Solar Panel Sizing Calculator
Work out how many solar panels you need, the array size in kW, roof area, yearly output, savings and payback.
Fill in your electricity use and the numbers update as you type.
- Array size needed
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- Panels needed
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- Installed array size
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- Estimated output / year
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- Average per month
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- Covers of your use
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- Roof area needed
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- Bill savings / year
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- Installed system cost
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- Simple payback
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- CO₂ avoided / year
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- Output per installed kW
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These are planning estimates, not a quote. Real output depends on roof pitch and orientation, shading, snow, dust, inverter clipping and the weather that year. Get a site survey and a written quote before you buy, and check what your utility pays for exported electricity.
How it works
This calculator uses the standard PVWatts-style derate method. Yearly output is
kWdc × peak sun hours per day × 365 × (1 − losses), so the array you need
is yearly kWh wanted ÷ (peak sun hours × 365 × (1 − losses)). "Peak sun hours" is the
number of hours per day of full 1,000 W/m² sunshine that would deliver your location's daily solar
energy — a site with 5 peak sun hours receives 5 kWh/m² per day on the array. The 14% default loss
figure is the PVWatts standard bundle of soiling, shading, wiring, connections, mismatch, inverter
efficiency, light-induced degradation and availability.
Panel count is that array size divided by your panel rating, rounded up to a whole panel, which is why the installed array is usually a little bigger than the minimum. Roof area is the raw glass area of the panels (length × width × count) — add roughly 20–30% in practice for row spacing, setbacks and walkways. Savings assume you are credited at your retail rate for the kWh you actually use, and payback is simple payback with no financing, incentives, degradation or rate inflation. CO₂ avoided assumes a grid intensity of 0.4 kg CO₂ per kWh. Location presets are typical annual average peak sun hours for a well-oriented array; use a local solar resource map for a precise figure. Everything is computed in your browser — nothing you type is uploaded, and there are no sign-ups or ads.
Frequently asked questions
How many solar panels do I need?
Divide the electricity you want to cover each year by your site's yearly yield per kilowatt, then divide by your panel rating. Yearly yield is peak sun hours × 365 × (1 − system losses), so at 5 peak sun hours and 14% losses each kW produces about 1,570 kWh. A home using 10,800 kWh a year needs roughly 6.9 kW, which is 18 panels of 400 W once you round up to whole panels.
What are peak sun hours and where do I find mine?
Peak sun hours are the number of hours per day of full 1,000 W/m² sunshine that would deliver the same energy as your location's real daily sunlight — a site with 5 peak sun hours receives 5 kWh/m² per day on a well-oriented array. Typical annual averages range from about 2.6 in the UK to 6.0 in the US Southwest. Pick the closest location preset or type a figure from a local solar resource map.
Why does the calculator subtract 14% for losses?
14% is the standard PVWatts default loss bundle: soiling, light shading, wiring and connection resistance, module mismatch, inverter efficiency, light-induced degradation and system availability. Increase it for a shaded, dusty or badly oriented roof and lower it for a clean, unshaded, optimally tilted array. The results are planning estimates, not a quote.