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Solar System Size Calculator

Size a solar array in kilowatts from annual consumption, sun hours and system losses.

About Solar System Size Calculator

Peak sun hours condense a whole day of varying irradiance into equivalent full-sun hours. The derate factor of about 20 per cent covers inverter, wiring, soiling and temperature losses. Sizing on nameplate watts without derating overstates generation by a fifth, which is the most common cause of disappointing solar performance.

Formula

Solar System Size

System kW = annual kWh ÷ (365 × peak sun hours × derate factor)

peak sun hours
Daily equivalent hours at full 1,000 W/m² irradiance
derate
Combined inverter, wiring, soiling and temperature losses

Worked example

With the default values below, Solar System Size Calculator returns System Size Required of 4.403. Change any input above and every figure here is recalculated in your browser.

Inputs used

Annual electricity consumptionminimum 0 kWh
4,500 kWh
Average peak sun hours per dayaccepts 0.1–12 h
3.5 h
System lossesaccepts 0–100 %
20 %
Share of consumption to offsetaccepts 0–100 %
100 %

Results

System Size Required
4.403 kW
Panels Needed at 430 W Each
11
Daily Generation
12.329 kWh
Annual Generation
4,500.0 kWh
Approximate Roof Area Needed
22.00 m2

Step by step

  1. 1Values used
  2. 2Solar System Size
  3. 3System Size Required= 4.403 kW
  4. 4Panels Needed at 430 W Each= 11
  5. 5Daily Generation= 12.329 kWh
  6. 6Annual Generation= 4,500.0 kWh
  7. 7Approximate Roof Area Needed= 22.00 m2

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 or fees or charges into account.
  • Figures are rounded for display to 3 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 System Size calculated?

System kW = annual kWh ÷ (365 × peak sun hours × derate factor) Peak sun hours condense a whole day of varying irradiance into equivalent full-sun hours. The derate factor of about 20 per cent covers inverter, wiring, soiling and temperature losses.

Why does Solar System Size matter?

Sizing on nameplate watts without derating overstates generation by a fifth, which is the most common cause of disappointing solar performance.

What values do I need to enter?

This calculator takes 4 inputs: Annual electricity consumption, Average peak sun hours per day, System losses, Share of consumption to offset. 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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