Solar Power Calculator
Use our free solar power calculator to quickly estimate your system size (kW) and how many panels you'll need, based on key inputs like monthly electricity use (kWh), peak sun hours, and panel wattage. Perfect for homeowners and small businesses.
Calculator Interface
Knowing how to calculate your required system size is as simple as using this formula. Your output shows the array capacity needed to offset your usage under local sunlight conditions. Not a fan of doing the math? Skip the manual work and plug your numbers into our free Solar Power Calculator—you'll get instant results and see exactly how large a system you need.
How this solar power calculator works
This calculator converts your monthly electricity use into daily kWh, then divides that by available sunlight (peak sun hours) and a performance ratio that captures real-world losses (temperature, inverter and wiring losses, soiling, shading). It's designed to provide a fast, easy way to gauge the PV capacity required before getting professional quotes or a site assessment.
When to use this solar power calculator
When budgeting for a new residential or small-business solar installation
To compare panel wattages and roof-space needs (e.g., 350 W vs 450 W)
While planning for battery backup or EV charging readiness alongside PV
For quick pre-quote sizing before contacting installers
To model different performance ratios or partial-offset scenarios
Want to make your solar planning smarter?
Use ServiceAgent.ai to automate estimates, manage proposals, and track profitability—all from one dashboard.
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Average Solar System Sizes by Property Type
Understand how typical energy needs translate into array sizes. Use these ranges as a quick starting point; your site-specific sun hours and efficiency will change the final result.
Apartment / Small home
Small–medium home
Medium home
Large home / high usage
These benchmarks help homeowners and small businesses evaluate whether their preliminary sizing aligns with common ranges before scheduling a professional site assessment.
Frequently Asked Questions
This calculator gives a solid preliminary estimate using your inputs and standard assumptions. Final system designs may change after a site audit, shading analysis, and local code checks.
The performance ratio accounts for real-world losses (inverter, wiring, temperature, soiling). Typical values are 0.75–0.85; use 0.80 if unsure.
Peak sun hours are the average equivalent hours of full sun per day for your location. If unknown, start with 4–5 hours and refine using local solar maps or installer data.
Yes. Multiply your monthly kWh by the desired offset percentage (e.g., 0.70 for 70%) and use that adjusted value in the formula.
Shading and orientation reduce output. Lower your performance ratio or apply a shading reduction factor when estimating to reflect expected losses.
Panel wattage changes the number of panels required for a given kW size, but total kW (array capacity) drives energy output. Higher-watt panels reduce panel count for the same kW.
No. This tool sizes only the PV array. Battery systems are sized separately based on required backup duration and peak loads.
No — this calculator sizes the system. For payback or ROI, combine this sizing output with local electricity rates, incentives, and installation costs (ServiceAgent.ai can help automate that stage).