Solar Panel Calculator
Determine the optimal solar PV system size for your rooftop. Estimate yearly energy yield, financial savings, and payback period by entering roof area, solar resources, and installation costs.
Measure contiguous south-facing roof segments; remove shading zones.
Modern monocrystalline panels range from 19–23% efficiency.
Obtain from solar resource maps (US average 4–6 hours).
Includes inverter, wiring, temperature, and soiling losses.
Residential averages range from $2.5–$3.5 per watt before incentives.
System size
24 kW
Panels required
60
Annual energy
37,230 kWh
Roof coverage
85%
Annual savings
$5,584.5
Payback period
11.17 years
How to Use This Calculator
Measure your roof
Estimate the area available for panels, excluding chimneys, vents, and shaded sections.
Enter sunlight and system details
Use solar resource data for your city and adjust efficiency, losses, and costs to match your project.
Review production and finances
Compare the calculated energy output with your household consumption and evaluate payback time.
Formula
System Size = Panels × 400 W ÷ 1000
Annual Energy = System Size × Sun Hours × 365 × (1 − Losses)
Payback = (System Size × 1000 × Cost/W) ÷ (Annual Energy × Price)
Example: 25 panels × 400 W = 10 kW. With 5 sun hours/day and 15% losses, annual production is roughly 15,500 kWh.
Adjust panel wattage or area constants if using modules of different sizes.
About the Solar Panel Calculator
Rooftop solar PV remains one of the most accessible renewable energy options for homeowners. This calculator combines spatial planning with performance modeling to provide instant insights into system sizing and financial outcomes.
When to Use This Calculator
- Early project planning: Size your system before contacting installers.
- Budgeting: Estimate total costs and payback for capital planning.
- Educational use: Teach solar performance concepts in classrooms.
- Policy analysis: Evaluate rooftop solar potential across neighborhoods.
Why Use Our Calculator?
- ✅ Practical defaults: Uses typical panel specs with room for customization.
- ✅ Financial modeling: Provides savings and payback estimates.
- ✅ Spatial awareness: Accounts for roof coverage and spacing.
- ✅ Quick scenarios: Test different sunlight and cost assumptions instantly.
Common Applications
Homeowners: Evaluate DIY vs. professional installations.
Schools: Plan solar arrays for campuses or sustainability projects.
Community programs: Demonstrate impact of group purchasing initiatives.
Tips for Best Results
- Get local irradiance data from sources like PVWatts or Solargis.
- Increase losses for flat roofs or partially shaded arrays.
- Consider incentives and tax credits when evaluating payback.
- Compare annual energy production to your utility bills to size appropriately.
Frequently Asked Questions
What if my panels have a different watt rating?
Adjust the constant (400 W) to match your module specification. Most modern panels range from 350 to 450 W.
How accurate are the loss assumptions?
Real-world losses depend on inverter efficiency, temperature, soiling, and wiring. Use a higher loss percentage if you expect significant shading or heat.
Do incentives affect payback?
Yes. Subtract tax credits or rebates from the installation cost before calculating payback to get a shorter timeline.
Can I offset 100% of my usage?
Check with your utility. Some net-metering policies limit system size to annual consumption. Use your past bills to ensure the system size aligns with rules.