Solar PV Derating
A practical, hands-on guide to estimating real cable surface temperature under rooftop solar panels.
Cable temperature under a rooftop solar array during a sunny day is meaningfully higher than general outdoor ambient temperature — enough that skipping this specific correction is one of the most common PV cable undersizing mistakes.
Panels absorb solar radiation and re-radiate a portion as heat, warming the roof surface and the confined air space directly beneath them where DC cable typically runs. That microclimate can run 20–30°C (or more) above general shaded ambient air temperature on a hot, sunny day.
| Scenario | Rough guidance |
|---|---|
| Cable well-ventilated beneath panels, decent clearance | Moderate elevation above ambient — better than fully enclosed conduit |
| Cable in conduit directly on a dark roof surface | Significant elevation above ambient — among the hottest realistic scenarios |
| Cable shaded and elevated with good airflow | Closer to general ambient, though rarely equal to it |
When in doubt, use a conservative (higher) temperature assumption for rooftop DC cable rather than defaulting to general outdoor ambient — the cost of a slightly larger conductor is far smaller than the cost of a genuinely undersized one baking in a hot rooftop microclimate for 25 years.
Enter your best estimate of real cable temperature in the PV/Solar DC Cable Sizing calculators.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks