How many solar panels do I need?

1. The sizing formula

Sizing a solar array is an energy-balance problem, and the same method used by NREL's PVWatts tool and every reputable installer comes down to two short lines. First, the system size: kilowatts = daily energy use ÷ (peak sun hours × performance ratio). Then the panel count: number of panels = system watts ÷ the wattage of one panel, rounded up to a whole number. Everything else is reading the right figures off your bill and your location.

The logic is straightforward once the three inputs are clear. Your daily use is how much electricity you need the array to cover. Peak sun hours convert a whole day of changing sunlight into an equivalent number of hours at full strength. And the performance ratio recognises that no real system delivers its nameplate rating — there are always losses. Multiply sun hours by the performance ratio and you get the “useful” production each kilowatt of panels delivers per day; divide your daily need by that and you have the kilowatts of panels to install.

2. Start with your electricity use

The single most important number is your own consumption, and it is printed on your power bill in kilowatt-hours. Use a recent bill, or better, average a full year so summer cooling and winter heating both count. If your bill is annual, divide by twelve for a monthly figure; the calculator then divides by the average days in a month (about 30.44) to get a daily average. A home on 900 kWh a month is using roughly 29.6 kWh a day, which is the figure the rest of the sum works from.

Size to what you actually use, not to a generic “average home,” because consumption varies enormously — an all-electric house with heat pumps and an electric vehicle can use three or four times what a small, gas-heated flat does. If you expect your use to rise (an EV on the way, or you plan to electrify your heating), size for that future figure rather than today's, since adding panels later is rarely as cheap per watt as fitting them at install.

3. Peak sun hours — the location factor

Peak sun hours are not the hours of daylight. They are the number of hours per day during which sunlight would have to shine at a full 1,000 watts per square metre to deliver the same total energy your location actually receives. It is a way of compressing a day of dawn-to-dusk, cloud- and-clear sunlight into one tidy number you can multiply. Typical values run from around three hours a day in cloudier, higher-latitude regions to five or six in sunny ones, and they swing with the season — lower in winter, higher in summer.

Because system size is directly proportional to this figure, it pays to use a value for your own location rather than a national average. The same house needs a meaningfully bigger array where there is less sun. Solar-resource maps and the PVWatts tool give location-specific figures; once you have yours, type it into the calculator. If you want to guarantee winter performance, size on a winter sun-hours figure; if you only need to cover annual use with net metering, an annual average is usually fine.

4. The performance ratio

A solar panel rated at 400 watts is measured under Standard Test Conditions — 1,000 W/m² of light at a cell temperature of 25 °C — which a working roof almost never matches. In the real world the inverter loses a little turning DC into AC, the wiring loses a little, panels run hotter than 25 °C and lose efficiency, dust and dirt shade them slightly, and any nearby tree or chimney takes a bit more. Added up, these losses commonly come to 15–25%, so a performance ratio of about 0.80 (80%) is the long-standing default, and it is the figure the calculator starts with.

This is also the honest answer to “does a 400 W panel produce 400 W?” — only fleetingly, in ideal conditions. The performance ratio is precisely what stops you from over-promising. A clean, well-ventilated, unshaded, modern system might justify a slightly higher ratio; an older or partly-shaded one a lower one. Adjust it in the calculator if your installer gives you a specific figure, but 80% is a sound planning number.

5. Panels per home, at a glance

This chart shows the system size and panel count for a range of monthly electricity use, all at 4 peak sun hours, an 80% performance ratio and 400 W panels— the calculator's defaults. Change any of those three and the numbers shift, which is exactly why the tool lets you edit them.

Read it as a planning guide, not a quote. At sunnier sites with higher peak sun hours, the same home needs fewer panels; at cloudier ones, more. The solar panel calculator recomputes all three columns for your own figures.

6. Choosing the panel wattage

Once you know the system size in kilowatts, the panel count is just that figure divided by the wattage of the panel you choose. Modern residential panels cluster around 400 watts, but they run from roughly 350 to over 450. Higher-wattage panels give you the same kilowatts in fewer units, which matters when roof space is tight or you want a cleaner layout; lower-wattage panels mean more units and sometimes a lower price per panel. The kilowatts are what determine your output — the wattage only changes how many physical panels make up that total.

Roof area follows directly from the count. A typical 400 W panel occupies about 1.95 m² (around 21 square feet), so a 24-panel array needs roughly 47 m² (about 500 ft²) of clear, well-oriented roof, plus room for spacing and access. If the sum says more panels than your roof can hold, either choose higher-wattage panels, trim non-essential loads, or accept that the array will offset only part of your use.

7. From estimate to install

This calculation gets you a sound, defensible system size to compare quotes against — it is not a substitute for a site survey. A real design accounts for roof pitch and orientation, shading through the day, the specific inverter and its loading ratio (the “20% rule” installers mention), local interconnection limits, and whether you are adding a battery. Inverter and wiring choices are electrical work; if you are weighing cable runs for the array or a new circuit, our voltage drop calculator handles that side, and the electrical hub collects the rest of the electrical tools.

Note too that this guide deliberately deals only in energy and hardware, not money: we do not quote prices, payback periods or incentives, because those change constantly and vary by region and provider. Use the panel count and system size here to have an informed conversation with installers, and check current incentives with your official local programme. Get the size right first, and the rest of the decision rests on solid ground.

Common questions

How do I calculate how many solar panels I need?

Size the system from your electricity use, then divide by the panel rating: system kW = daily use (kWh) ÷ (peak sun hours × performance ratio), and panels = system watts ÷ panel watts, rounded up. A home on about 900 kWh a month (≈29.6 kWh a day) at 4 peak sun hours and 80% efficiency needs roughly a 9.2 kW system — about 24 panels at 400 W each.

Does a 400W solar panel produce 400W?

Only at Standard Test Conditions — 1,000 W/m² of light at 25 °C — which real roofs rarely meet. Heat, dust, wiring and inverter losses and any shading typically take 15–25% off, which is what the performance ratio (around 0.80) in the formula accounts for. So a 400 W panel usually delivers noticeably less than 400 W in everyday conditions.

What is the 20% rule for solar?

It usually means the inverter loading ratio: installers often fit an array up to about 20% larger than the inverter's rating (a DC-to-AC ratio near 1.2), because panels seldom hit their full rating, so a slightly oversized array keeps the inverter well fed. It is a rule of thumb, not a code requirement — confirm the design with your installer.

How do I calculate what solar panel I need?

Find the total system size first (daily use ÷ peak sun hours ÷ performance ratio), then pick a panel wattage and divide. Higher-wattage panels mean fewer panels for the same kW — useful on a small roof — while lower-wattage panels mean more units. Enter your panel rating in the calculator to see both the kW and the panel count.