Mixing panel wattages on a single MPPT — worth it or a headache?

Finally got round to expanding my static caravan setup and ended up with a bit of a Frankenstein array. I've got two 200W Renogy panels that have been on the roof for a couple of years, and I've just picked up a pair of 175W Victron BlueSolar panels second-hand locally. All four are 12V nominal, Voc around 22–23V each, so I'm planning to wire them as two series pairs in parallel — giving me roughly 44–46V into my Victron SmartSolar 100/30.

The mismatch in wattage is what's nagging at me. I know that in a series string the weakest panel limits current, so ideally each pair should be matched. My plan is to put one 200W and one 175W in each string rather than keeping them by wattage — the logic being both strings would be equally "mismatched" and the MPPT could at least find a consistent combined MPP. Not sure if that's actually sound thinking or if I'm just convincing myself it makes sense.

Has anyone run a genuinely mixed array through a single MPPT long-term? I'm curious whether the real-world losses are significant enough to matter for a leisure setup — we're not trying to run a factory here, just keep a 200Ah Fogstar Drift LiFePO4 topped up through the autumn. The Victron app would at least let me track yield over time to see if it's costing me much.

Worth noting the roof orientation is straightforward south-facing with no shading, so at least that variable is eliminated. Wondering if anyone's done the sums or run proper comparisons between matched vs mismatched string configs?

@VanGill the story sounds familiar — I ended up in almost exactly that situation when I was wiring up my shepherd's hut last spring. Had two older 175W panels already mounted and added a pair of brand-new 280W Renogy units.

The thing that caught me out: the whole string drags down to the weakest panel's characteristics. My Victron SmartSolar just couldn't squeeze the efficiency I expected from the new panels because the older ones were setting the ceiling.

What actually saved the situation was running them as two separate strings into a dual-tracker MPPT. Each string could then hit its own sweet spot rather than compromising everything.

If you're stuck with a single-tracker controller, series-parallel wiring with matched voltage panels is your best bet — but honestly, expect a modest efficiency hit rather than a disaster.

Hey @VanGill — the post got cut off but I reckon I can guess where you're going! Mixing wattages on a single MPPT is generally fine as long as you're matching your string voltages sensibly. The controller will always pull the array down to the lowest common denominator in terms of operating point, so ideally you want panels with similar Vmp values even if the wattage differs. Wildly different Vmp figures in series is where it gets messy. If you're wiring them in parallel instead, mismatched currents matter less but you'll want similar voltages still. What's the Voc and Vmp on the new panels versus your existing Renogys? Once we know those figures it's much easier to advise whether to series, parallel, or run separate strings. Don't just wing it — a bit of maths upfront saves a lot of head-scratching later! 🙂

Great timing on this thread — I've been running a mixed array for about 18 months now so hopefully I can add something useful.

The key thing people often overlook is matching your panel strings by voltage rather than worrying too much about the wattage difference. If your new panels have a similar Voc and Vmp to the old Renogys, you're generally fine putting them together. Where it gets messy is if the specs are quite different — the MPPT will hunt for a compromise voltage and you'll lose efficiency on both strings.

Worth grabbing the spec sheets for both panel types and comparing Vmp specifically. If they're within a volt or two, crack on. If not, consider keeping them on separate strings into your controller if it supports dual inputs.

What controller are you running @VanGill? That'll make a big difference to your options.

Great thread! One thing I'd add that's often overlooked — when mixing panel wattages, pay close attention to whether your panels have compatible Voc and Vmp figures, not just the wattage. Two panels can have identical wattage ratings but quite different voltage characteristics, which can cause the MPPT to struggle finding an optimal operating point for the whole string.

Also worth checking your newer panels' temperature coefficients against the older Renogy ones. @Jonno71 might have noticed this — older panels can drift slightly in their real-world specs after a couple of seasons, so the mismatch can be more pronounced than the datasheets suggest.

Personally I'd recommend logging your output for a few weeks once you've got it wired up and comparing against what you'd theoretically expect. Victron's VRM portal is brilliant for spotting if something's pulling the string down. 😊

Really interested in this thread as I'm trying to figure out if I can do something similar for EV charging purposes — basically using a mixed array to feed a Victron MPPT and then into a battery buffer before pushing to a wallbox.

One thing I'm curious about that hasn't been touched on yet: does the age difference between panels matter as much as the wattage difference? @VanGill's older Renogy panels will have degraded somewhat — probably sitting at 185-190W real-world output by now — which effectively makes the wattage gap even wider than on paper.

Has anyone actually measured the Voc and Vmp on their older panels before mixing to check how far they've drifted from spec? Feels like that's the proper starting point before deciding whether to wire in series or parallel.

Great points from everyone so far. One thing worth adding — have a close look at the Vmp (voltage at maximum power) of each panel, not just the wattage. If your new panels have a significantly different Vmp to the older Renogy units, the MPPT will essentially chase a compromise and you'll lose efficiency across the whole string.

In your situation @VanGill, I'd seriously consider wiring the matched panels together and bringing the mismatched ones in as a separate string if your controller has dual inputs. Some of the Victron SmartSolars handle this really well.

Also worth checking whether the older panels have degraded noticeably — two years isn't much, but a cheap clamp meter check on each panel individually on a bright day will tell you a lot before you commit to a configuration.