Maximizing mppt without damage

Running a Victron SmartSolar 100/30 on my motorhome and trying to squeeze every last watt out of it without cooking the thing — anyone done the maths properly on this?

Current setup is a single 200W panel, but I've got roof space for another and I'm wondering whether to go series or parallel before I hit either the voltage ceiling or the 30A input limit.

The bit that's doing my head in: panel specs are quoted at STC (lovely lab conditions, not a Tesco car park in Wolverhampton), so real-world Voc on a cold January morning can creep up noticeably. Tried running the numbers through one of those AI tools and got three different answers, which is exactly as useful as asking a golden retriever.

Specifically wondering:

• If I add a second identical 200W panel in series, am I genuinely safe on cold-day Voc, or am I gambling?
• Would parallel be the smarter play to stay under the current limit instead?
• Is there a proper UK-sourced calculator or wiring guide that accounts for realistic winter temps rather than just STC figures?

Panels I'm eyeing are from Renogy — decent enough for the money but the spec sheets aren't always the clearest on temperature coefficients.

Any seasoned motorhomers or van dwellers who've worked this out properly, I'd love to know how you approached it rather than just winging it and hoping Victron's protection kicks in before anything goes pop.

@BaySoul the 100/30 is rated 30A output, so your battery voltage dictates max panel wattage — at 12V that's roughly 360W theoretical ceiling, but Victron themselves recommend staying around 10-15% headroom to account for cold-temperature Voc spikes.

The bit people miss is the 100V input limit — series strings can easily breach that on a cold morning. Check your panels' Voc at -10°C, not STC. I learned this the hard way before properly reading the datasheet.

What's your panel configuration — series or parallel? And are you on lithium or lead? Makes a difference to how aggressive you can push the absorption stage without stressing the controller.

@DucatoProject is right on the 360W ceiling, but there's a bit more nuance worth knowing.

The 100/30 input limit is right there in the name — 100V Voc max. That's the one that'll actually kill your controller if you ignore it, especially on a cold morning when panel Voc spikes. Victron's own guidance says derate by around 25% for cold climates.

On a motorhome I'd also factor in partial shading from roof vents, skylights, etc. Real-world harvest is rarely what the numbers suggest.

If you're running 12V LiFePO4 (Fogstar Drift or similar), absorption voltage is lower so your charge current window is shorter — worth knowing when you're chasing efficiency.

What panels are you actually running? Voc and Isc figures would help give you a proper answer rather than guesswork.