Reading a Thermal PV Inspection Report: Hotspots, Diodes and What to Fix First
A thermal drone flies over your PV park, and a week later a PDF lands in your inbox: sixty pages, orange and purple images, a table of anomalies with codes you have never seen. Most owners skim it, forward it to the maintenance contractor and hope. This article is for reading it yourself, because the difference between findings is the difference between losing a few euros a year and losing a whole string of production.
We covered why drones beat ladders and handheld cameras for this work in our comparison of drone inspection versus traditional methods. This is the follow-up: what the results actually mean.
How thermal inspection finds faults
A radiometric thermal camera measures the temperature of every point on every panel. A healthy panel under load runs warm but uniform. Faults show up as temperature patterns, and the pattern is the diagnosis. That is the key insight: the shape of the anomaly tells you what is wrong, usually before any electrical testing.
The flight happens in good sun, typically above 600 W/m² irradiance, because the faults only reveal themselves under load. A survey flown on an overcast day is a survey wasted, which is one of several reasons to ask how, not just whether, your provider flies.
The patterns, from cheap to expensive
Single hotspot on one cell. The classic finding. One cell running visibly hotter than its neighbours, often caused by soiling, bird droppings, a leaf, or localised cell damage. If cleaning fixes it, it cost you almost nothing. If it persists after cleaning, the cell is damaged and worth watching: chronic hotspots degrade and can eventually burn.
A hot diagonal or patchwork across the module. Multiple hot cells scattered on one panel usually mean cell mismatch or micro-cracking, often from transport or hail. Production loss is modest per panel but permanent. These go on the warranty claim list if the park is young.
One third of a panel warm, uniformly. The signature of a bypass diode fault. Panels are usually built in three substrings, each protected by a diode; when a diode fails or activates permanently, its substring stops producing and shows as a uniform warm band. You are losing a third of that panel’s output, every hour of sun. Diode faults are cheap to fix and expensive to ignore, and they are invisible from the ground.
A whole panel hot. The panel is disconnected and not producing at all, sitting at open-circuit temperature. Broken connector, failed junction box, or an installation fault. One dead panel in a string can also drag down its neighbours depending on design.
A whole row or string at ambient temperature, cooler than everything around it. The expensive one. A string outage: a blown fuse, a failed connector, an inverter input down. Nothing in that string has produced since the fault occurred, and without inspection or granular monitoring, string outages routinely go unnoticed for months.
What to fix first
Priority is production loss times persistence, not visual drama:
- String outages: full loss, fix immediately, usually a simple electrical repair.
- Diode faults and dead panels: large permanent loss per unit, quick wins.
- Persistent cell hotspots: small loss but degradation and, rarely, fire risk; monitor or replace.
- Soiling patterns: schedule cleaning when the pattern, not the calendar, says so.
A good report arrives with exactly this prioritisation, GPS-tagged per panel, so the maintenance crew walks straight to the fault with a map instead of hunting. If your report is just pretty pictures without locations and severity ranking, you paid for half a product.
When to fly
Twice a year suits most parks in Greece: once after winter to catch storm and moisture damage, once in early summer under strong irradiance for the sharpest thermal contrast. After any hailstorm, add an ad-hoc flight; micro-cracks from hail are precisely what thermal imaging catches early and warranty claims require documented.
We fly PV thermal inspections under EASA rules with certified pilots as part of our drone services, and deliver reports built for maintenance crews, not for decoration.
Frequently Asked Questions
What does a hotspot on a solar panel mean?
A localised area running hotter than the rest of the panel, caused by soiling, shading or cell damage. Cleaning resolves many; persistent hotspots indicate damaged cells that degrade over time.
Why is one third of my panel warmer than the rest?
That is the classic signature of a bypass diode fault. One of the panel’s three substrings has stopped producing. It is a small repair and a significant permanent production loss if left.
How often should a PV park be thermally inspected?
Twice a year is a sound baseline, plus an extra flight after severe weather such as hail. Always fly under strong sun, above roughly 600 W/m² irradiance, or faults will not show.
Editorial note: This article was researched and drafted with the assistance of Claude (Anthropic), and reviewed and approved by Amazing Projects before publication.
