Solar System Size Calculator

How It Works

The sizing formula

A solar array has to cover your daily energy use despite real-world losses. The system size in kilowatts is:

Dividing that by the rating of one panel and rounding up gives the panel count, and the panel count times the panel rating gives what you actually end up installing:

Worked example

A household uses 900 kWh a month (about 29.6 kWh a day), sits in a region with 4.5 peak sun hours, expects 18% system losses, wants to cover 100% of its use, and is fitting 400 W panels.

• Daily use = 900 ÷ 30.44 = 29.57 kWh
• Effective sun hours = 4.5 × (1 − 0.18) = 3.69 h
• System size = 29.57 ÷ 3.69 = 8.01 kW
• Panel count = ⌈8013 ÷ 400⌉ = ⌈20.0⌉ = 21 panels
• Installed capacity = 21 × 400 ÷ 1000 = 8.4 kW, needing roughly 42 m² (452 ft²) of roof
• Annual output = 8.4 × 3.69 × 365 = 11,314 kWh

Loss factor explained

The loss factor is the gap between a panel's nameplate rating and what actually reaches your load. Typical contributors: inverter efficiency (about 4%), DC and AC wiring resistance (about 2%), temperature derating (about 4.5% averaged over a year), soiling (about 2%), shading (about 2%) and module mismatch (about 2%). PVWatts v8's own default balance-of-system figure is about 14% and excludes temperature; the 18% default here is that budget plus an annual-average temperature loss, so it can be used on its own. Installs with microinverters or optimisers and short cable runs sit below it.

The temperature figure is the one most often overstated. A panel running at 45 °C loses about 7% of its output, but that is a summer midday cell temperature, not a yearly one. Across a British or northern-European year, cold-season operation runs at or below the 25 °C rating condition and loses nothing at all, which pulls the annual weighted figure down to roughly 3 to 4.5%.

If you use the panel output calculator as well, do not carry this 18% across to it. That tool takes 14% and applies temperature separately, and entering 18% there would count temperature twice. The two are deliberately different numbers describing the same system, and they agree to within 0.1% when that tool is given an annual-average cell temperature of about 38 °C. Its 45 °C default is a summer midday figure, so leaving it there makes the annual answer about 2.5% pessimistic.

To run the calculation the other way, starting from an array you already have or have been quoted and ending at kilowatt-hours, use the solar panel output calculator, which separates cell temperature out of the loss budget rather than blending it in. Cable losses are the one part of that budget you control after the panels are chosen. Size the DC runs with the solar cable size calculator and check the drop with the DC voltage drop calculator.