Solar System Size & Panel Count Calculator

How to calculate your solar system size

System size — the kilowatt (kW) rating of your panel array — is the headline number of any solar project, and it follows directly from two things: how much electricity you use, and how much sun your location receives. Everything else (cost, savings, roof space, the number of panels) scales from it. Getting it right means neither overbuilding, which wastes money on capacity you can't use, nor underbuilding, which leaves savings on the table. The calculation is simple arithmetic once you have your numbers.

Getting the size right is worth a little effort because it’s the decision everything else hangs on. Price scales almost linearly with system size, so an array that’s 20% too big is roughly 20% more expensive than it needed to be; one that’s too small leaves you buying grid power you could have generated. Installers, understandably, often lean toward larger systems, so understanding the calculation yourself is the best defence against paying for capacity you can’t use — or being talked into a system that doesn’t fit your roof or your tariff. The aim isn’t the biggest array that fits; it’s the array that best matches your consumption, your sunlight and the way your utility pays for exports.

Whether you're offsetting a home electricity bill or planning a larger installation, the method is the same; only the inputs change. The calculator above does it instantly, but understanding the logic lets you sanity-check any quote an installer gives you.

The core formula

Your annual use comes from your electricity bills (sum a year, or multiply a typical month by 12). Peak sun-hours is the daily average of full-strength sunlight your location gets — typically 3 to 6. The performance ratio (~0.8) accounts for the real-world losses between the panel's lab rating and your meter: inverter conversion, wiring, heat, dust and shading. A system rarely delivers its nameplate output, so this factor keeps the estimate honest.

Worked example

A home using 9,000 kWh a year in a location with 4.5 peak sun-hours and a 0.8 performance ratio needs 9,000 ÷ (4.5 × 365 × 0.8) ≈ 6.8 kW. At 400 W per panel, that's about 17 panels, needing roughly 31 m² (≈330 ft²) of unshaded roof. Move the home to a sunnier 5.5-sun-hour location and the same usage needs only ~5.6 kW; move it somewhere cloudier at 3.5 sun-hours and it climbs to ~8.8 kW. Your location matters as much as your consumption.

Should you size to 100% of your usage?

Not always — and this is where honest sizing diverges from a default sales pitch. Covering 100% of annual usage makes sense under traditional net metering, where exported surplus is credited near the retail rate. But where exports are paid far less than retail — California's NEM 3.0, the UK, Pakistan, and increasingly elsewhere — a large array that dumps midday surplus for a fraction of its value is poor economics. There, sizing closer to your daytime self-consumption, or adding a battery, often beats a bigger array. The right size depends on your tariff as much as your roof, which is why we treat export rate as a key input rather than assuming retail-rate net metering.

Plan for the future

If an electric vehicle, heat pump or home extension is on the horizon, size for that future load now. Adding capacity to an existing system later is more expensive per watt than including it at the outset, because you pay again for scaffolding, labour and possibly a larger inverter. A modest amount of headroom is cheap insurance against outgrowing your array.

From system size to roof and budget

Once you know the kW you need, two practical checks follow. First, roof space: at roughly 1.8 m² per 400 W panel, multiply your panel count to see whether your usable, unshaded roof can hold the array — and if not, higher-efficiency panels fit more capacity into less area. Second, budget: cost scales closely with size, so the kW figure quickly translates into a price range once you know local cost-per-watt. These two checks sometimes push the practical size below the ideal one, in which case you simply offset a smaller share of your bill — still worthwhile, just partial. It’s better to install the largest sensible array your roof and budget allow than to abandon the project because you can’t reach 100% offset.

Frequently asked questions

Divide your annual kWh usage by your local peak sun-hours times 365 times a ~0.8 performance ratio. A typical home lands somewhere between 4 and 9 kW depending on usage and climate. Enter your figures above for a specific size and panel count.

Roughly 6 × peak sun-hours × 365 × 0.8. At 4.5 sun-hours that's about 7,900 kWh a year; at 5.5 sun-hours, about 9,600 kWh. The same system produces noticeably more in a sunnier location.

It depends on your export rate. Under full net metering, sizing to 100% of usage is sensible. Where exports pay poorly, slightly undersizing toward your self-consumption (or adding storage) is often better value than a large array selling surplus cheaply.

Because panels rarely deliver their lab rating in real conditions. Inverter losses, heat, wiring, dust and shading typically cost 15–25%, so a ~0.8 ratio turns nameplate capacity into realistic output. Ignoring it overstates production and undersizes the array.