Solar Panel Calculator
Estimate what a solar system produces — from its size, your sun hours and its efficiency — in kWh per day, month and year.
Solar output is system size times sun hours times efficiency — three numbers, all yours to enter. This calculator is that multiplication, per day, month and year.
Enter the system and the sun; the output follows.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Enter the system size in kW.
- Enter your peak sun hours per day.
- Enter the system efficiency and the panel wattage.
- Read the daily, monthly and yearly output, and the panels needed.
Worked example
| System | 5 kW |
|---|---|
| Sun hours | 4.5 per day |
| Efficiency | 80% |
| Daily | 5 × 4.5 × 0.80 = 18 kWh |
| Monthly | 18 × 30 = 540 kWh |
| Yearly | 18 × 365 = 6,570 kWh |
| Panels (400 W each) | ceil(5,000 ÷ 400) = 13 panels |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What are peak sun hours?
The hours of full sun a location gets on an average day — not daylight hours. A place with 4.5 peak sun hours gets the energy of 4.5 hours at full panel power. Find yours from a solar map or an installer.
Why is efficiency 80%?
Because panels lose energy to the inverter, heat, dirt and wiring — about 20% in a typical system. It is a planning figure; enter your own if your installer gives you one.
How many panels do I need for 5 kW?
It depends on the panel wattage: 5,000 W ÷ 400 W panels = 12.5, so 13 panels. This calculator rounds up — you cannot install half a panel.
Is this a guarantee of output?
No. It is an estimate from your own sun hours and efficiency. Real output varies with weather, shading, roof angle and the season. Get a professional survey for a real system.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.