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

Running a Victron SmartSolar 100/30 on the narrowboat with two 200W panels wired in series (Renogy mono, Voc 24.3V each, so ~48.6V string). Got a spare 175W panel left over from the static that I'm tempted to bolt onto the roof and wire in parallel with the existing string — but the Voc on that one is 22.1V, so the voltages don't match.

I've read the theory about mismatched panels dragging down the higher-voltage string, and I know the MPPT will hunt for a compromise operating point rather than the true MPP of either. What I can't find is anyone who's actually lived with a lash-up like this for a season and measured the real-world losses. Ballpark figures from simulation tools suggest maybe 8–12% efficiency penalty, but those assume ideal conditions and don't account for partial shading on the cut-price panel I'd inevitably get on a cloudy Leeds–Liverpool stretch.

Has anyone run mismatched panels on a Victron MPPT and pulled the data off VictronConnect to see what the hit actually looks like day-to-day? Alternatively, would I be better off putting that 175W panel on its own cheap PWM controller just to keep things clean — even accepting the PWM efficiency loss on a 12V bank?

MattJones | 847 posts | Midlands

@Scouse Done exactly this on my shed setup for about 18 months now. Mixed a 260W and two 180Ws all feeding the same Victron 100/50 — works absolutely fine in practice. The MPPT just finds the best operating point across the whole string, so yes, the 175W will drag things down slightly when it's the limiting factor, but honestly the real-world losses are pretty marginal compared to simply having that extra panel generating something.

Main thing to check is your Voc doesn't creep over the controller's 100V limit on a cold morning — that's where people catch themselves out with series strings. Parallel might actually suit you better here given the voltage headroom you're already using.

What's the 175W's Voc? Worth running the numbers before committing either way.

MidlandsBoater | 1,203 posts | Midlands

@Scouse I've been running mixed wattages on my boat's Victron 100/30 for going on two years now — different manufacturers too, which folk often worry about more than the wattage difference itself. The key thing nobody mentions is that in a series string, your lowest Voc panel effectively sets the ceiling, so double-check your 175W's spec sheet carefully before wiring it in. The MPPT will find the best compromise point regardless, but if that spare panel has a notably different Vmpp you'll leave some harvest on the table. Parallel might suit you better there depending on your current ratings. What's the 175W's Voc and Vmpp? Worth posting the figures before committing — someone here will run the numbers with you.

DeclanJohnson | 412 posts | South Wales

@Scouse The main thing to watch with that 175W addition is the Voc mismatch when wiring in series — your string voltage will be uneven across panels and the MPPT will track to whatever the weakest link allows. Worth checking the 175W's Voc spec carefully before committing.

Parallel is probably cleaner here given the wattage difference, though obviously your current then climbs — make sure your cabling to the controller handles it comfortably.

One practical tip: keep an eye on the Victron app's yield history after adding it. You'll quickly spot if the mixed string is dragging performance down versus what you were getting before. The SmartSolar logging makes it dead easy to compare week-on-week.

What's the 175W's Voc out of interest?

AlanWilson | 634 posts | Yorkshire

@Scouse Worth checking your Isc figures carefully before committing. The MPPT doesn't care much about wattage mismatches — it's current and voltage that matter. If you're adding that 175W in parallel with your existing series string, you'll be mixing voltage architectures, which is a different headache altogether. What's the Voc on the 175W panel? If it's closer to 24V nominal, you'd potentially need to restructure the whole string.

Also bear in mind the lowest-performing panel will drag the others down in a series arrangement. Victron's MPPT is forgiving, but you won't ever see the theoretical combined wattage. That said, for a narrowboat where space is tight and you're just topping up a leisure bank, "good enough" is often genuinely good enough. What's your battery bank capacity?

MidlandsNomad | 634 posts | Array

Worth adding — the Voc of that 175W panel is the bit that'll catch you out. If it's a different cell count to your Renogys, your string voltage could get awkward fast. Check the datasheet carefully before wiring it in series.

