Customers are often puzzled when their 10kW system peaks at 7.5 or 8kW in June, even on a clear afternoon. The panels aren’t faulty. They’re hot.
- Panels are rated at 25°C cell temperature, which rarely happens on a Pakistani roof.
- On hot days, cells can reach 60 to 70°C.
- Output drops roughly 0.3 to 0.4% for every degree above 25°C.
- Airflow under the panels helps more than anything else.
The 25°C problem
A panel’s rated wattage, say 580W, is measured under Standard Test Conditions, with the cells at 25°C. On a Pakistani roof in summer, with air at 40 to 45°C and the sun beating down, the cells themselves often sit at 60 to 70°C.
Temperature coefficient
Every panel datasheet lists a temperature coefficient of Pmax, typically around -0.29% to -0.36% per °C. That’s how much power is lost for every degree above 25°C.
Example: a panel with -0.35%/°C running at 65°C is 40 degrees above test conditions. That’s about 14% less power than its rating. Add a bit of dust and inverter losses, and a 10kW array giving around 7.5 to 8kW at peak is completely normal.
But summer still wins overall
Longer days and a high sun mean summer still produces more total units per day than winter, even with heat losses. The peak is just lower than the panel sticker suggests.
What helps panels stay cooler
- Raised structures: panels mounted with a good gap above the roof let air flow underneath. Flush-mounted panels on a hot concrete slab suffer most.
- Lower temperature coefficient: N-type panels typically lose a bit less in heat than PERC. See our comparison.
- Light roof surface: white or reflective roof paint keeps the area under the panels cooler.
- Clean panels: dust traps heat as well as blocking light.
Should you spray water on panels to cool them?
We don’t recommend it. Spraying cold water on hot glass in the afternoon can stress the panel, leaves mineral spots, and wastes water. Clean panels in the early morning instead.
Don’t forget the inverter
Inverters also reduce output when they overheat. Keep yours in the shade with plenty of airflow. In June, an inverter in a closed, hot room may limit its power to protect itself, and you’ll see less than the panels can offer.
For the winter side of the story, read solar output in winter.
Wondering if your summer output is normal?
Send us your system size and a screenshot of peak power. We’ll tell you if it looks right.