Inverter Derates Only on Hottest Days vs Fan Failure: Decision Tree

Why this matters

A string inverter that clips or derates production only on the hottest afternoons of the year is doing thermal protection, which is normal up to a point. The diagnostic question is whether the derate is expected ambient-driven behavior, a failed or fouled cooling fan, or a degraded heatsink path that needs intervention. Calling a normal thermal fold-back a fan failure leads to a wasted parts swap; calling a real cooling fault "just hot weather" lets the inverter run hot enough to age electrolytic capacitors and IGBTs prematurely, shortening its life by years. Both errors cost money. A senior tech separates these in one site visit by reading internal temperatures against ambient and watching whether the cooling system responds.

Symptom presentation

Production is flat or normal across most of the year. On days where ambient air exceeds roughly the mid-90s F (mid-30s C), the inverter caps output below where irradiance and array size should put it, often logging a temperature-derate, over-temperature, or thermal fold-back event around solar noon. Output recovers as the day cools. The derate may show as a smooth power ceiling rather than a hard trip. Inverters mounted in direct sun, in unventilated garages, or against west-facing walls present this earliest in the season. A failing fan version of this shows the same curve but at lower ambient thresholds each successive hot day, as the cooling capacity drops.

Quick checks

Read the inverter's internal heatsink and ambient sensor temperatures from the display or portal during the derate. Compare internal temperature to the manufacturer's derate-onset spec (commonly internal temps in the 60 to 85 C range trigger fold-back depending on model). Listen and feel for fan airflow at the vents during peak heat. Inspect intake and exhaust vents for dust, pollen, spider webs, or wasp nests blocking airflow. Check the mounting location and sun exposure. Verify the unit has the manufacturer-required clearance around it. A quick irradiance check (clear sky, panel near rated temperature-corrected output) confirms the array could produce more if cooling allowed.

Isolation tree

Branch one, is the fan running? Many models run the fan only above a temperature threshold, so verify the inverter is actually hot enough to call for cooling before concluding the fan is dead. With internal temp above the fan-on setpoint, confirm airflow. No airflow with high internal temp points to a failed fan, seized bearing, blocked intake, or a fan-drive fault. Branch two, fan runs but derate persists. Compare internal temperature rise above ambient. A healthy unit holds a modest delta; a large delta with the fan running means the heatsink airflow path is fouled or the thermal interface has degraded. Branch three, internal temp tracks ambient with a normal delta and fan responding. The derate is expected ambient-driven protection; the fix is environmental (shade, ventilation, relocation), not a part. Branch four, fan-on threshold never reached but unit still derates, which suggests a sensor fault or firmware setpoint issue, verifiable against logged temperatures.

Confirming diagnosis

Confirm a fan failure when internal temperature climbs past the fan-on setpoint, no airflow is present at the vents, and clearing or replacing the fan drops the temperature and restores output. A useful sub-test: command the fan on through the inverter's service menu if the model allows, then confirm it both spins and moves air; a fan that draws current but does not turn (seized bearing) or turns but moves no air (blocked intake) both read as airflow loss but need different fixes. Confirm a fouled-airflow case when the fan spins but the heatsink-to-ambient delta is abnormally high and cleaning the vents and fins reduces it; a healthy unit at full output holds the heatsink within a bounded rise above intake-air temperature, so a delta well beyond the model's normal range with the fan running points squarely at a fouled fin stack or a partially blocked path. Confirm normal ambient derate when the fan runs, the temperature delta is within spec, internal temp simply rides high because ambient is extreme, and no cleaning or part change moves the derate threshold; in that case the array could only produce more if the ambient were cooler, and the protection is working as designed. Logging temperature and output across a full hot afternoon, before and after any intervention, is the decisive test, because it shows whether the intervention actually lowered the operating temperature and lifted the production ceiling rather than just coinciding with a cooler day.

Remediation

For a failed fan, replace it with the manufacturer part and verify the new fan energizes at the correct setpoint and clears the over-temperature condition. For fouled airflow, clean intake and exhaust vents and heatsink fins, then re-torque any loose heatsink hardware. For an environmental derate, improve ventilation, add shading over the inverter, or relocate the unit out of direct sun and into manufacturer-specified clearances. For a sensor or setpoint fault, update firmware and confirm the logged temperatures are plausible before trusting the protection logic. Document peak internal temperatures so the customer understands whether the unit is healthy or was running hot enough to warrant the fix.

References

• Manufacturer inverter installation manual, thermal derating curve and cooling-fan operation (SolarEdge, SMA, Fronius per model)
• UL 1741, inverter thermal and over-temperature protection requirements
• IEEE 1547-2018, continuous operation and ride-through behavior
• NEC 2023 Article 690.4 and 110.13, equipment installation and clearance for cooling
• SolarEdge installation guide, ambient temperature derating and mounting clearance specifications