Panel Sizing Formula
Adjusted daily energy = daily usage / system efficiency. Panel output per day = wattage x peak sun hours / 1000 kWh. Panels needed = adjusted energy / panel output per day (rounded up). System size = panels x panel wattage. Add 15-25% buffer for shading, soiling, and seasonal variation.
Frequently Asked Questions
The number of panels depends on your daily energy usage, your location's peak sun hours, and the wattage of the panels you choose. A typical US home uses about 30 kWh per day. At 5 peak sun hours with 400W panels: 30,000 Wh / (5 hours x 400W) = 15 panels, plus a 20-25% buffer for losses.
Peak sun hours measure the total daily solar irradiance at a location, expressed as equivalent hours of full sunlight at 1,000 W/m². The US average is about 4-5 peak sun hours per day, with the Southwest receiving 6-7 and the Northwest 3-4. Your utility bill or the NREL PVWatts tool can give a precise estimate for your location.
Solar systems lose 15-25% of potential output due to panel degradation, soiling, temperature, shading, inverter losses, and wiring losses. A 20% de-rating factor is a common industry default. This means panels produce about 80% of their rated wattage in real-world conditions.
The inverter should be sized to handle the maximum output of your solar array. A common guideline is to match the inverter to 100-110% of the total panel wattage. For 6,000W of panels, a 6,000-7,000W inverter is appropriate.
A standard residential solar panel is approximately 65 x 39 inches (5.4 x 3.25 feet), covering about 17.5 square feet. A 15-panel system would require roughly 260 square feet of usable south-facing roof area, free from significant shading.
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