Reference project
Reference: PV thermography of a commercial roof in Celje
Thermographic inspection of a 100 kWp rooftop system on a commercial building in Celje, flown in line with IEC TS 62446-3. Result: one failed string and 19 modules needing action, each with its position.
- Object
- Rooftop system on a commercial building in Celje, Slovenia, 100 kWp, 426 modules
- Method
- Aerial thermography in line with IEC TS 62446-3, irradiance 731 W/m², about 12 mm per pixel
- Equipment
- DJI Matrice 4TD with radiometric thermal camera and RTK, evaluation with dronalise
- Result
- Failed string with 17 modules, 19 modules rated defect, finding per module with coordinate
Starting point
The operator wanted to know the condition of the modules on a rooftop system on a commercial building in Celje, without scaffolding, without anyone walking on the roof and without shutting the system down. The system has 100 kWp and 426 modules.
How it was done
The flight took place on 29 September 2026 under a cloudless sky, with no wind and an irradiance of 731 W/m² on the horizontal plane. IEC TS 62446-3 requires at least 600 W/m², so the conditions were met.
The flight with the DJI Matrice 4TD took twelve minutes and produced 249 radiometric thermal images and as many images in the visible spectrum. At a distance of about 12.5 m the resolution was around 12 mm per pixel, while the standard allows at most 3 cm.
The data was evaluated with our partner dronalise. Every module was detected individually, placed on a 3D model of the system and recorded with its coordinate. For seven modules the thermal images could not be assigned automatically because of the turning manoeuvres. They were checked by hand, and one of them showed a hotspot. The report says so as well.
Result
The key finding is a failed string. 17 modules were delivering no power at the time of the measurement, and in the thermal image they stand out as a group that is uniformly warmer than their neighbours, because they are not delivering current. From the ground, a failure like this cannot be seen.
In addition, 19 modules were rated defect, two of them with a bypass diode fault pattern, 6 modules were rated warning and 16 were marked for observation. 385 modules showed no findings and none was rated critical.
What the operator received
- Inspection report with the measurement conditions and the temperature difference normalised to 1000 W/m²
- Finding per module with thermal image, visible spectrum image and coordinate
- 3D model of the system with every affected module marked
- Service report for the team on site, in German and Slovenian
Next steps
A thermal image shows where something is wrong, the cause is confirmed by a qualified electrician on site. The report recommends an electrical check of the failed string and the bypass diodes. After the repair, a short control flight can show that the string is working again, with the same module numbering as the first flight.