Mppt input restrictions

Been mulling over something similar with my current redesign and thought it worth throwing open for discussion here.

I'm running three 200W panels in series — combined Voc sits around 112V, which comfortably fits within the 100/50's input ceiling. The Victron MPPT calculator keeps recommending the 100/50, but honestly I'm questioning whether that's overkill given real-world output figures versus nameplate ratings.

In practice, I'm rarely seeing more than 60–65% of theoretical max current on the output side, even on clear summer days in my location. Clouds, orientation, temperature derating — it all chips away at those headline numbers. Which leads me to wonder: is the 100/30 a legitimate option here, or am I playing with fire by undersizing the controller relative to what the calculator suggests?

The key question I keep coming back to is whether the MPPT input restrictions are about protecting the controller's MOSFET switching components or the output wiring/fusing. If it's primarily the latter, then running a 100/30 with appropriately rated output cabling feels more defensible. If the former, then the calculator's recommendation is non-negotiable regardless of observed real-world figures.

Has anyone here deliberately sized down their Victron MPPT based on observed rather than theoretical output — and lived to tell the tale? Particularly interested in setups running LiFePO4 (I'm on Fogstar Drift cells) where absorption phases are short and bulk current demand is sustained but predictable.

Would also be curious whether Victron's warranty position shifts if you're technically within Voc limits but pushing output amps beyond the rated maximum during a freak cold, clear day.

@ZoeGrant that 112V Voc is fine until a cold Scottish morning drops it another 10% and your Victron suddenly becomes a very expensive paperweight 🏴󠁧󠁢󠁳󠁣󠁴󠁿❄️

@OffGridHamish makes a fair point on temperature correction — worth running the numbers properly rather than guessing.

On the MPPT side, the actual limit that catches people out isn't just absolute Voc but what the controller sees at startup before the battery pulls it down. I've seen a Victron 100/30 get confused when panels were connected cold with a near-depleted battery — it logged an overvoltage fault even though nominal Voc was technically within spec.

For the narrowboat I use a 150/35 with two strings, and I always leave myself at least 15% headroom above calculated cold Voc. Victron's own sizing tool accounts for this but people skip it.

Worth checking your specific controller's absolute maximum input — some Victron units have a hard cutoff that's lower than the model number implies for sustained input.

@ZoeGrant to add to what @OffGridHamish is getting at — most Victron datasheets quote an absolute maximum input voltage, typically 150V on their SmartSolar range, but that's not the same as a safe operating ceiling. You really want your temperature-corrected Voc sitting comfortably below that, ideally with a 10-15% buffer built in.

As @Spud79 says, run the proper calculation using your panel's temperature coefficient (usually expressed as %/°C on the datasheet) against your likely record low ambient temp. For most of the UK that's around -10°C but further north you'd want to be more conservative.

Also worth checking whether you're series-stringing into an MPPT rated for that voltage class — some of the smaller Victron units have a 100V max, which your 112V Voc would already breach before any cold correction.

@ZoeGrant learned this the hard way with my shepherd's hut setup — cold mornings are brutal on Voc. The Victron SmartSolar range has a hard cutoff and if you trip it even once you can fry the MPPT input stage, warranty gone too.

Worth using the Victron MPPT calculator — plug in your lowest expected temp and it'll spit out the corrected Voc. For most of the UK that means calculating down to around -10°C to be safe.

If you're right on the edge, dropping to two panels in series and adding a third string in parallel might be the cleaner solution rather than gambling on it.