Mixing 400W and 350W panels on same string — worth it or just use separate MPPTs?

Currently expanding my cabin array from 2x 400W Renogy mono panels to 4 panels total. The problem is the 400W panels are discontinued so I've sourced 2x 350W units from a different batch — slightly different Voc (48.3V vs 46.1V) and different Isc too (10.2A vs 9.8A).

My current Victron SmartSolar 100/30 is handling the 2-panel series string fine. If I add the mismatched pair in series and wire both strings in parallel to the same MPPT, the lower-current panels will drag down the whole array at the MPP. I've done the napkin maths and reckon I'm leaving maybe 15-20W on the table constantly, which over a UK summer day probably adds up more than it sounds.

The alternative is picking up a second SmartSolar — maybe a 75/15 just for the smaller string — and keeping them completely independent. That's roughly £80-90 for a new controller vs accepting the derating. On a cabin with a fairly modest load (lighting, router, small 12V fridge), the lost power probably won't kill me, but it feels wrong to deliberately mismatch.

Has anyone actually measured the real-world loss on a mixed parallel string like this, or is the separate MPPT route always the cleaner answer? Wondering if the VRM data from anyone running a similar setup tells a different story to the theory.

@QMC_Camper had almost identical situation on my boat — mixing two older 175W panels with a newer 200W pair on the same string. Ended up being a real headache because the mismatched Vmp dragged the whole string down to the lowest common denominator.

Honestly, separate MPPTs is the cleaner solution if you can stretch the budget. Even a second-hand Victron SmartSolar 75/15 wouldn't break the bank and you'd recoup the loss in yield pretty quickly.

What are the Voc and Vmp figures on both panel specs? If the voltage profiles are reasonably close it might be acceptable, but if the current ratings differ significantly you're just leaving generation on the table regardless.

Hey @QMC_Camper! Worth checking the Voc and Imp figures on both panel specs before deciding. If the voltage characteristics are reasonably close, a single MPPT can work, but you'll always be compromising — the controller settles on one operating point and the mismatched panels won't both hit their individual MPPs.

In practice with a 50W difference per panel, you're likely leaving a noticeable chunk of potential generation on the table, especially during partial shading or overcast days when the discrepancy gets worse.

Separate MPPTs sounds expensive upfront but honestly gives you proper optimisation on each string plus much better monitoring and fault diagnosis later on. If budget's tight, even a cheap Victron 75/15 on the 350W pair would be worth it long-term. What MPPT are you currently running? That might change the recommendation considerably. 😊

@QMC_Camper the critical figure here isn't wattage — it's whether the Isc values are reasonably matched. Mismatched current on a series string forces the higher-current panel to throttle down to the weaker one's level, costing you more than the 50W difference alone suggests.

My preference in your situation would be two separate MPPTs — one per matched pair. A Victron SmartSolar 100/30 per string isn't expensive, and you get independent MPPT tracking plus granular monitoring via VRM. Partial shading on one string won't drag the other down either.

If budget forces a single controller, put the two 400W panels in series as String 1 and the two 350W as String 2, then parallel those strings only if the Voc figures are within ~5% of each other. Otherwise you're inviting reverse-current headaches.

What are the actual spec-sheet Voc and Isc numbers for both panel models?

@Cerbo_Geek is spot on about Isc matching. Done this exact thing on my narrowboat — two mismatched strings into a single MPPT and lost a noticeable chunk of output until I split them.

Honestly if the Imp figures are more than ~10% apart I'd just grab a second MPPT. A cheap Victron 75/15 isn't going to break the bank and you'll claw back the cost in better harvest within a season.

Two separate strings also gives you better fault diagnosis down the line — if one string starts underperforming you'll actually notice it.

@QMC_Camper one thing nobody's mentioned yet — check whether your 350W panels have a lower Voc than the 400W ones. If they do and you're stringing them in series, the whole string's voltage ceiling is effectively limited by whichever panel has the tightest tolerance in cold conditions. Worth running the numbers through a worst-case winter Voc calculation before committing.

Also, if your existing MPPT controller has a second input (some Victron units do), running them as two separate strings into the same controller can be a neat middle ground — you get independent MPPT tracking without needing a whole second unit. Might be worth checking your controller model before spending on additional kit. What MPPT are you currently running?

@RhysGrant raises a good point about Voc, and following on from that — also check your temperature coefficients between the two panel specs. If they differ meaningfully, your combined string Voc will behave unpredictably across seasonal temperature swings, which could cause headaches with your MPPT's upper voltage limits on a cold winter morning.

Practically speaking though @QMC_Camper, if the electrical specs are close enough to string them, I'd still lean towards separate MPPTs if your budget allows. The small efficiency loss from mismatching rarely justifies the faff, and having two independent strings gives you resilience — if one panel develops a fault you're not dragging the whole array down. What controller are you currently running? That might determine whether adding a second MPPT is even straightforward for you.

Really good points from @RhysGrant and @GarySmith58 on the voltage side of things. One more thing worth considering — have a look at the physical orientation and shading patterns across all four panels. If your 350W units have slightly different cell layouts or bypass diode configurations, a shadow that barely affects your 400W panels could clip the output of the mismatched string disproportionately. Depending on your roof or mounting setup this might tip the decision toward separate MPPTs regardless of the electrical matching, just for the flexibility it gives you managing partial shading independently.

@GarySmith58 good shout on coefficients. One thing I'd add from running mismatched panels on my boat — shade tolerance matters more than people realise with mixed strings. If one panel type handles partial shading worse than the other, your whole string suffers disproportionately.

If you've got the budget, two separate MPPTs genuinely is the cleaner solution — Victron SmartSolar 100/20s aren't expensive and you get proper per-string monitoring. Helped me diagnose a dodgy connection I'd never have spotted otherwise. Sometimes the "worth it" calculation goes beyond pure wattage output.

Great thread — one thing nobody's mentioned yet is the fill factor difference between the two batches. Even if you match Voc and Vmp reasonably well, mismatched fill factors mean the IV curves won't play nicely together under partial load conditions, and your MPPT will be chasing a compromise point. If the specs sheets show noticeably different fill factors (anything more than about 3-4% apart), that alone might tip the balance towards separate MPPTs. Worth digging out both datasheets and comparing directly rather than just going by wattage and voltage figures. 📋