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We took this pic at 7am a couple of days ago. The frost shows that one panel is significantly warmer than the others. Anyone know why that might be?

The top-right 6 panels are one string on AC-inverter. Then other 8 are a 4x2 DC array, so the melted one is one end of one of the two parallel DC strings. Looks like 2 more panels are just starting to melt the frost off, from the centre.

The flashing for all the cabling is under this panel, so I suppose if it was letting out warm air from the loft that might melt this panel first, but that would be half-way up, not the whole panel, so seems unlikely to be the cause.

I can only guess:-

Assuming the 'warm' panel is in a string of 4 with the three below it, and the four lower panels on the right are the other dc string.

There is a tree shadow on the other three of that string so it would naturally be warmer, but also (depending on the minimum startup voltage for the d.c string), may be generating current, whereas the other three are just passing the current via their bypass diodes (which may explain their starting to melt in the middle).

The upper row would naturally be warmer anyway, as the rising sun would hit them 1st, and as the other 6 panels are on an ac inverter, I would speculate that the ac inverter has a higher startup threshold than the dc controller, so they haven't woken up and passed as much current as the dc strings.

Why the other dc string isn't warming up is harder to rationalise.

The temperature difference is probably tiny, and once the frost starts to melt, it absorbs the sunlight better and the panel will warm up much more quickly, so if there is a small heat leak under that panel, that could be enough to make the difference

Actually they are physically wired alternately, so the partly-shaded one immediately below is the start of the other DC string. The one to the right of that is the 2nd one in the 1st string and so on, in an A/B wiring pattern.

The orange lines in this diagram show how it's wired.

The sun is providing maybe 100W to that panel, of which 20W could be turned into electricity if it’s usefully connected to an mppt inverter, the other 80W turning to heat. If the inverter doesn’t use it for some reason - eg. it’s not actually wired up or it’s shorted out or the current is too high for it (bit of shade?) then the blocking diodes take over and the whole 100W turns into heat.

robl wrote: ↑Sat Mar 22, 2025 8:48 am The sun is providing maybe 100W to that panel, of which 20W could be turned into electricity if it’s usefully connected to an mppt inverter, the other 80W turning to heat. If the inverter doesn’t use it for some reason - eg. it’s not actually wired up or it’s shorted out or the current is too high for it (bit of shade?) then the blocking diodes take over and the whole 100W turns into heat.

Yes operating panels run cooler.

I did an experiment back in 2021,
https://camelot-forum.co.uk/phpBB3/view ... f=11&t=290