MPPT-sizing at upper Voltage-limit

Been staring at my Victron MPPT spec sheet for the past hour like it's going to apologise to me personally.

So here's the thing — ran the Victron MPPT calculator for my static caravan setup, factoring in a proper British winter (not the -20°C drama of continental Europe, but I plugged it in anyway for worst case). Calculator spits out a Voc that sits right on the bleeding edge of the controller's rated maximum.

Now I know in practice Scotland rarely hits -20°C, but "rarely" and "never" are very different words when your kit is sat unattended for a fortnight in January.

My actual question for the thread:

How close to the absolute maximum Voc are people actually running their MPPTs in the UK?

I've got a 150/35 and I'm tempted to push the string sizing a bit further, but the datasheet says "absolute maximum" like it means it. I've seen folk online treating that figure as a hard brick wall, others treating it as more of a... strong suggestion.

For context — UK ground temperatures can still dip nastily overnight even if air temps stay modest. Anyone factoring in ground-mounted array temps differently to roof-mounted?

Also curious whether anyone's used the Renogy or EPever calculators and got meaningfully different results to Victron's own tool — or is it all roughly the same maths underneath?

Don't want to fry a £300 controller because I was optimistic about British weather. That's what the weather itself is for.

@NotAnElectrician80 What's your actual Voc figure and which MPPT are you looking at? The key thing people miss is that Victron's max PV voltage limit is an absolute cold-weather ceiling — you need to derate for your lowest realistic ambient temp using the temperature coefficient on your panel datasheet.

For UK installs I typically add a decent safety margin below the rated limit because January mornings can push Voc noticeably higher than STC figures suggest.

Quick checklist:

• What's your panel's Voc at STC?
• What's the temp coefficient (usually around -0.3%/°C)?
• Lowest expected temp at your site?

Run those numbers and you'll likely find you're cutting it closer than the calculator implied. What's the actual string configuration you're considering?

@NotAnElectrician80 what's the actual array config though? Series, parallel, mixed? That changes everything.

Victron's calculator is decent but it assumes worst-case Voc at low temps — which in the UK actually matters unlike somewhere sunny. Cold January morning your Voc spikes higher than the label says.

Had this exact headache with my static caravan setup. Ended up going one size up on the MPPT just to stop sweating about it. Slightly annoying cost-wise but cheaper than a fried controller.

Don't cut it fine on the voltage ceiling. Victron are clear — exceed the absolute max Voc even once and warranty's gone, controller's potentially gone too.

@NotAnElectrician80 the bit everyone forgets: Victron's max PV voltage limit isn't your operating headroom, it's your absolute never-exceed brick-the-controller cliff edge.

Your actual sizing constraint is Voc × temperature coefficient correction for your coldest expected morning. In the UK that's typically -10°C for most locations, and panels get spectacularly chatty at cold temps — Voc climbs noticeably above STC figures.

Rule of thumb I use: take your string Voc at STC, multiply by ~1.12-1.15 for UK winter conditions, then make sure that figure sits comfortably below the MPPT's maximum input voltage with actual margin remaining.

Victron's own sizing tool applies this correction automatically if you enter your location — worth double-checking what temperature floor it's assuming though.

@ZFS_OffGrid is right that series vs parallel totally changes the maths here.

Good points all round, but one thing worth adding: that cold-weather Voc spike is the killer nobody budgets for properly.

Take your panel's Voc, apply the temperature coefficient (usually around -0.3%/°C), then calculate at -10°C — brutal but realistic for a UK winter. If that figure nudges above your MPPT's absolute max PV input, you're potentially destroying the input stage on a cold clear January morning when irradiance is high and panels are still frigid.

For a static caravan specifically, panels aren't shaded by nearby structures typically, so that scenario will occur.

I ran this exact calculation before wiring my garden office array — ended up dropping one panel from a series string precisely because the corrected Voc crept too close to the 150V ceiling on my Victron SmartSolar 100/30. Margin isn't optional here, it's protection.