Solar System Size Calculator
Find the kW solar system your home needs, plus the panel count and yearly output.
Size your solar system
Your system size estimate
System sizes for different monthly usage
System size, panel count and yearly production at 4.5 peak sun hours, using your system losses figure. Your monthly usage is highlighted.
How to size a home solar system
This free solar system size calculator finds the kW system your home needs from your monthly electricity use and local sun hours. It is for homeowners planning rooftop solar, and it runs entirely in your browser with no sign-up and no data uploaded. Enter your monthly usage, peak sun hours and system losses, then read the system size, panel count and yearly output.
How big a solar system do I need for my home?
Divide your daily electricity use by your peak sun hours, then adjust for system losses to find the size in kW. A home using 30 kWh a day with 4.5 peak sun hours and 14 percent losses needs about 7.75 kW of panels, which is roughly 20 standard 400 watt panels on a typical roof.
The formula is: system size in kW = (monthly usage / 30) / (peak sun hours x (1 – losses / 100)). Daily use is monthly usage divided by 30, and the losses factor accounts for the energy lost between the panels and your home.
What are peak sun hours?
Peak sun hours measure how much usable sunlight a location receives, counted as hours at full 1,000 watts per square metre intensity. Most of the United States averages 4 to 5 peak sun hours a day, with sunny southwestern states above 5 and cloudier northern states near 3.5, so your location changes the system size directly.
Peak sun hours are not the same as daylight hours. A place can have 12 hours of daylight but only 4 peak sun hours, because the sun is low and weak in the morning and evening and strong only around midday.
How many solar panels do I need?
Divide the system size in watts by the wattage of one panel, then round up to a whole panel. A 7.75 kW system needs 20 panels at 400 watts each, while the same system would need 22 panels at 350 watts. Roof space, shading and panel efficiency can change the count, so treat this as a planning estimate.
Most modern home panels are 400 to 450 watts. Higher wattage panels mean fewer panels for the same system size, which helps on small roofs, but they cost more per panel than standard 400 watt models.
How much electricity will my solar system produce each year?
A well placed residential system in the United States produces about 1,200 to 1,500 kWh per kW each year, according to 2024 data published by the National Renewable Energy Laboratory. A 7.75 kW system therefore produces roughly 10,000 to 11,000 kWh a year, which covers the typical home using about 10,700 kWh a year.
Yearly production in this calculator is system size x peak sun hours x 365 x (1 – losses / 100). Production is highest in summer and lowest in winter, so monthly output swings widely even when the yearly total matches your use.
What system losses should I expect?
Most home systems lose 10 to 15 percent of their panel output to inverter losses, heat, wiring, shading and dust, so this calculator uses 14 percent by default. Losses rise on hot roofs and shaded sites and fall with efficient inverters and clean panels, but 14 percent is a realistic planning figure for a typical installation.
If your roof is partly shaded, raise the losses figure to 20 percent or more when planning. A shaded panel can drag down a whole string of panels, so ask your installer about optimisers or microinverters for shaded roofs.
Frequently asked questions
How do I find my monthly electricity use?
Check your electricity bill, which shows kWh used each month. Add up the last 12 months and divide by 12 for a fair average, because summer air conditioning and winter heating can double use in some months.
What size solar system do I need for a 2,000 square foot home?
Home size alone does not decide it. What matters is how much electricity you use, which depends on heating, cooling, appliances and how many people live there. A typical 2,000 square foot home uses 800 to 1,000 kWh a month and needs roughly a 7 to 8 kW system.
How many panels is 8 kW?
An 8 kW system needs 20 panels at 400 watts each. With 450 watt panels it needs 18, and with 350 watt panels it needs 23. This calculator rounds the panel count up, because you cannot install part of a panel.
Do I need a battery with my solar system?
No, a battery is optional. Without one, excess daytime solar power goes back to the grid, and you buy power at night. A battery lets you use your own solar power in the evening and keeps essentials running in an outage. Size one with our Solar Battery Size Calculator.
How accurate is this solar system size estimate?
It is a planning estimate, usually within about 10 percent for a clear, south facing roof. Shading, roof angle, panel brand and local weather can shift the real output, so treat the result as a starting point and confirm it with an installer site survey.
What happens on cloudy days?
Solar panels still produce power on cloudy days, usually 10 to 25 percent of their sunny day output. The yearly production figure already averages sunny and cloudy days together, which is why peak sun hours are an annual average rather than a summer figure.
Does this calculator store my data?
No. The calculator runs entirely in your browser. Nothing you type is uploaded, stored, or shared.
More solar tools
Once you know the size, work out the cost with the Solar Panel Cost Calculator, check how many panels fit your roof with the How Many Solar Panels Do I Need Calculator, see when it pays for itself with the Solar Payback Period Calculator, and size storage with the Solar Battery Size Calculator.
