How Many Solar Panels Do I Need? Sizing Guide

One of the first questions homeowners ask about solar is "how many panels do I need?" Here's how to figure it out.

The Quick Answer

The average American home needs 15–25 solar panels (a 5–8 kW system) to fully offset its electricity usage. But your specific number depends on three factors:

1. Your energy consumption (kWh/month)
2. Your local peak sun hours
3. Your available roof space

Step 1: Find Your Energy Usage

Check your electric bill for your monthly kWh usage. The US average is about 900 kWh/month, but this varies widely:

• Small apartment: 300–500 kWh/month
• Average home: 800–1,200 kWh/month
• Large home with AC: 1,500–2,500 kWh/month

Step 2: Find Your Peak Sun Hours

Peak sun hours (PSH) measures how much solar energy your area receives. Check our state solar guides for your state's average PSH:

• Arizona/Nevada: 6.5 PSH
• California/Texas: 5.0–5.5 PSH
• New York/Washington: 3.5–4.5 PSH

Step 3: Calculate System Size

Use this formula:

System Size (kW) = Monthly kWh / (PSH × 30 × 0.82)

The 0.82 factor accounts for system losses (inverter efficiency, shading, temperature).

Example: 900 kWh/month in a 5.5 PSH area:

• 900 / (5.5 × 30 × 0.82) = 900 / 135.3 = 6.6 kW system

Step 4: Calculate Number of Panels

Most residential panels are 400–450 watts. To calculate:

Number of panels = (System Size in watts) / Panel wattage

Example: 6.6 kW system with 400W panels:

• 6,600W / 400W = 17 panels

Step 5: Check Roof Space

Each panel needs about 17.5 square feet. 17 panels need ~300 square feet of usable roof space.

• South-facing roofs are ideal
• East/west roofs work but produce 15% less
• Shading reduces usable space

Use our Roof Solar Potential Calculator to see how many panels fit on your roof.

Quick Reference Table

| Monthly Bill | Monthly kWh | System Size | Panels (400W) | |---|---|---|---| | $75 | ~500 kWh | 3.7 kW | 10 | | $150 | ~1,000 kWh | 7.3 kW | 19 | | $250 | ~1,600 kWh | 11.7 kW | 30 | | $400 | ~2,500 kWh | 18.3 kW | 46 |

Assumes $0.15/kWh and 5.5 PSH

What If My Roof Is Too Small?

If you can't fit enough panels on your roof:

1. Use higher-efficiency panels (450W+ instead of 400W)
2. Consider a ground-mount system if you have yard space
3. Offset a portion of your bill — solar doesn't have to cover 100%
4. Add battery storage to maximize self-consumption

Get a Personalized Recommendation

The best way to determine your exact system size is to run our free AI Home Energy Advisor. Enter your ZIP code and monthly bill, and we'll calculate the ideal system size, panel count, and estimated savings for your specific home.

You can also use our Solar Savings Calculator and Roof Solar Potential Calculator for quick estimates.