How to use this solar panel calculator
- Electricity use, enter your average monthly use in kWh. Don't know it? Tick the toggle to enter your monthly bill and electricity rate instead, and the tool will work out the kWh for you.
- Peak sun hours, enter how many you get on an average day. Most homes sit between 4 and 6, so 5 is a safe starting point if you are unsure.
- Panel wattage, enter the rating of the panels you are considering. Modern residential panels are commonly around 400 watts.
- Cost per watt is optional. Leave the default in place for a rough installed price, or change it to match a quote you have.
- Read your system size, panel count and production figures. They update instantly as you type.
Nothing is submitted or stored: your numbers never leave your device, so you can model different scenarios privately.
How solar system size is calculated
The maths behind a solar estimate is simpler than it looks. First the calculator converts your monthly usage into a daily figure by dividing by 30.4, the average number of days in a month. It then needs to know how much useful sunlight your roof gets, which is measured in peak sun hours. One peak sun hour is an hour of full, bright sun at 1,000 watts per square metre, and most regions average somewhere between 4 and 6 of them per day across the year.
Real panels never deliver their full rated output, so the tool applies a system losses factor of about 20 percent(multiplying by 0.8) to account for inverter losses, wiring, heat, dust and panel tolerance. Dividing your daily usage by your peak sun hours times that 0.8 factor gives the system size in kilowatts. To turn kilowatts into a panel count, the calculator multiplies the system size by 1,000 to get watts, then divides by your panel wattage and rounds up so you have enough capacity. Production is the reverse of the same sum: system size times peak sun hours times 0.8.
Estimate, not a quote: These figures are a planning guide only. Real output and pricing depend on your roof angle, shading, orientation, local weather, equipment and installer. Always get a site-specific quote before buying.
Frequently asked questions
How many solar panels do I need?
It depends on your electricity use, peak sun hours and panel wattage. As a rough guide, a home using about 900 kWh a month at 5 peak sun hours needs roughly a 7.4 kW system, which is about 19 panels rated at 400 watts each.
What is a peak sun hour?
One peak sun hour is an hour of sunlight at 1,000 watts per square metre. Most locations average 4 to 6 peak sun hours a day across the year, even though the sun is up far longer than that.
Why does the calculator add a losses factor?
Real systems never deliver their full rated output. Inverter losses, wiring, heat, dust and panel tolerance typically cut production by around 20 percent, so the tool multiplies by 0.8 for a more realistic result.
How is system size in kW calculated?
The tool divides your monthly kWh by 30.4 for a daily figure, then divides that by your peak sun hours times the 0.8 losses factor to find the kilowatts of panels needed.
Is the cost estimate accurate?
No. It simply multiplies system size by your cost per watt. It is a ballpark for planning only and not a quote, real pricing varies by installer, equipment, incentives and your roof.