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Solar Panel Size Calculator

Work out how many panels of a given wattage are needed for a target output.

About Solar Panel Size Calculator

Each panel produces its rated wattage times sun hours, reduced by system losses. Rounding up means actual output slightly exceeds the target, which provides useful margin. Panel wattage has risen steadily, so a modern array needs far fewer panels and less roof area than an equivalent system from a decade ago.

Formula

Solar Panel Size

Panels = daily kWh ÷ (panel watts × sun hours × derate ÷ 1,000)

panel watts
Rated output at standard test conditions

Worked example

With the default values below, Solar Panel Size Calculator returns Panels Required of 11. Change any input above and every figure here is recalculated in your browser.

Inputs used

Daily energy neededminimum 0 kWh
15 kWh
Panel rated powerminimum 1 W
430 W
Peak sun hours per dayaccepts 0.1–12 h
4 h
System lossesaccepts 0–100 %
20 %

Results

Panels Required
11
Total Array Size
4.730 kW
Daily Output per Panel
1.3760 kWh
Approximate Roof Area
22.00 m2
Actual Daily Output
15.136 kWh

Step by step

  1. 1Values used
  2. 2Solar Panel Size
  3. 3Panels Required= 11
  4. 4Total Array Size= 4.730 kW
  5. 5Daily Output per Panel= 1.3760 kWh
  6. 6Approximate Roof Area= 22.00 m2
  7. 7Actual Daily Output= 15.136 kWh

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Solar Panel Size calculated?

Panels = daily kWh ÷ (panel watts × sun hours × derate ÷ 1,000) Each panel produces its rated wattage times sun hours, reduced by system losses. Rounding up means actual output slightly exceeds the target, which provides useful margin.

Why does Solar Panel Size matter?

Panel wattage has risen steadily, so a modern array needs far fewer panels and less roof area than an equivalent system from a decade ago.

What values do I need to enter?

This calculator takes 4 inputs: Daily energy needed, Panel rated power, Peak sun hours per day, System losses. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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