Solar Panel Efficiency Explained: What the Numbers on the Datasheet Really Mean

Panel efficiency is the most quoted and least understood number in a solar quotation. It does not tell you how much electricity you will get. It tells you how much roof you will need.

What efficiency measures

Module efficiency is the share of sunlight falling on the panel that leaves it as electricity, measured under standard test conditions: 1000 W/m², 25°C cell temperature and a fixed light spectrum. A 22 per cent module converts 22 per cent of that laboratory sunlight.

Two modules of the same wattage produce the same power. The more efficient one simply does it in a smaller area, which only matters when roof space rather than budget is your constraint.

The specifications that move your annual units

• Temperature coefficient: output falls as cells heat up, and a module rated -0.30%/°C loses less on a 45°C afternoon than one rated -0.40%/°C
• Degradation: good modules lose about 1 to 2 per cent in the first year, then roughly 0.4 to 0.55 per cent annually
• Low-light performance: relevant through foggy December and January mornings here
• Soiling: dust can cost 5 to 15 per cent between cleanings in this region, often more than the gap between two module brands

Performance ratio: the honest metric

Performance ratio compares what your system actually delivered against what the modules should have delivered in the same conditions. A well-designed North Indian rooftop runs at 75 to 80 per cent. If yours is below that, the cause is usually shading, dirty glass, undersized cable or a clipped inverter — not the efficiency printed on the datasheet.

What to do with this

Ask your installer for expected annual generation in units, not just panel efficiency, then track the real number on your inverter's monitoring app each month. We hand over every system with monitoring configured and show the owner how to read it.