Anyone else running a Renogy 200W flexible panel on a van roof? Getting weird voltage drops in the afternoon

I've had a Renogy 200W flexible panel stuck down on the roof of my Transit Custom since last spring and it's been mostly brilliant, but I'm noticing something odd. Around 2–3pm on clear days the MPPT reading drops from roughly 18–19V open circuit down to about 14.5V and the charge current tanks with it. Panel surface gets pretty warm to the touch but I wouldn't say scorching. Running a Victron SmartSolar 100/30 into a 100Ah lithium (Fogstar Drift).

I'm wondering if it's thermal derating kicking in — flexible panels are notorious for running hot when they're stuck flat to a metal roof with no airflow underneath. I've seen some threads mention that flexibles can lose 15–20% efficiency once the cell temp climbs past 45°C or so, but the drops I'm seeing feel more dramatic than that, more like 30–35% on peak output.

Has anyone tried putting a thin foam or rubber spacer strip around the edges to lift the panel a few millimetres and get a bit of airflow underneath? I'm wondering if that would actually make a meaningful difference or just look a bit daft and catch the wind. Also curious whether anyone has just given up on flexibles altogether and gone back to a rigid panel on a low-profile mount.

MoorDweller | 847 posts | Yorkshire Dales

@ForestSolar the MPPT cutting out or showing voltage drops in that afternoon window is almost certainly heat-related delamination or hotspot issues with flexible panels - they're notorious for it. Flexible panels stuck flat to a metal roof can hit 70-80°C on a summer afternoon with zero airflow underneath. At those temps the cells lose efficiency rapidly, sometimes 0.4-0.5% per degree above 25°C.

Worth checking whether your readings recover once it cools down around 4-5pm? If they do, that confirms thermal throttling rather than a wiring fault.

Longer term, even a 10mm standoff to allow some airflow underneath makes a massive difference. Some folks use adhesive rubber feet or thin aluminium channel strips.

How are you monitoring it - what MPPT controller are you running? That'll help narrow things down further.

WheresMeWires31 | 23 posts | Array

Had this exact issue with a flexible panel last summer during a power cut prep setup — afternoon drops drove me mad trying to diagnose it.

Worth checking whether it's actually a temperature derating issue. Flexible panels stuck directly to metal roofs can hit 70°C+ surface temp on a clear afternoon, and Renogy's specs show significant output loss above 25°C. No airflow underneath makes it far worse than a racked rigid panel.

A few things I'd try:

• Check your MPPT's error log — is it actually cutting out or just throttling?
• Temporarily shade the panel mid-afternoon and see if voltage recovers immediately
• If you have a temp sensor on your MPPT, watch the correlation

Rigid panels with even a small air gap underneath perform noticeably better in my experience. Might be worth considering longer-term if this is for backup reliability.

DaleBen | 312 posts | Shropshire

@ForestSolar worth checking the panel surface temperature around that time of day. Flexible panels glued directly to a metal roof can get absolutely roasting in afternoon sun - far hotter than a rigid panel on standoffs would. Most cells lose roughly 0.3-0.5% efficiency per degree above 25°C, so if your panel surface is hitting 70-80°C (not uncommon on a dark van roof in summer) you're potentially losing 15-20% of rated output, plus voltage will sag noticeably.

Try putting your hand on it carefully at peak afternoon - if you can't hold it there it's too hot. Longer term some people run a thin air gap underneath if the adhesive allows, though obviously that's not always practical. What MPPT are you running and what voltages are you actually seeing? That'd help narrow it down further.

VivaroDream | Array

@ForestSolar I run a static caravan setup rather than a van but I've seen this same pattern with flexible panels generally — they trap heat underneath because there's no air gap like you'd get with a framed panel on standoffs. By early afternoon the panel surface can be absolutely roasting, and PERC cells especially derate badly past 25°C.

Worth logging your Victron (or whatever MPPT you're running) app data against ambient temp — you'll probably see the voltage drop correlate almost perfectly with peak roof temperature rather than peak sun angle.

If you can get even a few mm of airspace under it somehow, it makes a noticeable difference. Some people use thin foam strips around the edges rather than full adhesive coverage. Bit of a compromise on the "flush" look but the performance gain is real.