Solar Panel Calculator

Sizing a solar system is a matching problem: the array has to generate roughly as much electricity over a year as the household consumes, and how much a given number of panels produces depends mostly on where you live and how the roof faces. The starting point is consumption in kilowatt-hours, which appears on every electricity bill, usually as a monthly or annual total. Turning consumption into system size needs two figures. The first is peak sun hours, the number of hours per day when the sun is strong enough for a panel to produce its rated output — typically four to six in temperate regions and higher nearer the equator. The second is the performance ratio, which accounts for everything that stops a real system reaching its nominal rating: inverter losses, wiring, dirt, heat and shading. Around eighty per cent is a reasonable working figure for a well-installed system. Dividing daily consumption by peak sun hours and the performance ratio gives the system size in kilowatts, and dividing again by the wattage of one panel gives the panel count. That count also needs somewhere to live: an average panel occupies around twenty square feet, so a four-kilowatt array needs a good eighty to a hundred square feet of unshaded, reasonably south-facing roof. Shade, orientation and roof pitch all reduce output, and a north-facing roof may be useless in some climates. This calculator estimates system size, panel count, required roof area and annual generation, and prices the output at the tariff you enter. It assumes an unshaded, well-oriented roof and ignores battery storage, export tariffs and incentives. Get a site survey before committing, since a professional will measure the actual shading and roof condition.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.