system to cover your electricity use
  • Checking how many panels a roof will hold
  • Estimating yearly generation before getting quotes
  • Working out a rough payback period

Frequently asked questions

What are peak sun hours?

The equivalent hours per day at full rated irradiance, not daylight hours. It ranges from about 2.5 in northern Europe to 6 in sunny regions.

Why is the efficiency only 80 per cent?

Inverter losses, wiring, heat, dust and degradation all take a share between the panel rating and the meter. 75 to 85 per cent is normal.

Will I really save the full output?

Only if you use the power as it is generated. Exported power is usually paid at a lower rate, so a realistic saving depends on your usage pattern and your tariff.

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Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.