Solar Panel Calculator

The short answer

A 400 W panel in a location with four peak-sun-hours a day produces about 1.6 kWh a day, or roughly 584 kWh a year. Offsetting a $150 monthly electricity bill of about 900 kWh a month needs around 18 to 20 kW of panels in a good solar region — 45 to 50 panels, occupying roughly 1,000 square feet of roof or ground.

What it depends on

Peak sun hours vary enormously by location, season and orientation, and they are the largest single variable — the same panel in Arizona produces far more than in the Pacific Northwest. Shading, tilt, azimuth, temperature and soiling all reduce the nameplate figure, and system losses from inverter and wiring take another 10 to 20% off. Your consumption profile relative to solar production determines how much of the output you actually use.

Where this stops being useful

This is a sizing estimate. Grid connection rules, net metering or feed-in arrangements, permits, utility approval, structural roof capacity and incentive eligibility are all jurisdiction-specific and change frequently. A licensed solar contractor can assess your site and quote a system. General information and planning only, not engineering, structural or code advice.

Panel output depends on orientation and shading far more than on panel size. A south-facing roof in a clear-sky location will produce meaningfully more than an east-facing one, and a panel that loses the afternoon to a neighbouring tree may produce a fraction of the quoted figure. The calculator uses peak sun hours as a proxy for location, which is a useful average but hides seasonal variation — real output in December can be a third of the June figure even in a good location.

A solar calculation done before choosing the panels tends to produce a more honest answer than one done afterwards, because it is easier to question the assumptions than the result. The orientation and shading questions are worth spending an afternoon on before any money changes hands.

Generation in kilowatt-hours is only half the question; the system also needs to be sized against consumption. Exporting surplus is usually worth far less than the electricity you consume directly, so a system matched to your usage is worth more than a larger one. Check the roof orientation and the shading pattern at several times of day before ordering. If the roof is shaded by trees that will grow, factor that in now rather than discovering it in five years.

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.