Solar Panel Power Output
Understanding Watts and kWh
Learn Solar PowerA "400-watt" solar panel doesn't produce 400 watts all day—that's its peak rating under ideal conditions. Understanding how solar power is measured and what factors affect output helps you size systems correctly and set realistic expectations.
Standard Test Conditions (STC)
Panel ratings are measured under standardized conditions:
- Irradiance: 1,000 W/m² (bright sun)
- Cell temperature: 25°C (77°F)
- Air mass: AM 1.5 (sun angle through atmosphere)
Real-world conditions rarely match STC, so actual output is typically lower than rated power.
Typical Panel Ratings
| Panel Type | Wattage | Dimensions | Efficiency |
|---|---|---|---|
| Residential (standard) | 350-400W | 65" × 40" | 19-21% |
| Residential (premium) | 400-450W | 65" × 40" | 21-23% |
| Commercial | 450-550W | 75" × 40" | 20-22% |
| Utility scale | 500-700W | Larger formats | 20-22% |
From Watts to kWh
The Calculation
Daily energy = Panel watts × Peak sun hours × System efficiency
Example
A 400W panel in a location with 5 peak sun hours:
- Theoretical: 400W × 5h = 2,000Wh = 2 kWh
- With ~85% system efficiency: 2 × 0.85 = 1.7 kWh/day
- Monthly: 1.7 × 30 = 51 kWh
- Annually: 51 × 12 = ~612 kWh
Peak Sun Hours by Region
| Location | Peak Sun Hours/Day |
|---|---|
| Arizona, Nevada | 6.5-7.5 |
| California, Texas | 5.5-6.5 |
| Florida, Colorado | 5.0-5.5 |
| Midwest US | 4.0-5.0 |
| Northeast US | 3.5-4.5 |
| Pacific Northwest | 3.0-4.0 |
Factors Affecting Output
Reduces Output
- Temperature: Panels lose ~0.4%/°C above 25°C
- Shade: Even partial shade dramatically reduces output
- Dust/dirt: 2-5% loss if not cleaned
- Angle/orientation: Non-optimal reduces production
- Inverter losses: ~3-5%
- Wire losses: ~1-2%
Combined System Efficiency
Real-world systems typically achieve 75-85% of theoretical output.
System Size Examples
| Home Size | Usage (kWh/mo) | System Size (kW) | Panels (400W) |
|---|---|---|---|
| Small apartment | 300-500 | 2-4 | 5-10 |
| Average home | 800-1,000 | 6-8 | 15-20 |
| Large home | 1,500-2,000 | 10-15 | 25-38 |
| With EV | 2,000-3,000 | 15-20 | 38-50 |
Conclusion
Solar panel wattage (e.g., 400W) indicates peak output under ideal conditions. Real-world energy production depends on location (sun hours), temperature, shading, and system efficiency. A typical 400W panel produces about 1.5-2 kWh per day in most US locations. To estimate system needs, divide monthly electricity use by peak sun hours and account for 15-25% system losses.