Solar panels are rated in watts — but understanding what those watts mean in real-world energy production is the key to making smart decisions about solar. From panel ratings to system sizing, power conversion is at the heart of solar energy.
Solar Panel Ratings
Modern solar panels are rated between 350–450 watts each under standard test conditions (STC: 1000 W/m² sunlight, 25°C cell temperature). A typical residential system uses 15–25 panels for a total of 5–10 kW.
Panel efficiency ranges from 17–22%. Higher efficiency panels produce more power per square meter, which matters when roof space is limited.
From Watts to Kilowatt-Hours
Power (watts) × time (hours) = energy (watt-hours). A 400 W panel producing power for 5 peak sun hours generates 2,000 Wh = 2 kWh per day.
Real-world output is typically 75–85% of rated power due to temperature losses, wiring losses, inverter efficiency, and dust. A 10 kW system might produce 30–45 kWh/day depending on location and season.
System Sizing Basics
To offset 1,000 kWh/month usage, you need about 33 kWh/day. With 4 peak sun hours and 80% system efficiency: 33 ÷ (4 × 0.8) = 10.3 kW system needed.
Cost comparison: US average $2.50–3.50/watt installed. A 10 kW system costs $25,000–35,000 before incentives. The federal tax credit (30%) reduces this to $17,500–24,500.
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