Solar Panel Sizing Calculator
Estimate the size of solar array you need from your daily energy consumption, local peak sun hours and a real-world efficiency allowance. Get the system size in kW and a panel count.
How to use this calculator
- Enter your daily energy use — take a monthly bill's kWh and divide by 30.
- Enter your local peak sun hours (a solar map or installer can tell you).
- Keep efficiency around 80% for real-world losses; set your panel wattage.
Formula used
Peak sun hours is the equivalent hours of full 1,000 W/m² sunlight per day — not daylight hours. Efficiency (~75–85%) accounts for inverter losses, wiring, temperature, dust and panel degradation. Panels needed = system watts ÷ panel wattage, rounded up.
Example calculation
15 kWh/day, 4.5 sun hours, 80% efficiency: 15 ÷ (4.5 × 0.8) = 4.17 kW.
With 400 W panels that's 11 panels (4.4 kW installed), needing ~22 m² of roof and generating around 5,780 kWh a year.
Sizing solar realistically
Solar sizing starts from your energy need and works backwards through two big multipliers. Peak sun hours — the daily equivalent of full-strength sunlight — varies hugely by location and season, from around 3 in cloudy northern climates to 6+ in sunny regions, and it's the single biggest factor in how many panels you need. The efficiency factor then trims the theoretical output for the unavoidable real-world losses: inverters, wiring resistance, heat (panels lose output when hot), dust and gradual degradation.
The result is a planning estimate, not an installer's quote. Roof orientation and tilt, shading from trees or chimneys, and seasonal variation all shift real output, and covering 100% of annual use on paper still means importing power on dark winter days and exporting surplus in summer — which is where battery storage and grid export tariffs enter the economics.
Why use this calculator?
- Translate your actual energy use into a realistic array size.
- Includes efficiency losses that back-of-envelope estimates ignore.
- Panel count and roof area to check feasibility.
Frequently asked questions
How many solar panels do I need?
Divide your daily kWh use by (peak sun hours × efficiency) to get system kW, then divide by panel wattage. A 15 kWh/day home in decent sun typically needs ~10–14 panels (4–5 kW). Enter your figures for an estimate.
What are peak sun hours?
The number of hours per day equivalent to full-strength (1,000 W/m²) sunlight — not total daylight hours. It ranges from about 3 in cloudy northern climates to 6+ in sunny regions, and drives system size more than anything else.
Why apply an efficiency factor?
Real systems lose 15–25% to inverter conversion, wiring resistance, heat (panels underperform when hot), dust and gradual degradation. Sizing at 100% of the theoretical figure would leave you short, so ~80% efficiency is a realistic planning assumption.