Formula
Daily Wh = Panel watts × Peak sun hours × System efficiency
Example: 400W × 4.5 hours × 0.8 ≈ 1,440Wh/day
Use peak sun hours for your location (not daylight hours). Pair this estimate with your daily load plan when sizing batteries.
Calculator
Solar panel output calculator
Estimate daily energy from panel watts and peak sun hours.
Choose an efficiency factor
Use 0.75 to 0.85 to account for losses from heat, wiring, and inverter efficiency.
Losses also come from dust and shading. If your output is lower than expected, review low output troubleshooting and panel cleaning basics.