Solar Panel System Size Calculator
Calculate the right solar panel system size for your home. Enter your monthly electricity usage, peak sun hours for your location, and panel preferences to get a personalized recommendation for system size, number of panels, and estimated roof space needed.
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How Solar System Sizing Works
Step 1: Understanding Your Electricity Usage
The foundation of solar system sizing is your electricity consumption. Check your utility bill for your average monthly kilowatt-hour (kWh) usage. The U.S. average is approximately 900 kWh per month, but this varies widely — homes in hot climates with heavy air conditioning may use 1,500+ kWh, while energy-efficient homes might use only 400-600 kWh.
For the most accurate sizing, average your usage over 12 months to account for seasonal variations. Your utility company may provide an annual summary or you can add up 12 monthly bills.
Step 2: The Role of Peak Sun Hours
Peak sun hours (PSH) measure the intensity of solar energy your location receives. One peak sun hour equals one hour of sunlight at 1,000 watts per square meter. This is different from total daylight hours — a location might have 12 hours of daylight but only 4.5 peak sun hours because the sun's intensity varies throughout the day.
In the United States, peak sun hours range from about 3 hours in the Pacific Northwest to over 6.5 hours in the desert Southwest. You can find your area's peak sun hours using the NREL PVWatts Calculator or solar irradiance maps.
Step 3: How System Losses Work
No solar system operates at 100% theoretical efficiency. Real-world losses include:
- Inverter conversion: ~3% loss converting DC to AC power
- Wiring and connections: ~2% resistance losses
- Soiling and dust: ~2% from dirt on panels
- Temperature effects: ~3-5% (panels lose efficiency in heat)
- Shading: ~2-3% from nearby objects
- Panel degradation: ~0.5% per year
The industry-standard total system loss factor is 14%, which is the default used by NREL's PVWatts tool. You can adjust this based on your specific installation conditions.
Step 4: The Sizing Formula
This calculator uses the following methodology, aligned with NREL PVWatts:
System Size (kW DC) = Daily kWh Needed ÷ (Peak Sun Hours × (1 − System Loss ÷ 100))
Number of Panels = System Size in Watts ÷ Panel Wattage (rounded up)
Annual Production = System Size × Peak Sun Hours × 365.25 × (1 − System Loss ÷ 100)
Step 5: Roof Space Requirements
A standard 400W residential solar panel measures approximately 6.5 ft × 3.3 ft, occupying about 17.5 square feet of roof space. However, you'll need additional space for setbacks (fire codes typically require 3-foot pathways), vent pipes, skylights, and other roof obstructions. As a rule of thumb, plan for about 20-25% more roof area than the raw panel area to account for spacing and obstructions.
The roof space estimate in this calculator uses the raw panel footprint of 17.5 sq ft per panel. Discuss actual layout with your installer who can assess your specific roof geometry.
Frequently Asked Questions
It depends on your electricity usage, not house size. A typical 2,000 sq ft home uses 800–1,200 kWh/month, which usually requires a 5–8 kW system (13–20 panels). Use this calculator with your actual utility bill for an accurate estimate.
Peak sun hours represent the equivalent number of hours per day when solar radiation averages 1,000 W/m². It's not just daylight hours — it accounts for sun intensity. In the US: Southwest gets 5.5–6.5, Southeast gets 4–5, Midwest gets 3.5–4.5, Pacific NW gets 3–4.
System losses account for real-world efficiency reductions: inverter conversion (~3%), wiring losses (~2%), soiling/dust (~2%), temperature effects (~3–5%), shading (~2–3%), and panel degradation. The 14% default is the industry standard used by NREL PVWatts.
Not always. Many utilities cap net metering credits or have tiered rate structures. An 80–100% offset is typical. Some homeowners oversize to account for future EV charging or electric heating. Check your utility's net metering policy.
As of 2026, residential solar costs $2.50–$3.50 per watt before incentives. A typical 8 kW system costs $20,000–$28,000 before the 30% federal tax credit (Inflation Reduction Act), bringing the net cost to $14,000–$19,600.