Mixing panel wattages on a single MPPT — is it actually worth the hassle?

Currently sorting the solar setup on my narrowboat and I've got two 175W panels already mounted on the roof, but I've managed to get hold of a decent 200W panel cheaply that I want to add to the array. All three would feed into a Victron SmartSolar 100/30.

From what I've read, mixing wattages in a series/parallel string causes the MPPT to track at the lowest common denominator, so you lose efficiency from the higher-rated panel. Is that accurate, or is it more nuanced than that? I've seen conflicting stuff about whether it matters if they're in parallel vs series.

The Voc and Vmp on the 200W panel are reasonably close to the 175W ones — within about 3V — so I'm wondering if that changes things. Has anyone actually done this and measured the real-world loss, or is it negligible enough that I shouldn't stress about it?

@KellyRobinson the main thing to watch is Voc and Vmp — mixing wattages is generally fine as long as the panels are matched in voltage, not wattage. Chuck mismatched voltages in series and you're asking for trouble.

If you're wiring them in parallel, the 200W will effectively be dragged down to perform like the 175W panels anyway — the MPPT finds one compromise point for the whole string. Not ideal but not catastrophic either.

Had a similar bodge-job on my shepherd's hut setup for a while. Worked, just wasn't squeezing every watt out. Eventually separated them onto two MPPTs and it made a noticeable difference on cloudy days.

What controller are you running? A Victron SmartSolar with two trackers would solve the problem neatly if you're not already on one.

@KellyRobinson worth adding to what @GazAllen was saying — the bigger practical issue on a narrowboat is shading. With mismatched panels in series, one shaded panel drags the whole string down badly. Given how often bridges, trees and lock structures cast shadows on boat roofs, I'd seriously consider running that 200W panel on a separate MPPT input if your controller supports it, or even a small dedicated controller just for that panel. Keeps things independent so shading on one doesn't murder output from the others.

Also check your 200W panel's Imp against the 175W panels — if the current ratings are quite different and you're wiring in parallel, the lower current panel becomes the limiting factor. What controller are you running? That'll help narrow down the best wiring config for your situation.

The narrowboat roof curse strikes again — you'll get more shade from passing bridges than a vampire on holiday, so if you haven't already, wire those mismatched panels in parallel rather than series and let your MPPT sort itself out.

@KellyRobinson one thing nobody's mentioned yet — check your MPPT's input voltage range before anything else. Adding that 200W panel will change your combined Voc, and on a narrowboat you're probably working with a fairly compact controller. If you're wiring all three in parallel you'll be fine voltage-wise, but you'll lose some efficiency because the 200W panel will likely have a slightly different Vmp to your 175W pair. The controller can only chase one maximum power point at a time, so one panel will always be operating slightly off its optimum. Not a dealbreaker by any means — you'll still harvest more energy overall than running just the two — but worth knowing going in rather than being disappointed by the figures. What controller are you running?

@BrianKnight86 makes a solid point on voltage — worth running the numbers properly, not just guessing.

On the mixing question itself: I've done similar with my setup and the main thing people don't realise is the 200W panel will likely have a different Vmp. String them together and your MPPT tracks a compromise voltage, so both panels underperform slightly.

Parallel wiring is your friend here if the Vmp values are close enough — losses are much smaller that way. Victron's MPPT calculator is decent for checking if it's viable.

Honestly though, on a narrowboat where roof space is the limiting factor, squeezing in that 200W for cheap is probably worth a small efficiency hit. Free watts are free watts.

@BrianKnight86 and @DodgyDrifter have the voltage side covered, so I'll focus on the wattage mismatch itself.

Done this exact thing on my van build — mixed a 200W with two 175W panels on a Victron SmartSolar 100/30. The critical thing people overlook isn't wattage, it's Vmp matching. If those panels have sufficiently close Vmp figures (within ~5%), stringing them together works reasonably well in practice. Where it genuinely hurts you is partial shading scenarios — the lower-performing panels drag the string's operating point away from the 200W panel's true MPP.

On a narrowboat specifically, @PanelRob's bridge-shade point is worth taking seriously here. If one panel regularly drops into shade, that mismatch penalty compounds noticeably.

Worth checking: are all three panels the same cell technology (mono vs poly)? Mixing those amplifies the Vmp drift across temperature ranges considerably.