Size your solar system. This free solar panel calculator turns your electricity use and peak sun hours into system kW, the number of panels, daily generation, roof area and an optional cost.
Your needs
kWh/yr
h
%
W
$/ W
Enter the electricity you want to cover and your local peak sun hours (roughly 3–4 in cloudier regions, 5–6 in sunny ones). Performance ratio (~75–85%) accounts for real-world losses.
System size needed
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Enter your usage
Panels
—
Daily generation
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Roof area (approx)
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Estimated cost
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System size (kW) = annual kWh ÷ (peak sun hours × 365 × performance ratio). Panel count = system watts ÷ panel wattage, rounded up. Roof area assumes about 1.9 m² per panel. These are planning estimates — a local installer will refine for roof, shading and orientation.
Use this free solar panel calculator to size a solar system. Enter the electricity you want to cover and your sun hours to get system size, panel count, generation and roof area.
What this solar panel calculator does
A solar panel calculator works out the system size needed to cover your electricity, then converts it into a number of panels, daily generation, roof area and an optional install cost.
How the system size is calculated
system kW = annual kWh ÷ (peak sun hours × 365 × performance ratio)
panels = system watts ÷ panel wattage
daily generation = system kW × sun hours × performance ratio
To cover 10,000 kWh a year at 4.5 sun hours and 80% performance you’d need about a 7.6 kW system — roughly 20 panels of 400 W.
What the inputs mean
Input
Typical value
Peak sun hours
3–4 (cloudier) to 5–6 (sunny)
Performance ratio
75–85%
Panel wattage
350–450 W
Area per panel
~1.9 m²
What affects real output
Roof orientation, tilt, shading and temperature all change real generation, which is what the performance ratio captures. South-facing (in the northern hemisphere), unshaded roofs perform best.
How to use the solar panel calculator
Enter the electricity to cover, per year or month.
Set sun hours, performance ratio and panel wattage.
Read system size, panels and roof area.
Solar Panel FAQ
How many solar panels do I need?
Divide your system size in watts by the panel wattage. A 7.6 kW system at 400 W panels is about 20 panels.
How is solar system size calculated?
System kW = annual kWh ÷ (peak sun hours × 365 × performance ratio). For 10,000 kWh at 4.5 sun hours and 80%, that's about 7.6 kW.
What are peak sun hours?
The equivalent hours per day of full-strength sunshine — roughly 3–4 in cloudier regions and 5–6 in sunny ones.
What is performance ratio?
It accounts for real-world losses from heat, wiring, shading and inverter efficiency — typically 75–85% of the panels' rated output.
How much roof space do I need?
About 1.9 m² per panel, so a 20-panel system needs roughly 38 m² of suitable, unshaded roof.
Is the solar panel calculator free?
Yes, this solar panel calculator is completely free, needs no sign-up, and gives instant results in your browser.
