MPPT 250/100 - loading panels

Been mulling this one over since I saw a similar conversation doing the rounds elsewhere, and it's relevant to something I've been planning for the narrowboat's shore-power setup over winter.

Running a Victron MPPT 250/100 and wondering about how aggressively you can load the panel strings — specifically when you've got panels split across orientations.

My situation is slightly different: I'm looking at east/west splits on a static installation rather than a boat roof, but the principle's the same. When your panels are on opposing faces, you're rarely getting both strings hammering at full tilt simultaneously. Which begs the question — can you lean harder into the panel wattage than you'd otherwise dare?

The 250/100 is rated to 5,800W input on a 48V system. Straightforward enough on paper. But if you've got, say, 3kW east and 3kW west, and they're staggered throughout the day, are people finding they can push the nameplate figures higher than the controller's rated output without grief?

I'm sceptical of the "just oversize and it'll clip harmlessly" school of thought — I've seen that go sideways when you get unexpected clear days in February and everything peaks together.

Anyone running a similar split-array setup with a 250/100 here in the UK? Curious what your real-world harvest looks like versus what the calculator promised, especially through autumn and winter when the angles matter a lot more.

What's your string configuration and have you seen the controller struggle at all?

Not sure if this applies to your specific situation @Spider, but worth flagging — the 250/100 refers to max open-circuit voltage (250V) and max charge current (100A). The bit that catches people out is that your panel array's Voc at lowest expected temperature mustn't exceed that 250V ceiling. Cold days push Voc up noticeably, and I've read of people inadvertently killing their MPPT that way.

What's your actual panel configuration? Series, parallel, or mixed strings? That'll determine whether you're at risk on the voltage side. For a narrowboat winter setup I'd be fairly cautious — moored somewhere exposed could mean genuinely cold mornings.

@Spider this is something I've been wrestling with for my own boat setup too! Quick question — are you planning to keep those panels in a single string or split them across two MPPTs?

I ask because on my tiny house project I went with a Victron SmartSolar 250/100 and the string voltage headroom in winter (when Voc climbs on cold mornings) caught me out more than I expected. Worth running your actual panel specs through the Victron MPPT Excel calculator — it accounts for low-temperature Voc spikes which can be surprisingly significant on a narrowboat moored somewhere exposed.

Did @Clive1978's point about Voc limits change your wiring plans at all, or were you already accounting for that in your original configuration?

Something I learned the hard way with my shepherd's hut build — those voltage figures aren't just theoretical ceiling numbers. On a sharp February morning in the Lakes, my panels were pushing open-circuit readings I hadn't anticipated because I'd only tested in August.

@Spider the thing that caught me out was calculating string voltage at Standard Test Conditions rather than at realistic low-temperature peaks. Victron's MPPT calculator lets you plug in your minimum expected ambient temp, which gives a much more honest worst-case Voc figure.

Worth doing that calculation before you commit to a string configuration — the headroom between your calculated peak Voc and that 250V ceiling shrinks faster than you'd expect once you factor in a proper cold snap.

The bit nobody mentions: VOC drops as panels heat up, so your "dangerously close to 250V" string in winter morning cold is actually your worst case — factor in -10°C and VOC can spike 10-15% above the STC rating on the datasheet.

Victron's MPPT calculator will tell you the temperature-corrected max, don't just eyeball it.

Also worth remembering the 100A is the output current to your batteries — you can absolutely wire in more panel wattage than "100A × battery voltage" suggests, because the controller clips at its rated output regardless. Run the numbers properly rather than leaving headroom by accident like I did first time round with my motorhome array. 😅