Solar Panel System Size Calculator
Determining the right solar system size depends on your electricity consumption, geographic location, roof orientation, and panel efficiency. This calculator estimates the system capacity and number of panels needed to offset your electric bill.
Solar System Sizing
Actual production depends on roof orientation, tilt angle, shading, and local weather patterns. A professional site assessment with shade analysis provides precise estimates. Net metering policies and utility rate structures vary by state.
How Solar Sizing Works
The Core Formula
Solar system sizing starts with your energy consumption:
- Monthly kWh = Monthly bill ÷ electricity rate
- Daily kWh = Monthly kWh ÷ 30
- System kW = Daily kWh ÷ peak sun hours ÷ system efficiency
- Number of panels = System kW × 1,000 ÷ panel wattage
Peak Sun Hours by Region
Peak sun hours represent the equivalent number of hours at full 1,000 W/m² irradiance. A location with 5.0 peak sun hours might have 10 hours of daylight, but the total energy equals 5 hours of peak intensity. This number is the single biggest variable in system sizing.
| Region | Peak Sun Hours | Representative States |
|---|---|---|
| Excellent | 5.5–6.5 | Arizona, Nevada, New Mexico |
| Very Good | 5.0–5.5 | Florida, Southern California, Colorado |
| Good | 4.5–5.0 | Texas, Georgia, North Carolina |
| Average | 4.0–4.5 | Virginia, Illinois, Kansas |
| Below Average | 3.5–4.0 | New York, Pennsylvania, Michigan |
| Low | 3.0–3.5 | Washington, Oregon, Minnesota |
Roof Space Requirements
Modern residential panels are approximately 5.4 ft × 3.25 ft (17.5 sq ft each). A typical 8 kW system with 400W panels needs 20 panels, requiring about 350 sq ft of unshaded roof area. South-facing roofs with a 15–40° tilt produce the most energy in the Northern Hemisphere.
Frequently Asked Questions
What is net metering?
Net metering allows solar customers to send excess electricity back to the grid and receive credits on their electric bill. When your panels produce more than you consume (typically midday), the meter runs backward. Policies vary significantly by state and utility — some offer full retail rate credits, while others use lower wholesale rates.
Should I size for 100% offset?
Most installers recommend 80–90% offset. Utility rate structures often include fixed charges that solar cannot offset, and oversizing may result in excess credits that expire unused. An 80% offset system typically eliminates the variable energy charges while avoiding an oversized investment.
How much roof space do I need?
Plan for approximately 17–18 sq ft per panel. Setback requirements from roof edges (typically 3 ft), vents, skylights, and HVAC equipment reduce usable area. Fire code pathways may also be required, especially in states following the 2018 IFC or later.