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☀️ Solar Panel Calculator PRO

Estimate system size, energy output, savings, and payback period for solar panel installations.

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Dimension Diagram — updates as you type

Roof face with panel layout. Usable area after pitch and shading decides the panel count.

panels = usable area ÷ panel area

1 Roof Dimensions

Angle affects usable area via cos(angle). 0° = flat, 45° = steep.

2 Panel Specifications

3 Environment & Cost

Average peak sun hours. 4-5 hrs typical, 6-7 in desert climates.

Results

Roof Analysis

Gross Roof Area
Angle Adjustment
Usable Roof Area

System Specifications

Panel Area
Shading Factor85%
Panel Count
System Size

Energy Output

Daily Output
Monthly Output
Annual Output

Savings & ROI

Annual Savings
Payback Period
25-Year Savings

Cost Breakdown

Panel Cost
Installation Cost
Total Cost
Cost per Watt

⚠️ Pre-Export Checklist

  • Roof dimensions verified on-site
  • Roof structural capacity confirmed for panel weight
  • Shading from trees/neighboring buildings assessed
  • Local solar permits and HOA requirements checked
  • Utility net metering policy confirmed
  • Electrical panel capacity verified
  • Panel and inverter warranty terms reviewed

Professional Solar Estimation Guide

📐 Formulas & Calculations ▼

Usable Roof Area

UsableArea = Length × Width × cos(angle) × 0.85 (shading factor)

The shading factor accounts for obstructions, setbacks, and orientation losses. Adjust your roof angle to see the impact on usable area.

Panel Count

Panels = UsableArea ÷ PanelArea

Each panel is roughly 17-18 sq ft. A typical 5kW system needs 12-15 panels.

Annual Output

Annual = SystemSize(kW) × SunHours × 365

Output varies by location. 4 sun hours/day is average US, 6+ in Southwest.

Payback Period

Payback = TotalCost ÷ AnnualSavings

Most residential systems pay back in 6-12 years with 25-30 year panel life.

💡 Pro Tips ▼
  • South-facing roofs produce 15-25% more energy than east/west in the Northern Hemisphere.
  • Micro-inverters are better for partially shaded roofs; string inverters for unshaded arrays.
  • Battery storage adds 30-50% to system cost but provides backup power during outages.
  • Federal tax credits can offset 30% of total cost — check current incentives in your area.
  • Net metering policies vary by utility — confirm buyback rates before calculating savings.
☀️

Solar Installation — 3D Preview

Drag to rotate — Scroll to zoom

Roof: × @ °
Panels: × W
System: kW
Legend
Solar Panels
Roof
Mounting Rails
Inverter

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Client & Quote (prints on PDF & Excel)

Adjustments (applies to exports)

Contingency is added as a separate line on the exported cost summary. Waste % is printed as an advisory reminder.

Project notes (prints on PDF & Excel)

Result compare (A/B)

Item Pinned Current Change

Saved rates auto-apply from your Price Library. Advanced details stay in your browser — nothing is uploaded except inside your own PDF/Excel exports.

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Solar energy is a clean, renewable power source that reduces electricity bills and carbon footprint. Installing solar panels on your home or commercial property allows you to generate your own electricity from sunlight. This solar calculator helps you determine the right system size, number of panels needed, estimated energy output, and total installation costs based on your electricity consumption and location. The calculator considers your average monthly electricity usage, the amount of sunlight your location receives (peak sun hours), the efficiency of available solar panels, and available roof or ground space. System sizing involves balancing your energy needs with available space and budget. A typical residential solar system ranges from 3kW to 10kW, while commercial systems can be much larger. Solar panel efficiency has improved significantly over the years, with modern panels converting 18 to 22 percent of sunlight into electricity. The cost of solar has dropped by over 70 percent in the last decade, making it increasingly affordable. Federal and state incentives, tax credits, and rebates further reduce the net cost. Most solar installations pay for themselves within 5 to 8 years through electricity savings. The calculator provides detailed estimates including the number of panels required, total system capacity in kilowatts, annual energy production in kilowatt-hours, the roof area needed, and the total cost before and after incentives. Understanding these figures helps you make informed decisions about going solar and maximize your return on investment.

📖 How to Use the Solar Panel Calculator

1

Enter your average monthly electricity bill or consumption in kilowatt-hours.

2

Select your location or enter peak sun hours for your area.

3

Choose the solar panel wattage you plan to install.

4

Enter the cost per watt for solar installation in your area.

5

Click Calculate to get system size, panel count, and cost estimates.

6

Review the detailed breakdown including energy output and payback period.

