Estimate your solar PV system size, annual and daily electricity generation, roof area needed and potential bill savings from your panel count and wattage.
Optional: estimate bill savings
Good to know: These figures are planning estimates. Real output depends on your exact location, roof angle and direction, shading and the specific equipment. Treat the numbers as a realistic ball-park, not a guarantee.
Your solar array (also called a PV system, short for photovoltaic) is sized in kilowatts-peak (kWp). That is simply the number of panels multiplied by each panel's wattage, divided by 1,000. A 10-panel system of 400 W panels is 4 kWp. The calculator then estimates how much electricity that system will make in a year.
Annual generation is worked out as:
Peak sun hours is the average number of hours per day that sunlight hits full strength (1,000 W/m²) at your location — think of it as "full-power sunshine hours". The performance ratio accounts for real-world losses (inverter, cabling, heat, dust/soiling), typically around 0.75–0.85.
| Region | Peak sun hours/day |
|---|---|
| UK, Ireland & Northern Europe | ~2.8 |
| Central Europe | ~3.4 |
| Southern Europe / Mediterranean | ~4.6 |
| Northern US / Canada | ~3.8–4.0 |
| Southern US / Sun Belt | ~5.5 |
| Australia / India | ~5.0–5.2 |
Ten 400 W panels in the UK with a typical performance ratio:
A modern residential panel is around 1.7–1.8 m² (about 18–19 ft²). Allowing for mounting rails and small gaps, this tool budgets about 1.9 m² (roughly 20 ft²) per panel, so 10 panels need about 19 m² (around 205 ft²) of usable roof. Always leave clearance from roof edges and obstructions.
Enter your electricity unit price (in pence or cents per kWh) to see a rough annual saving. Bear in mind you only save the full rate on the electricity you use as it is generated; power you export is usually paid at a lower rate. Adding a battery lets you use more of what you make.
The roof-area result switches between square metres and square feet using the toggle at the top. All other figures (kWp, kWh) are the same worldwide.
It depends on your electricity use and your panel wattage. As a rough guide, a 4 kWp system (about ten 400 W panels) suits a typical family home and generates roughly 3,000–4,200 kWh a year depending on location. Enter your panel count and wattage above for a tailored estimate.
A single 400 W panel produces roughly 300–450 kWh a year in a sunny climate and around 300 kWh in cloudier regions such as the UK and Northern Europe. Output scales with panel wattage, peak sun hours and system losses, all of which this calculator accounts for.
kWp (kilowatts-peak) is the maximum rated power of your system under ideal test conditions — it describes system size. kWh (kilowatt-hours) is the actual energy generated or used over time. This tool converts your panel details into kWp, then estimates yearly kWh.
Allow around 1.9 m² (about 20 ft²) per panel including mounting gaps. So a 10-panel system needs roughly 19 m² (around 205 ft²) of clear, unshaded roof. Leave a margin from the edges and any vents or chimneys.
It gives a realistic planning estimate using your system size, regional peak sun hours and a performance ratio for real-world losses. Actual output varies with roof angle and direction, shading, temperature and equipment quality, so treat the result as a ball-park rather than a guarantee.
