Solar Panel System Sizing Calculator
Energy Usage
Location & Roof Details
South: 5-6, Midwest: 4-5, North: 3-4
Account for vents, chimneys, shading
System Specifications
Accounts for inverter, wiring, shading losses
Solar Sizing Guide
- Calculate daily energy usage from bill
- Divide by peak sun hours to get system size (kW)
- Account for system efficiency losses (15-20%)
- Determine number of panels based on wattage
- Southwest (AZ, NM, NV): 6-7 hours
- Southeast (FL, GA, SC): 5-6 hours
- Midwest (IL, IN, OH): 4-5 hours
- Northeast (NY, PA, MA): 3.5-4.5 hours
- Equipment (panels, inverters): 40-50%
- Installation labor: 15-20%
- Permits & inspections: 5-10%
- Overhead & profit: 25-30%
Tip: Federal tax credit covers 30% through 2032. Additional state/local incentives may be available.
About This Calculator
Solar Panel System Sizing Calculator is designed to reduce manual errors and give repeatable outputs when you need quick, reliable answers.
Inside solar & energy, this tool gives you a practical way to model scenarios, compare outcomes, and make better next-step decisions without spreadsheet overhead.
If your workflow expands, pair this calculator with Solar Panel ROI Calculator and Solar vs Grid 25-Year Cost Calculator to cross-check assumptions and build a stronger analysis chain.
Formula
Daily kWh needed = Monthly usage / 30 | Array kW needed = (Daily kWh / Sun hours) / Derate factor | Number of panels = (Array kW × 1000) / Panel wattage | Actual system size = Number of panels × Panel wattage / 1000
Example Calculation
The worked example below demonstrates how the input fields translate into the final output. Use it as a quick validation pass before entering your own numbers.
- Average monthly kWh usage: 6
- Average daily sun hours: 8
- System derate factor (0.75-0.80): 12
- Panel wattage: 3.5
Explanation of Results
Result Interpretation
The solar panel system sizing calculator returned calculated value based on Average monthly k Wh usage 6, Average daily sun hours 8, System derate factor (0.75 0.80) 12, and Panel wattage 3.5. Use this result as a baseline, then adjust one input at a time to understand how sensitive your outcome is before making decisions.
FAQ
How should I validate the solar panel system sizing calculator result?
Run a second scenario with rounded numbers, then compare the direction and magnitude of the change before using the value operationally.
What formula is this based on?
This page uses the following formula logic: Daily kWh needed = Monthly usage / 30 | Array kW needed = (Daily kWh / Sun hours) / Derate factor | Number of panels = (Array kW × 1000) / Panel wattage | Actual system size = Number of panels × Panel wattage / 1000
Can I bookmark this solar & energy tool?
Yes. Use the canonical URL /construction-architecture/solar-energy/solar-panel-system-sizing-calculator to return to this calculator in the Construction & Architecture library.
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