Power at the inverter's AC terminals falls when the inverter itself is hot, and it falls again when the air around it is thin. Both effects are cooling problems. The silicon and the magnetics can pass only the current the heatsink can reject. The curve that says when full power ends, and how fast the limit drops after that, is printed in the manual for that model. A percent copied from another brand, or from a forum, is a number this installation was never given.
Heat moves the limit before the module does
A solar module also loses power as its cells warm, and that loss has its own article, how heat affects a solar module. The inverter's derating is a second, separate curve. On a hot afternoon both can be true at once: the array is offering less, and the inverter is willing to take less. Blaming only the roof, or only the inverter, mixes two nameplates.
Read the axis the manufacturer actually used. Some curves are drawn against the air temperature in the shade beside the enclosure. Some are drawn against a sensor inside the box. A unit in full sun on a south wall can be much hotter than the shaded air temperature a weather app reports, so the curve and the mounting chapter have to be read together. Clearances above the fins, a filter that is packed with dust, and a closet door that was shut after commissioning all change the temperature the sensor sees. The manual's clearance is the clearance. A smaller gap because the wall was convenient is a different thermal design.
Where those lines sit on the page, and which footnote applies above a stated elevation, is part of how to read an inverter datasheet. If the datasheet shows full rated power only up to a temperature it names, believe that temperature for that model. Do not slide the knee of the curve to a round number that feels memorable.
Thin air carries less heat
Fans and natural draft move heat by moving air. At higher elevation the same volume of air has less mass, so each pass across a heatsink picks up less heat. Manufacturers answer that with an altitude note: full power up to a stated elevation, then a reduction, or a hard maximum elevation beyond which the listing stops. The elevation and the reduction belong to that note. Two inverters on the same ridge can carry different notes. The one with the larger fan is not automatically the one with the kinder altitude line.
Humidity, dust, and a clogged insect screen sit beside altitude. They are site facts the curve assumes away when the lab test used clean air and the clearances in the drawing. If the installation cannot match those assumptions, the person who signs the job has to treat the published curve as a best case and follow whatever extra limit the manufacturer gives for a restricted site. Inventing a spare ten percent of headroom, because the last job seemed fine, does not create that limit.
What to do when the curve and the site disagree
Move the enclosure out of the sun, restore the clearances, and clean only what the manual says an owner may clean. The placement choices are in where to mount a solar inverter. If the site is simply above the elevation the manual allows, a different model, or a model whose note covers that elevation, is the honest fix. Stacking external fans that the listing never mentioned can disturb the internal path or blow dust into a board the enclosure was meant to protect.
Derating is the inverter protecting itself. A unit that quietly folds its output on hot afternoons can be inside its curve, and a unit that faults on temperature every mild morning can be outside it: a dead fan, a blocked vent, or a sensor. Those signs belong with the failure article, and a qualified person is the one who opens the cover to tell them apart. Until that visit, leave the screws in.
