Derating
The automatic reduction of an inverter's output power to protect its internal components, most commonly triggered by high temperature but also by high grid voltage or by an out-of-range input. The unit keeps running at reduced capacity instead of shutting down.
Thermal derating is the usual case: once ambient temperature climbs past the manufacturer's threshold, output falls progressively, and it does so on exactly the hot, clear afternoons when the array is producing hardest. Mounting on a shaded, ventilated north-facing wall — south-facing in the southern hemisphere — and respecting the specified clearances often recovers more annual energy than choosing a nominally more efficient model. Grid-voltage derating is the other frequent cause and points at a different problem: a long or undersized AC cable run pushing the voltage at the inverter terminals towards its upper limit.