📐 Solar Energy Formulas

Solar system sizing follows several key formulas. The system capacity formula is: System Size (kW) = Monthly Consumption (kWh) ÷ (Peak Sun Hours × 30 days × Panel Efficiency). For example, if you use 900 kWh per month with 5 peak sun hours, you need approximately: 900 ÷ (5 × 30 × 0.80) = 7.5 kW system. The number of panels is calculated by dividing the system size by the individual panel wattage. For a 7.5 kW system using 375W panels: 7,500 ÷ 375 = 20 panels. The required roof area is the number of panels multiplied by the panel dimensions, typically 1.7 m² per panel, so 20 panels need approximately 34 m² of roof space. Annual energy production is estimated by multiplying the system capacity by the annual peak sun hours. A 7.5 kW system in an area with 1,500 peak sun hours per year produces: 7.5 × 1,500 = 11,250 kWh annually. The total cost is calculated by multiplying the system size by the cost per watt, then adjusting for incentives. With a cost of $2.50 per watt and a 30 percent federal tax credit, a 7.5 kW system costs: 7,500 × $2.50 × 0.70 = $13,125 net cost. The payback period is determined by dividing the net cost by the annual electricity savings. If you save $1,500 per year on electricity, the payback period is: $13,125 ÷ $1,500 = 8.75 years. After payback, the system generates free electricity for the remaining 15 to 20 years of its lifespan.

⚠️ Common Solar Calculation Mistakes

✕

Using current electricity bill without accounting for future rate increases and consumption changes.

✕

Not considering shading from trees, buildings, or other obstructions that reduce panel output.

✕

Ignoring roof condition and orientation which affect panel placement and efficiency.

✕

Forgetting to account for inverter efficiency losses of 5 to 10 percent in energy calculations.

✕

Not including all costs such as permitting, interconnection, and monitoring systems in total cost estimates.

💡 Professional Solar Installation Tips

✓

Get a professional site assessment to accurately determine your solar potential and identify any shading issues.

✓

Consider battery storage if you want energy independence or live in an area with time-of-use electricity rates.

✓

Shop multiple quotes from licensed solar installers to ensure competitive pricing and quality workmanship.

✓

Check for all available incentives including federal tax credits, state rebates, and utility net metering programs.

✓

Ensure your roof is in good condition before installation, as panels will last 25 to 30 years.

✓

Consider panel orientation and tilt angle for maximum energy production in your latitude.

✓

Monitor your system performance regularly to ensure it is producing at expected levels.

❓ Frequently Asked Questions About Solar Panels

The number depends on your electricity consumption and panel wattage. A typical home using 900 kWh per month needs approximately 20 to 25 panels rated at 350 to 400 watts each. A professional assessment can determine the exact number based on your specific situation.

The average cost for residential solar is $2.50 to $3.50 per watt before incentives. A typical 6kW system costs $15,000 to $21,000 before the 30 percent federal tax credit. After incentives, the net cost is usually $10,000 to $15,000.

Most solar panels come with 25 to 30 year warranties. They continue producing electricity beyond this period, though output gradually decreases by about 0.5 percent per year. After 25 years, panels typically produce 80 to 85 percent of their original output.

Yes, solar panels produce electricity on cloudy days, though at reduced efficiency. They typically generate 10 to 25 percent of their rated output on heavily overcast days. Diffuse sunlight still reaches the panels and produces usable electricity.

Solar panels require minimal maintenance. Keep them clean by rinsing with water occasionally to remove dust and debris. Annual inspections to check connections and monitor performance are recommended. No moving means virtually no wear and tear.

🏗️ Real-World Solar Installation Examples

A typical residential solar installation demonstrates the practical application of these calculations. A 3-bedroom home consuming 900 kWh monthly in an area with 4.5 peak sun hours would need approximately a 6.8 kW system. This requires about 18 panels rated at 380 watts each, occupying approximately 30.6 m² of roof space. The system would produce roughly 10,200 kWh annually, offsetting about 94 percent of the home's electricity consumption. The total cost for this system at $2.80 per watt would be $19,040 before incentives. After the 30 percent federal tax credit of $5,712, the net cost is $13,328. With electricity savings of $1,400 per year, the payback period is approximately 9.5 years. Over the 25-year system lifespan, the total savings exceed $35,000, representing a return on investment of over 250 percent. Commercial installations benefit from economies of scale with lower per-watt costs. A 100 kW commercial system for a small business might cost $2.00 per watt ($200,000) before incentives, producing approximately 150,000 kWh annually. The business could save $20,000 or more per year on electricity, with a payback period of 5 to 7 years depending on local electricity rates and available incentives.

Use our solar panel calculator to determine the right system size, number of panels, and estimated costs for your home or business solar installation.

✅Method, Sources & Review

Maintained by theBuildingCalc methods desk· Last reviewedSeptember 23, 2026. This calculator implements the standards and references below; every figure can be rechecked against the formulas and worked example on this page.

NEC Art. 690 (PV) PVWatts methodology

Results are estimates, not engineering advice. Verify against local codes and licensed professionals for structural, electrical, and gas work. See ourTerms. Spotted an error? Report it— corrections ship with the next review.