Solar Panel Size Calculator
How big a solar system do you actually need? Enter your monthly usage and local sun hours and get the recommended system size, number of panels, and roof area — instantly.
From your bill — or monthly bill ÷ price per kWh.
Most modern residential panels are 390–450 W.
Recommended system size
0 kW
to produce 0 kWh/year
Estimates only. Roof orientation, tilt, and shading change the answer — a professional site survey gives the final design.
The next step: confirm it with installers
Now you know roughly what you need. Local installers will confirm the size against your actual roof orientation, tilt, and shading, then quote real prices — and comparing three quotes is the single best way to avoid overpaying. Once you have a proposed system size, price it out in the solar savings & payback calculator to sanity-check the return.
How this solar panel size calculator works
It works backwards from your usage: system kW = annual kWh ÷ (peak sun hours × 365 × 0.78). The 0.78 performance ratio bakes in real-world losses — inverter efficiency, heat, wiring, and dust — so the recommendation reflects what systems actually deliver, not lab numbers.
Panel count is simply the system size in watts divided by your chosen panel wattage, rounded up, and roof area assumes about 2 m² per modern panel including mounting gaps. Once you have a size in mind, run it through the solar savings & payback calculator to see what it's worth.
Solar panel size FAQ
How many solar panels do I need to run a house?
A typical home using 850 kWh a month in a temperate region (4 peak sun hours) needs roughly a 9 kW system — about 22–23 panels at 400 W each. Homes in sunnier regions or with lower usage need fewer; the calculator adjusts for both.
How is solar system size calculated?
System kW = annual kWh ÷ (peak sun hours × 365 × 0.78). The 0.78 performance ratio accounts for real-world losses from inverters, temperature, wiring, and dust. Panel count is system watts divided by panel wattage, rounded up.
How much roof area do solar panels need?
A modern residential panel occupies roughly 2 m² (about 21.5 sq ft) including mounting gaps. A 22-panel system needs around 44 m² (roughly 475 sq ft) of usable, unshaded roof.
What wattage solar panel should I choose?
Most residential panels today are 390–450 W; 400 W is a sensible default. Higher-wattage panels reduce the count and roof area needed but cost more each — total system kW, not panel wattage, determines production.
Should I oversize my solar system?
Slight oversizing (10–20%) is common if you expect usage to grow — an EV, heat pump, or pool. But export credits are usually worth less than retail rates, so very large systems have diminishing returns. Check your utility's net-metering rules first.