← All guides · 2026-06-10

How Many Solar Panels Do I Need? A Simple 3-Step Method

Start with your electricity bill

The only number that matters at the start is your annual consumption in kilowatt-hours (kWh). Find it on your utility bill or multiply a typical monthly figure by twelve. A European household averages 3,500–5,000 kWh per year; in the US it is closer to 10,500 kWh; an air-conditioned home in a hot climate can exceed 15,000 kWh.

Divide by your location's specific yield

Specific yield is how many kWh one kilowatt-peak (kWp) of panels produces per year at your location. It ranges from roughly 800 kWh/kWp in northern Europe to 1,900 kWh/kWp in desert climates. You can read this number for free on our analyzer — click your roof on the map and check the 'kWh/kWp/year' figure. Divide annual consumption by specific yield to get the system size: a 4,500 kWh household in Madrid (about 1,600 kWh/kWp) needs 4,500 ÷ 1,600 ≈ 2.8 kWp.

Convert kWp to panel count

Modern residential panels deliver 420–460 W each. Our 2.8 kWp example therefore needs 2,800 ÷ 440 ≈ 6–7 panels, occupying roughly 14 m² of roof. Add 10–20% headroom if you plan to buy an EV or heat pump: oversizing slightly at install time is far cheaper than extending later.

Common mistakes

Three errors dominate: sizing from peak power instead of annual energy, ignoring shading (a single chimney shadow can cut a string's output by 20%+), and forgetting that self-consumed kWh are usually worth 2–3× more than exported ones. If your goal is bill reduction, match the system to your daytime consumption profile, not your total annual number.

Check your own roof in 10 seconds

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