Anyone else noticed their MPPT dropping efficiency when temps hit 35°C+?

Been noticing something odd with my Victron SmartSolar 100/30 over the past few weeks during this heatwave. When ambient temps push above 35°C, my controller seems to be pulling noticeably less power than I'd expect given the irradiance levels.

I've got 400W of panels on a south-facing roof and the MPPT is mounted inside a small timber outbuilding — probably not ideal, granted. The Victron Connect app is showing yield figures that feel low for the solar hours we've been getting.

A few questions for anyone who's experienced similar:

• Is this thermal throttling on the controller itself, or something else going on?
• Does it make sense to add a small fan or heatsink to the enclosure, or is relocating the unit the better fix?
• Anyone know at what internal temperature the Victron SmartSolar units actually start deroating?

I've read that MPPT controllers can derate output to protect themselves from heat damage, but I can't find a clear threshold in Victron's documentation. The manual mentions derating above 40°C ambient but my outbuilding is definitely getting hotter than that internally on a sunny day.

Also wondering whether panel-side heat plays into this — I know PV output drops roughly 0.4% per °C above 25°C, so the panels themselves are probably losing a fair bit too. Could the two effects be compounding?

Has anyone actually measured the difference in yield between a shaded/ventilated controller versus one in a hot enclosure? Would love some real-world data rather than just spec sheet numbers.

@SparkyHiker This is totally expected behaviour, and Victron are actually quite transparent about it in their datasheets.

The SmartSolar 100/30 will derate its output current above roughly 40°C case temperature — not ambient. If it's mounted in a confined space or in direct sun, the case could easily be 15-20°C above ambient, so you're probably hitting thermal derating territory.

Key things to check:

• Mounting position — vertical mounting massively improves convective cooling
• Direct sunlight on the unit itself — shade it if possible
• VRM/app data — the SmartSolar logs case temperature; worth reviewing

On my narrowboat I had exactly this issue until I relocated the controller to a ventilated cabinet away from the engine bay. Gained back roughly 8-10% harvest on peak summer days.

The controller isn't faulty — it's protecting itself. Worth reading Victron's derating curve in the MPPT datasheet.

@SparkyHiker yeah I've seen the same on my SmartSolar 75/15 out at the static van. Mine's mounted in a fairly enclosed space under a bench though so I always wondered if that was making things worse than they needed to be.

Does yours have decent airflow around it? I relocated mine after noticing the derating and it made a difference — not huge but noticeable.

Also worth checking — is it the controller that's the bottleneck or are your panels themselves dipping? Because panels lose efficiency in heat too, roughly 0.4% per degree above 25°C or something like that. Could be both things compounding at once.

What are your actual panel temps reading at when this happens?

@SparkyHiker Worth checking the actual derating curve in the Victron datasheet — the 100/30 starts throttling output current above roughly 25°C ambient to protect internal components. By 35°C you're already meaningfully into the derating zone.

In my shepherd's hut setup I solved this by mounting the controller on an external north-facing timber panel rather than inside. Dropped operating temps by around 12°C on hot days and recovered nearly full rated output. Aluminium backplate between controller and timber helps enormously as a heatsink.

Also worth logging via VictronConnect — the MPPT efficiency figure and internal temperature are both exposed. If your controller is hitting 60°C+ internally whilst ambient is only 35°C, that's a ventilation problem compounding the issue rather than pure ambient temperature being the culprit.

Has anyone actually measured how much difference mounting location makes in practice? I've got my 100/30 bolted to a metal bracket inside the van, and I'm wondering whether moving it somewhere with better airflow would be worth the hassle.

@LiamFrost70 — useful point about the datasheet, I'll dig that out. What I'm curious about is whether there's a rough rule of thumb for how many degrees you can realistically knock off the controller temp through better ventilation versus active cooling.

Also wondering whether the panel-side losses compound the issue — presumably the panels themselves are outputting less Voc in high heat too, so the MPPT is dealing with a degraded input and throttling its own output simultaneously? That seems like it could add up to a meaningful chunk of capacity on a 35°C+ day.