Solar Panel Calculator

The calculation itself

System size in kilowatts is annual consumption divided by peak sun hours times 365 times a derate factor. Panel count is that size divided by the panel rating.

Peak sun hours is the parameter carrying the answer, and it is widely misunderstood. It is not daylight hours — it is the equivalent number of hours at 1,000 watts per square metre, which already folds in latitude, season, typical cloud and panel orientation.

In practice

12,000 kWh a year at 4.5 peak sun hours and a 0.8 derate needs 12,000 ÷ (4.5 × 365 × 0.8) = 9.13 kW, which is 23 panels at 400 W each and about 414 square feet of roof.

Move to a location with 6 peak sun hours and the same house needs 6.85 kW — 18 panels. The array scales inversely with sun hours, which is why a national average is close to useless for a specific roof.

Worsen the derate from 0.8 to 0.7, which shading can easily do, and the 4.5-hour case rises to 10.44 kW. Shade costs panels.

The derate factor is where optimism hides

Nameplate output is measured at 25°C under standard test conditions that rarely occur on a roof. Real systems lose to inverter conversion, wiring resistance, soiling, panel temperature — output falls as panels heat — and any shading at all.

0.75 to 0.85 covers most well-designed systems. Below that usually means a shading problem, and no factor rescues a badly shaded roof; partial shade on a string can cost far more output than the shaded area suggests.

Panels also degrade, typically a few tenths of a percent a year, so a twenty-year average is lower than year one.

Where the figure deceives

Simple payback divides installed cost by annual saving and ignores incentives, financing costs, tariff changes and degradation. It is a first sort, not a decision.

Sizing to cover 100% of annual usage is rarely optimal, because export rates are usually well below retail. A system sized to cover winter generates a summer surplus you sell cheaply, and the economics then depend on your tariff structure more than on your roof.

Annual figures also hide seasonality entirely. A system that matches yearly consumption can still leave you buying heavily in December.

Acting on it

Use twelve months of your own bills, not an estimate. Consumption varies more between households than sun hours vary between regions.

Get a location-specific sun hours figure rather than a national one, and get a shading assessment before trusting any derate.

Model self-consumption against export separately if your tariff distinguishes them, because it usually decides the right system size.