Solar Panel Calculator
Use our free solar panel calculator to quickly estimate your system size (kW), based on key inputs like monthly electricity use (kWh), peak sun hours, and panel wattage. Perfect for homeowners and small businesses.
Solar Panel Calculator Formula
Knowing how to calculate your system size is as simple as using this formula. Your output shows the array capacity needed to offset your usage under local sun conditions. Not a fan of doing the math? Skip the manual work and plug your numbers into our free Solar Panel Calculator—you'll get instant results and see exactly how large a system you need.
How this solar panel calculator works
This calculator estimates your required system size by converting monthly electricity use into daily kWh, then dividing by available sunlight (peak sun hours) and an efficiency factor called the performance ratio (accounts for temperature, wiring, inverter losses, dust, and shading). It's designed to provide a fast, easy way to gauge what size array you'll need before getting quotes.
When to use this solar panel calculator
When budgeting for a new residential or small-business solar project
To compare roof space and panel wattage options (e.g., 400 W vs 450 W)
While planning battery or EV-charger readiness alongside solar
For quick pre-quote sizing before contacting installers
To model savings under different sun-hour assumptions or mild shading
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Average Solar System Sizes by Property Type
Understand how typical residential needs translate into array sizes. Use these ranges as a starting point; your site-specific sun hours and efficiency will change the result.
Apartment/Small home
Small–medium home
Medium home
Large home/high usage
These benchmarks help homeowners and small businesses evaluate if their preliminary sizing aligns with common ranges before getting a professional site assessment.
Frequently Asked Questions
It provides a solid preliminary estimate using your usage, sun hours, and panel wattage. Final designs may change after a site audit, shading analysis, and local code checks.
It represents real-world system losses (temperature, inverter, wiring, soiling). A typical range is 0.75–0.85. If unsure, use 0.80.
Peak sun hours are the average daily equivalent of full sunlight in your location. If you don't know it, start with 4–5 hours and refine later using local solar maps.
Yes. Multiply your monthly kWh by your desired offset (e.g., 0.7 for 70%) before using the formula.
Shading and orientation reduce output. Lower the performance ratio or add a shading % reduction to compensate in your estimate.
Higher-watt panels reduce the number of panels for the same kW size, which can help when roof space is limited, but total kW drives energy production.
No. This calculator sizes the PV array only. Batteries are sized based on backup duration and peak loads and can be added later.
Usually, yes—savings depend on your tariff, export credits, and self-consumption. Use this to size the array, then check payback with your local rate structure.