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Solar Panel Sizing Calculator

Work out solar panel sizing instantly with clear inputs, formula shown and shareable results.

About Solar Panel Sizing Calculator

Array size is daily energy demand divided by peak sun hours and by the system efficiency. Losses of 20-25% are normal and cover inverter conversion, wiring, soiling, mismatch and the fact that panels run hotter than their 25 degree test condition.

Formula

Array sizing

array W = daily Wh / peak sun hours / (1 - system losses)

Worked example

With the default values below, Solar Panel Sizing Calculator returns Array size required of 1,282. Change any input above and every figure here is recalculated in your browser.

Inputs used

Daily energy demand
4,200 Wh
Peak sun hours
4.2 h/day
System losses
22 %
Panel rating
450 W

Results

Array size required
1,282 W
Panels needed
3 panels
Generation from that array
4,423 Wh
Daily surplus
223 Wh

Step by step

  1. 1Values used
  2. 2Array sizing
  3. 3Array size required= 1,282 W
  4. 4Panels needed= 3 panels
  5. 5Generation from that array= 4,423 Wh
  6. 6Daily surplus= 223 Wh

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 whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

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

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

What are peak sun hours?

The equivalent number of hours at 1,000 W/m2 that delivers the day's total irradiation. It is not daylight hours — a 12 hour day may give only 4 peak sun hours.

Why is 20% loss realistic?

Temperature alone costs 8-15% in warm climates, inverters 3-5%, wiring 2-3%, soiling and mismatch the rest.

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