It is the first question almost every homeowner asks about solar, and it has a refreshingly simple answer: the number of panels you need depends on how much electricity you use, how much sun your roof gets, and how powerful each panel is. Get those three numbers right and the rest is basic arithmetic.

Step 1: Find out how much electricity you actually use

Forget rules of thumb based on house size. The only number that matters for sizing is your real consumption, measured in kilowatt-hours (kWh), and the best source for it is your own electricity bill.

Look for the kWh figure on your bill — it is usually printed near the usage summary — and add up the last 12 months, then divide by 12. A full year matters because usage swings with the seasons: air conditioning in summer and heating in winter can easily double a household’s consumption in some months. Sizing from a single mild month in spring will leave you short in July.

If you cannot find 12 months of bills, your utility’s online account usually shows monthly history. As a rough benchmark only, the US Energy Information Administration (EIA) reports the average American home used about 874 kWh per month in 2025, at around 17 cents per kWh. Your home may be well above or below that — a small apartment might use 300 kWh a month, a large house with electric heating or an EV can pass 1,500 kWh. Use your own figure wherever possible.

Step 2: Know your peak sun hours

Panels are rated in watts under laboratory conditions, but the sun does not shine at full strength all day. Installers therefore use peak sun hours: the number of hours per day when sunlight averages 1,000 watts per square metre, the intensity panels are rated at. One peak sun hour is not the same as one hour of daylight — a 12-hour summer day might deliver only five peak sun hours once the weak morning and evening light is accounted for.

Across most of the continental United States, homes receive roughly 3.5 to 5.5 peak sun hours a day on an annual average, with the desert Southwest reaching 6 or more and the Pacific Northwest and parts of the Northeast sitting nearer 3.5 to 4. The UK is lower, typically 2.5 to 3.5 peak sun hours a day. Your exact figure depends on location; the National Renewable Energy Laboratory’s free PVWatts calculator can estimate it from your postcode or ZIP code.

Step 3: The sizing formula

With your daily usage and peak sun hours in hand, the formula is straightforward:

System size (kW) = daily kWh ÷ peak sun hours ÷ 0.8

The 0.8 is a derate factor. Real systems lose roughly 20% of their theoretical output to heat, inverter conversion, wiring, dust, and panels that are not perfectly angled. It is a planning allowance, not a flaw in any particular brand — even a well-installed system will not deliver its laboratory rating every hour of the day.

Once you have the system size in kilowatts, divide by the wattage of a single panel to get the panel count:

Number of panels = system size (W) ÷ panel wattage (W)

Most residential panels sold in 2026 are rated around 400 watts, though 420W to 450W panels are increasingly common. Higher-wattage panels mean fewer panels for the same system size.

A worked example: a 900 kWh-per-month home

Take a household that uses 900 kWh a month — close to the US average. That works out to 30 kWh a day (900 ÷ 30). Assume 4.5 peak sun hours a day and the usual 0.8 derate factor:

  • 30 kWh per day ÷ 4.5 peak sun hours = 6.7 kW
  • 6.7 kW ÷ 0.8 = 8.4 kW of panels needed
  • 8,400 W ÷ 400 W per panel = 21 panels

So this home would need roughly 21 panels of 400 watts, an 8.4 kW system. If the same home were in Arizona with 6 peak sun hours, the answer would drop to about 16 panels. In Seattle, at 3.5 peak sun hours, it would rise to about 27. Sunlight, more than anything else, decides the number.

Try the same sums with your own figures using our solar panel calculator.

How much roof space will that take?

A standard residential 400W panel measures roughly 65 by 39 inches, covering about 18 to 21 square feet. Allow for spacing between rows, walkways, and setbacks from the roof edge, and a 21-panel system needs on the order of 400 to 500 square feet of usable, unshaded roof. The constraint is usually usable space, not total roof area: dormers, chimneys, vents, and shaded sections all reduce what is available.

What changes the number?

Shade. Even partial shade from a tree or chimney can cut a panel’s output sharply. A shaded roof may need more panels, microinverters, or power optimisers — or may not be a good solar candidate at all.

Roof direction and tilt. In the Northern Hemisphere, a south-facing roof at a tilt roughly equal to your latitude performs best. East- or west-facing roofs still work well, typically producing around 10 to 20% less than south-facing, which means a few extra panels. A north-facing roof is usually a poor choice.

High-usage homes. Electric heating, a heat pump, a pool, or an electric vehicle can each add hundreds of kWh a month. An EV charged at home can add roughly 250 to 350 kWh a month for typical driving — the equivalent of about 6 to 8 extra panels. Size for the usage you expect, not just the usage you have today.

Your goal. Not everyone wants to cover 100% of their bill. Some homeowners deliberately size a system to cover 70 to 80% of their usage, which keeps the upfront cost down while still cutting the bill substantially. Others oversize slightly to allow for a future EV or heat pump.

A quick way to check any quote

When an installer proposes a system, run the formula backwards: divide the proposed system size by your daily usage and your peak sun hours, and see whether the numbers line up with the worked example above. A quote far bigger than your usage justifies should come with a clear explanation; a quote far smaller may be designed to look cheap rather than to meet your needs.

For a fast, personalised estimate, try our how many solar panels do I need calculator. Enter your monthly usage and location, and it will estimate your system size and panel count — a useful starting point before you talk to any installer.

Figures in this article: US average household use of 874 kWh per month and average residential price of 17.23 cents per kWh are from the EIA’s final 2025 data; US peak sun hours of roughly 3.5 to 5.5 a day reflect NREL irradiance data. See the EIA’s explanation of electricity prices for the official figures.