How Many Solar Panels Do I Need?
It comes down to four numbers: how much electricity you use, how much sun your location gets, how powerful each panel is, and the real-world losses between the panel and your plug. Get those right and the panel count almost calculates itself.
"How many panels do I need" is the first question almost everyone asks, and most online answers give you a single number that ignores where you live and how much power you actually use. The honest answer is that there's no universal figure — a home in sunny Arizona or Rajasthan needs noticeably fewer panels than the same home in cloudy Britain or northern Germany. But the method to find your number is simple, and you can do it in five minutes.
The four numbers that decide it
- Your electricity use — ideally your annual consumption in kilowatt-hours (kWh). A typical home uses somewhere between 3,000 and 11,000 kWh a year depending on the country, climate and household size.
- Peak sun-hours — the number of hours per day your location effectively receives full-strength sunlight (1,000 W/m²). This is usually 3–6 per day. It already bakes in your latitude, climate and the average across seasons.
- Panel wattage — modern residential panels are mostly 400–460 watts, with 400 W the most common choice in 2026.
- System losses — between the panel's lab rating and your home's AC supply you lose roughly 15–25% to inverter conversion, wiring, heat, dust and shading. A "performance ratio" of about 0.8 (i.e. 80%) is a sensible real-world assumption.
The calculation, step by step
Number of panels = system size (kW) × 1,000 ÷ panel wattage
The first line tells you the array size in kilowatts that will, over a year, generate roughly as much as you use. The second line just divides that capacity into panels.
Worked example
Suppose you use 6,000 kWh a year, your location gets 4.5 peak sun-hours a day, and you use 400 W panels with an 80% performance ratio:
- System size = 6,000 ÷ (4.5 × 365 × 0.8) = 6,000 ÷ 1,314 ≈ 4.6 kW
- Panels = 4,600 ÷ 400 ≈ 12 panels
Move that same home to a cloudier spot with 3.5 sun-hours and the array grows to about 5.9 kW (≈15 panels). Move it to a sunny 5.5-hour location and it shrinks to about 3.7 kW (≈10 panels). Same house, same usage — the sun does the rest.
How much sun does your location get?
Peak sun-hours are not the same as daylight hours. A location with 12 hours of daylight might only deliver 4–5 peak sun-hours, because early-morning and late-afternoon sun is weak. As a rough guide:
| Climate / region | Typical peak sun-hours/day |
|---|---|
| Very sunny (desert, low latitude) | 5.5 – 6.5 |
| Sunny temperate (most of the US Sunbelt, much of India) | 4.5 – 5.5 |
| Mixed temperate (central Europe, US Midwest) | 3.5 – 4.5 |
| Cloudy/northern (UK, northern Germany, Pacific Northwest) | 2.8 – 3.8 |
These are annual averages — winter will be lower and summer higher. For a home grid-tied with net metering, the annual figure is what matters, because summer surplus offsets winter shortfall over the year.
Don't forget roof space
A modern 400 W panel is roughly 1.8 m² (about 19 ft²). So a 12-panel, 4.6 kW system needs around 22 m² (≈240 ft²) of unshaded roof. If your usable roof is smaller than your ideal array, you have two options: use higher-wattage or higher-efficiency panels to fit more capacity into less space, or simply install what fits and offset a smaller share of your bill. Shading from chimneys, trees or neighbouring buildings reduces output more than people expect — even partial shade on one panel can drag down a whole string.
Should you size to 100% of your usage?
Not always — and this is where honest advice diverges from a sales pitch. Sizing to cover 100% of your annual usage makes sense when your exports are credited at or near the retail rate (traditional net metering). But where export is paid far less than retail — California's NEM 3.0, the UK's export tariffs, Pakistan's net billing — oversizing means dumping surplus to the grid for a fraction of its value. In those cases, sizing to your daytime self-consumption (or adding a battery) often beats a bigger array. The right size depends as much on your tariff as on your roof.
Quick rules of thumb
- Roughly 1 kW of panels per 1,200–1,600 kWh of annual use, in a moderately sunny climate.
- About 2.5 panels per kW at 400 W each.
- Around 1.8 m² (19 ft²) of roof per panel.
- Where export pays poorly, size to self-use, not total use.
Want your exact number? Enter your usage and location and let the calculator do the maths.
Open the system size calculator →Common mistakes
- Using daylight hours instead of peak sun-hours. This overstates output by 2–3×.
- Ignoring the performance ratio. A panel rarely delivers its lab rating in your home — plan on ~80%.
- Sizing to a single sunny month. Use the annual average, or you'll under-build.
- Forgetting future load. If an EV or heat pump is on the horizon, size for that — adding panels later is more expensive per watt than including them now.