A solar panel calculator that turns a few real numbers into a complete picture of a rooftop solar array: how many panels you need, the system size in kWp, the annual energy produced, and a realistic estimate of the money it saves each year. You can work in two directions. Start from your energy need — type the monthly kWh from a recent electricity bill and the tool tells you how many panels cover it. Or start from your roof area in m² or ft² and see how many panels physically fit and what fraction of your usage they would offset. It is built for homeowners comparing quotes, renters weighing a move to solar, and anyone who wants to sanity-check an installer’s sizing before signing.
How it works
The engine rests on one core relationship. A single panel’s daily output is its rating in kilowatts multiplied by your local peak sun hours and a performance ratio that accounts for real losses. Multiply by 365 for the annual figure:
Annual kWh per panel = (Wattage / 1000) × peak sun hours × performance ratio × 365
In energy mode the calculator takes your yearly demand (monthly kWh times twelve) and divides by that per-panel output, rounding up to whole panels. In roof mode it converts your roof area to square metres, applies a usable-fraction allowance for setbacks, vents and shading, then divides by the area of one panel to count how many fit. Either way it then sums the array: total panels give the system size in kWp (total watts divided by 1,000), the daily, monthly and annual output, and the economics. Savings split the generation into a self-used share — valued at your retail electricity price — and an exported share valued at your export tariff. Dividing the install cost (per-watt times system watts) by that annual benefit yields a simple payback in years. Everything recomputes instantly as you type and auto-saves in your browser; no figures are ever uploaded.
Worked example
A home using 350 kWh per month needs 4,200 kWh a year. With 430 W panels at 3.8 peak sun hours and an 80% performance ratio, each panel makes about 477 kWh a year. Dividing 4,200 by 477 and rounding up gives 9 panels — a 3.87 kWp system that produces roughly 4,300 kWh annually.
If you self-consume 50% at an electricity price of 0.28 per kWh and export the rest at 0.07 per kWh, the array saves about 602 on bills and earns about 150 in export income — an annual benefit near 750. At an install cost of 1.90 per watt the system costs around 7,350, giving a simple payback of roughly 9.8 years and a 25-year benefit well above 18,000 before any price rises.
| Monthly use | Sun hours | Panel | Panels | System | Annual output |
|---|---|---|---|---|---|
| 250 kWh | 3.5 | 400 W | 8 | 3.20 kWp | 3,270 kWh |
| 350 kWh | 3.8 | 430 W | 9 | 3.87 kWp | 4,295 kWh |
| 500 kWh | 4.5 | 450 W | 11 | 4.95 kWp | 6,504 kWh |
| 700 kWh | 5.0 | 450 W | 13 | 5.85 kWp | 8,541 kWh |
Every figure is calculated in your browser. Treat the savings as planning estimates, not a quote — orientation, shading, weather and tariffs all move the real numbers.