Anyone else running mismatched panels in series? Curious about real-world losses

Been running two 200W panels on my narrowboat for a couple of seasons — grabbed a second one cheap off eBay but it turned out to be a slightly different Voc (44.2v vs 41.8v on the original). Wired them in series into a Victron SmartSolar 100/30.

The MPPT handles it fine as far as I can tell, but I'm never quite sure if I'm leaving meaningful power on the table. On a decent summer day I might see 280-300W peak rather than anything close to 400W, though shade and angle are probably doing more damage than mismatched specs.

Has anyone actually measured the difference before/after swapping to matched panels? Or is the conventional wisdom about mismatch losses a bit overblown in practice when you're using a decent MPPT?

SteveGreen74 | 847 posts | ⚡ Solar Enthusiast

@MuddyFisher I ran a similar setup on my shed system for about 18 months — mismatched Vocs aren't the biggest issue in series actually, it's mismatched Isc (short circuit current) that really kills you. The panel with the lower current rating effectively becomes the bottleneck for the whole string.

With your voltage difference being relatively small, you're probably losing less than you'd expect — maybe 3-8% depending on conditions. The losses are worse at low light when the panels behave more differently from each other.

Worth logging your actual output against what the manufacturer specs suggest and seeing where you land. What MPPT controller are you running? A decent one will hunt around and find the best compromise point rather than just sitting at a fixed voltage. Makes a real difference with mismatched strings.

TrevorRoberts75 | 203 posts | 🔋 Off-Grid Learner

Watching this thread with interest — got a similar dilemma on my static caravan setup. Two 175W panels, both Renogy, but one's a newer batch with slightly different specs to the older one.

Quick question for anyone who knows: does the mismatch penalty get worse in partial shade, or is it mainly a full-sun issue? I've read conflicting things about whether an MPPT controller (running a Victron 75/15 here) compensates enough to make it a non-issue, or whether there's still meaningful loss regardless.

Also wondering — does the Voc difference matter more than the Isc difference when wiring in series? My gut says current matching is the real bottleneck but I'm not confident enough to state that as fact.

DriftMaster | 1,432 posts | ⚓ Liveaboard

@MuddyFisher The Voc difference is less of a concern than mismatched Vmp and Isc, honestly. Your real losses come from the current being dragged down to whichever panel produces less — typically the weaker one dictates the string. With an MPPT controller you'll claw back some efficiency compared to PWM, but you're still leaving a bit on the table.

Worth checking both panels' Imp figures specifically — if those are reasonably close (within ~5%), your real-world losses are probably only 3-8% rather than anything catastrophic. On a narrowboat you're also often dealing with partial shading from trees and bridges anyway, so the mismatch may be the least of your worries!

What controller are you running? That'll change the answer quite a bit.

FellLover | 612 posts | 🏕️ Off-Grid Tinkerer

@MuddyFisher worth checking your Vmp figures rather than just Voc — that's what actually matters when the panels are working. Had a similar situation on my static caravan setup, two panels with noticeably different Vmp and the MPPT (Victron 100/30) just tracked somewhere in the middle. Lost maybe 8-10% on sunny days but honestly less than I expected on overcast days which is most of the time in the UK anyway.

If you're on an MPPT controller you've got more protection than a PWM would give you. What are you running?