Solar & battery
Solar Panel Output Calculator
What one panel actually produces per day and per year, against its nameplate rating.
Result
System derate covers inverter losses, wiring, heat, soiling and panel mismatch. Around 0.80 is the figure most design tools use; a hot climate, a dusty site or a shaded roof pushes it lower. It is why a system never produces the sum of its panel ratings. Panel ratings are measured under standard test conditions at 25 degrees C. Real cells run far hotter than that on a roof and produce measurably less, which is part of what the derate accounts for.
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About this calculator
Why does a 400 W panel not make 400 W?
The rating is measured in a laboratory at a cell temperature of 25 degrees C with 1,000 W per square metre of light. On a hot roof the cell is far hotter and the output drops several percent.
How fast do panels degrade?
Roughly 0.5% a year for modern modules, with warranties typically guaranteeing 80% to 85% at 25 years. It is slow enough to ignore for a payback estimate and worth including over a full lifetime.
The formula
daily kWh = panel watts / 1000 x peak sun hours x derate
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