A flat power curve.
Not always the same problem.

Start with the operating limit, not the silhouette

An owner sees a flat-topped production curve and asks whether the inverter is failing. That is a useful question, but the chart alone cannot answer it. Normal clipping, temperature-related limiting, and an active plant-control setting can produce similar-looking output. More than one restriction may apply during the same interval.

For Central Valley commercial and agricultural systems, compare the observation with the installed design and actual conditions. Do not use a hotter afternoon, a different season, or a different inverter size as an uncontrolled benchmark for another unit.

Clipping: power available exceeds the conversion limit

Sandia's PV Performance Modeling Collaborative explains that inverter clipping occurs when available array power exceeds the inverter's operating capacity; many inverters limit the input rather than converting all available DC power. A design can intentionally use more DC nameplate capacity than AC inverter capacity.

Clipping is not automatically a defect. The useful owner questions are whether the amount matches the approved design model, whether limits have changed, and whether additional losses occur outside the expected clipped period. Adding inverter capacity is not a maintenance default: it needs an engineering review of electrical limits, equipment compatibility, protection, and the existing interconnection arrangement.

Temperature derating: protecting equipment from excessive heat

SMA's temperature-derating guidance describes power reduction when monitored components approach temperature limits. It identifies heat dissipation, airflow, clearances, and installation conditions as relevant. That mechanism should not be confused with the normal temperature dependence of PV module output.

Ask for the exact inverter model, its applicable derating curve, relevant temperature readings, event codes, and cooling-system condition. A cooling problem is one possible cause; ambient heat or the installation may also explain limiting. No universal temperature threshold applies to every inverter. Cleaning, fan work, enclosure access, and return-to-service decisions must follow the particular equipment manual and site safety procedures.

A short evidence matrix prevents the wrong purchase

• Repeatable plateau near a documented power limit: compare available DC power, the inverter specification, and plant-control settings before calling it a fault.
• Reduction accompanied by temperature events: review temperatures, airflow history, cooling condition, and the manufacturer's limiting behavior.
• Plant-wide limiting that follows load or dispatch: ask the controls engineer to review the applicable export or plant setpoint, communications, and event history.
• One unit underperforming its genuinely comparable peers: inspect input availability, sensor validity, alarms, and equipment differences as well as cooling.

These patterns prioritize investigation. None is a stand-alone diagnosis. A faulty sensor, missing data, different DC loading, shading, or a separate electrical issue can confuse the comparison. Keep the original data resolution and align time zones before comparing traces.

Define the diagnostic package before dispatch

Request a time-aligned package containing AC power, available DC operating values, inverter events, temperatures where available, irradiance or a suitable reference, and plant-limit records. Include equipment ratings, the original production model, recent setting changes, and dates of cooling maintenance. Record data gaps instead of interpolating them into an apparently complete result.

The service report should separate expected design clipping, confirmed thermal limiting, other controls, and unexplained loss. Ask for each conclusion to identify the evidence supporting it. Do not clear export restrictions, increase protection thresholds, or disable temperature safeguards to make a chart look better.

Verify the result under comparable conditions

After corrective work, inspect the relevant events and limiting behavior during suitable operating conditions. Compare against an agreed model or matched equipment, accounting for weather and configuration. Restored output on a cooler day does not, by itself, prove that a thermal issue was corrected.

DOE FEMP recommends comparing actual and expected production and documenting reacceptance testing after repair. For a site-specific review, use TerraSun's solar performance service or inverter maintenance and repair service. The next step should be a defined evidence review, not an unsupported replacement recommendation.

Technical references

• Sandia PVPMC: Inverter saturation and clipping
• SMA: Installation site, cooling, and temperature derating
• DOE FEMP: PV performance and reacceptance testing