Anyone else finding MPPT sizing confusing when mixing old and new panels?

Right then, bit of a head-scratcher here. I've got a cabin setup that's been growing organically over the past couple of years — started with two 200W Renogy panels wired in series, then last spring I bolted on a third panel from a different batch (slightly different Voc and Isc specs). The whole lot feeds into a Victron SmartSolar 100/30.

Here's the thing — when I sit down to properly calculate whether my MPPT can handle the combined array, I keep second-guessing myself on which figures to use. Do I calculate worst-case Voc using the coldest realistic UK morning temp (I'm in Scotland, so we're talking potentially -10°C on a hard frost) and apply the temperature coefficient to each panel individually, then add them in series? Or do I just use the spec sheet Voc and add a rough 25% buffer and call it a day?

The mismatched panel situation complicates things further. The newer panel has a slightly higher Voc at STC — 41.2V vs the older ones at 40.1V. In series that's only a volt difference at the moment, but at low temps the coefficients might push things differently depending on the panel's individual spec. Has anyone actually worked through this properly rather than just guessing and hoping Victron's input protection saves them?

@JackeryGuy mixing panels is basically the off-grid equivalent of wearing one old sock and one new one — technically works, but something's always going to feel off 😄

Key thing with mismatched panels in series: your MPPT will only ever pull current up to whatever your weakest panel can deliver, so you're essentially bottlenecking yourself.

Practically speaking, size your Victron SmartSolar to the combined Voc of your series string (with the standard 1.25 safety multiplier), not the nameplate wattage — that's where people come unstuck.

Separate arrays into their own MPPT inputs if the specs are wildly different; two smaller Victron units often outperforms one oversized one in real-world yield.

What are the actual Voc and Isc figures on both sets of panels? That'll tell us whether you're genuinely losing meaningful output or just worrying for nowt.

@JackeryGuy going through almost exactly this with my van build right now. What tripped me up was that the MPPT needs to be sized for the array's Voc at cold temperatures, not just the panel ratings at STC — caught me right out when I was speccing a Victron SmartSolar.

Worth checking: when you wire mismatched panels, does your weakest panel's Isc effectively become the limiting factor in series, or are you mixing series/parallel? That changes the calculation quite a bit.

Have you run the numbers through Victron's online MPPT calculator? It actually handles mixed arrays reasonably well and flags if you're undersized. Might save a lot of head-scratching before you commit to anything.

The bit people consistently get wrong is Voc temperature correction — your coldest expected morning temp in the UK can push panel voltages up by 10-15% above the rated figure. Worth grabbing Victron's MPPT calculator and plugging in your actual panel specs separately rather than treating mixed strings as a single array.

If your old Renogy 200W and newer panels have different Vmp values, series wiring is going to drag performance down to the weakest link regardless of what controller you fit. Ran into this myself when I bolted a couple of extra panels onto my original array — current pulled down noticeably until I rewired into separate MPPT inputs.

Parallel strings work better here if Voc stays within controller limits, or consider whether a second smaller MPPT just for the legacy panels is cheaper long-term than the efficiency losses.

@Shunt_Geek is absolutely right about the cold Voc correction — worth adding that the Victron MPPT calculator tool handles this automatically if you plug in your panel specs and postcode region. Dead useful.

The other thing I'd flag for @JackeryGuy specifically: mismatched panels in series is where it gets properly gnarly. Your older Renogy 200Ws will almost certainly have degraded slightly, so the string's output gets dragged down to the weakest panel's Isc. If your new panels have a higher current rating, you're essentially wasting capacity.

In my static caravan setup I ended up separating old and new panels onto independent MPPT inputs rather than fighting the mismatch. Two smaller MPPTs can actually work out cheaper than one oversized unit, and you get useful redundancy into the bargain.