Mixing panel wattages on a single MPPT — anyone actually done this long-term?

After finally getting round to expanding my static caravan setup, I've ended up with a bit of an awkward situation. I've got two older 175W panels that were already wired to a Victron SmartSolar 100/30, and I've just acquired a pair of 410W panels (Renogy mono, 41.4Voc each) at a price I couldn't turn down. Tempted to string the new ones together and run them into the same controller as a second pair in parallel with the existing string, but the Voc and Isc figures obviously don't match.

From what I understand, mismatched panels in parallel will pull the higher-performing string down to the weaker one's voltage, which sounds painful in winter when the 175s are already marginal. The 100/30's 100V input limit is also making me nervous — two 410W panels in series would hit 82.8Voc before temperature correction, which starts getting close when you factor in a cold January morning.

Has anyone actually run mismatched wattages long-term on a single MPPT and tracked the real-world losses? Or is the cleaner solution just to grab a second controller — possibly another SmartSolar — and keep the two arrays completely independent? With VE.Smart Networking the two Victron units would at least talk to each other and share temperature/voltage sensing, which seems like it'd offset some of the cost.

Keen to hear from anyone who's done proper monitoring on this rather than just guessing at the losses.

RussThomas | 847 posts

@Nessa yes, been running a mixed array on my SmartSolar 100/50 for about three years now — two 250W panels alongside an older 180W panel, all in series. Works absolutely fine day-to-day. The key thing people miss is that the MPPT will always chase the lowest common denominator in terms of current when panels are in series, so you want to check your string calculations carefully. Mismatched Voc and Isc figures matter more than the wattage labels themselves.

Worth downloading the Victron MPPT Excel spreadsheet calculator if you haven't already — plug your panel specs in and it'll tell you whether you're within safe limits. What are the actual specs on both sets of panels? Voc and Isc figures from the datasheet would help diagnose whether you'll see any meaningful losses. Generally though, Victron controllers handle mixed arrays better than most.

EdGrant91 | 312 posts

@Nessa the main thing worth watching with mixed wattages is that your controller will essentially track to whatever the lower-performing panels dictate — so if your older 175W panels are partially shaded or degraded, they can drag down the whole string. Worth checking if the Voc figures are reasonably matched before wiring them in series. If the voltages are quite different, parallel might serve you better depending on your 100/30's input limits.

I ran a similar bodge-job setup on my narrowboat for a couple of winters — two 200W and one 130W — and it worked fine honestly, just never quite hit the theoretical combined output. The Victron app is brilliant for spotting if something's underperforming. What are the Voc ratings on your new panels compared to the 175s?

BordersExplorer | 1,204 posts

One thing neither @RussThomas nor @EdGrant91 has touched on yet — pay close attention to your Voc and Isc figures across both panel generations, not just the wattage labels.

I ran a similar hybrid array on my shepherd's hut (two older Renogy 160W alongside a newer 200W panel) and the mismatch in temperature coefficients caused the MPPT to hunt noticeably on cold bright mornings in the Borders. Not damaging, just inefficient.

Worth pulling the datasheets and comparing:

• Vmpp — ideally within ~10% of each other
• Temperature coefficient of Pmax — older panels often perform relatively better in cold conditions

If the coefficients are quite different, consider whether a parallel rather than series arrangement suits you better. With a 100/30 the lower voltage headroom of parallel is usually fine for a static setup, and it reduces the mismatch penalty considerably.

AlexPalmer60 | 203 posts

Running a mixed array on my garden office setup — two 200W Renogy panels paired with an older 160W I had spare. Been about 18 months with no drama on a SmartSolar 75/15.

The thing I'd add beyond what @EdGrant91 mentioned — try to match your Voc values as closely as possible rather than worrying too much about wattage. If you're wiring in series especially, mismatched voltages can cause more headaches than mismatched power ratings.

Also worth checking your string calculations haven't pushed you too close to the controller's input voltage ceiling with the extra panels. Easy to overlook when you're just adding what you've got spare.

The Victron app gives decent visibility on harvest so you can judge whether it's actually worth the effort of a more uniform array down the line.

OffGridGeek | 847 posts

Done this on the narrowboat for three years — two mismatched Fogstar-era panels bodged onto a Victron 100/20 — and the only thing I'd add that nobody's mentioned is check your Voc totals in winter cold, because panels get spicy when temperatures drop and you'll be surprised how close to the controller's voltage limit you creep when you've mixed specs and stopped paying attention.

TransitConvert | 67 posts

Jumping in as someone who's just been through the exact same head-scratching process for my garden office build. One thing I haven't seen mentioned yet — did you check whether your older 175W panels have a noticeably different Vmp to the new ones? I found that even a couple of volts difference between panel strings caused my Victron to essentially optimise for one set and leave the other underperforming. The SmartSolar's VictronConnect history graphs made this pretty obvious once I knew what I was looking at. Worth pulling up your yield data and seeing if there's a plateau where you'd expect a curve. Might point to whether it's actually worth rewiring into separate strings rather than just accepting the efficiency hit.

VanGill | 1,204 posts

The key thing nobody's mentioned yet — string voltage compatibility matters far more than wattage matching. Before wiring those new panels in, check the Voc of your combined string doesn't push past the 100V input limit on that SmartSolar 100/30 when cold.

I've been running a mixed array on my static for two years (2× 180W older panels + 1× 250W newer one). The MPPT finds a compromise operating point, so you do lose a small percentage of potential yield — I'd estimate 5–8% in my case based on VictronConnect logging — but it's absolutely functional long-term.

Worth enabling the equalisation logging in VictronConnect to see exactly what you're sacrificing.

LindaPrice87 | 203 posts

Running a mixed array on my boat — two 200W Renogy panels paired with a leftover 160W — through a Victron SmartSolar 75/15 for going on 18 months now. @VanGill is absolutely right about voltage being the critical factor. What I'd add is watch your Voc in cold weather; my array nearly kissed the controller's limit on a frosty January morning on the Thames. Punch the numbers through Victron's MPPT calculator before committing. The wattage mismatch itself has been a non-issue day-to-day, honestly.

StuThompson | 412 posts

Done this for about three years now on my shepherd's hut — two 250W panels alongside a solitary 190W, all feeding into a Victron 75/15. The short answer is yes, it works, but you'll sacrifice a bit of efficiency since the MPPT tracks the lowest common denominator during partial shading. @VanGill is right that string voltage is your first check. What I'd add is: keep an eye on your Voc figures across all panels before committing. As long as you're safely within the controller's limits, the real-world losses are often smaller than the theory suggests. Three winters in and no complaints here.