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Solar Panel Sizing Guide: From Your Bill to Panel Count

Solar sizing is one honest calculation away from your electricity bill. This guide walks through the four numbers that determine how many panels your home needs.

Step 1: Know your consumption

Everything starts with your annual kWh. Find it on your bill or add up 12 monthly statements — never size from a single month. Electric heating, EVs and air conditioning create huge seasonal swings.

If you are planning an EV or heat pump in the next few years, add its expected consumption now: an EV driven 15,000 km a year adds roughly 2,500–3,000 kWh.

Step 2: Find your sun hours

Solar panels are rated in lab sunlight (1,000 W/m²). Your location delivers the equivalent of 2.5 to 7 “peak sun hours” of that per day, averaged over the year. Northern Europe: 2.5–3.5. Central Europe and the northern US: 3.5–4.5. Mediterranean and California: 4.5–5.5. Southwest US and Australia: 5.5–7.

This single number changes the answer by a factor of two — look it up for your exact location (PVGIS, NREL) rather than guessing.

Step 3: Apply real-world losses

Lab ratings never survive contact with a real roof. Inverters lose 3–5%, hot panels lose 10–15% in summer, dust and wiring take a few more. The industry-standard performance ratio of 0.75 bundles all of this.

The Solar Panel Calculator chains all three steps: monthly kWh ÷ 30 ÷ (sun hours × 0.75) = system size in kW, then divides by panel wattage for the count.

Step 4: Check your roof — and the payback

Each modern 400 W panel needs about 2 m² of unshaded roof. A 10 kW system — 25 panels — needs 50 m² facing roughly south (northern hemisphere). Shading on even one panel can cut a string’s output by a third; complex roofs may need microinverters.

For payback, combine production with your tariff using the Electricity Cost Calculator: a 10 kW system producing 11,000 kWh a year at $0.15/kWh offsets about $1,650 annually. And if outages are your concern, size a backup with the Generator Size Calculator — solar without batteries does not run during a blackout.