I ran a mismatched 3-panel string on my Fogstar-backed system for about a year. The MPPT just clamps to the weakest panel's characteristics — you lose some harvest but nothing blows up. Parallel is usually the safer option with mixed wattages if you're not fussed about the extra cabling.

@DeclanJohnson is right about the Voc mismatch concern. On a 100/30 the 150V input ceiling gives you headroom, but check your worst-case cold morning Voc before committing.

VRM logs will show you if you're actually losing meaningful yield — worth monitoring for a few weeks.

CE_Builds | 847 posts | Array

Done exactly this on my garden office — mixed a 160W with two 200W panels on a Victron 100/20 for about 18 months now. Works fine if you match the string configuration carefully.

The real gotcha isn't Voc (others have covered that) — it's partial shading behaviour. When panels have different Isc ratings, the weakest panel limits the whole string. On a boat you'll have masts, rigging, bridges all throwing shadows at odd angles.

Seriously consider whether that 175W works better as a separate parallel string if your MPPT input current headroom allows it. Keeps the mismatch losses isolated.

MountainBarry | 287 posts | Array

Done exactly this on my cabin setup for going on three years now. Mixed a 250W Renogy with two older 190W panels on a single SmartSolar 100/50 — all in parallel, different manufacturers, different ages. The MPPT just finds the best compromise MPP and cracks on.

The honest truth? You lose some theoretical harvest, maybe 5-8% on mixed-irradiance days when the panels aren't seeing the same conditions. On full clear days it barely matters.

@Scouse the bigger practical question is whether you're wiring that 175W in series with your existing string or parallel. Series changes your Voc arithmetic significantly — worth running it through Victron's MPPT calculator before anything gets wired up. Parallel keeps voltages cleaner but you'll want to check your string current stays within the 100/30's 30A input limit.

BoatMick | 412 posts | Array

Running mixed wattages on my shepherd's hut — 2× 180W Renogy alongside a 160W panel I had spare, all on a Victron 100/20. Been fine for 18 months. The thing nobody mentions is that mismatched Vmp values can quietly drag your string down more than the wattage difference suggests. Worth plugging your actual panel specs into the Victron MPPT calculator before committing. Also check your 175W panel's temperature coefficients — if they differ significantly from the Renogy pair, cold winter mornings can throw your Voc higher than you'd expect.

DerekDixon | 412 posts | Array

Been running a mixed string on my narrowboat for about eighteen months — a 200W Renogy alongside a 150W panel I salvaged from the static when I upgraded it. The thing nobody tells you upfront is that it's not really the wattage mismatch that bites you, it's when the current ratings diverge significantly. Mine were close enough that I've lost maybe 8-10% theoretical output, which on a cruising boat where you're constantly shifting angle and shade anyway barely registers against the other variables. @MidlandsNomad is dead right about Voc — check that first, everything else is secondary.

Callum1982 | 156 posts | Array

Worth flagging something nobody's mentioned yet — before you wire that 175W in series, double-check its Voc matches reasonably closely with your existing panels. Your string Voc is already sitting at 48.6V, and adding a third panel in series could push you uncomfortably close to or over your SmartSolar's 100V input limit depending on temperature (Voc rises in cold weather, remember). Parallel might be the safer route here if the specs don't align well. Chuck the datasheets side by side first. What's the 175W panel's Voc?

SarahClark75 | 287 posts | Array

Good thread this. One thing I'd add — keep an eye on your temperature coefficients between the panels if they're from different manufacturers. If they diverge significantly, your MPPT will struggle to find a stable MPP on cold mornings when Voc spikes, and you'll lose more than just the wattage mismatch. I spent a frustrating winter wondering why my harvest was poor before I spotted it. Worth pulling the datasheets and comparing. @Callum1982 makes a fair point too — check those specs carefully before committing the wiring.