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

Got a bit of a Frankenstein situation developing on my garden office roof. Started with two 200W Renogy mono panels wired in series, running into a Victron SmartSolar 100/20. Works fine. But I've just been offered two 175W panels from a mate clearing out his static caravan setup — same Voc roughly (around 44V each), slightly different Imp.

Tempted to add them as a second string in parallel with the first, but the mismatched wattage is nagging at me. I know the MPPT will find the combined peak, but in practice does the lower-output string drag things down or does each string effectively do its own thing current-wise?

The 100/20 can handle the combined Isc fine on paper, and the Voc in series stays well within limits. My main concern is whether I'm leaving meaningful generation on the table compared to just selling the 175s and saving up for two matched 200W panels.

Anyone running mismatched strings on a Victron or similar and got real-world data on the efficiency hit?

Lefty91 | 📍 West Yorkshire | ⚡ Posts: 847

@BoxerProject yes, done exactly this for about 18 months now. I've got two mismatched panels feeding my SmartSolar 75/15 — different ages, slightly different wattages — and honestly the Victron handles it without complaint.

The thing to watch is keeping your Voc figures compatible when wiring in series, not obsessing over matching wattages. The controller will simply harvest whatever the weakest link allows in series. Parallel adds a bit more complexity with the current mismatch but it's manageable.

What panels are you planning to add? If you can share the spec sheets, particularly the Voc and Imp figures, that'd help assess whether it's sensible. The Victron Connect app is brilliant for spotting any oddities once you've got it running.

MoorHermit | 📍 Array | ⚡ Tiny House / Emergency Backup

Running a mixed array here for going on two years — 2x 175W older Renogy panels plus a 250W mono I sourced secondhand, all feeding a Victron SmartSolar 100/30.

The key thing nobody mentions: check your Voc figures carefully before wiring in series. Mismatched wattages matter far less than mismatched voltages. My 250W panel has a slightly higher Voc, so I did have to recalculate my string voltage to stay safely within the controller's 100V input limit.

Performance-wise, the MPPT finds a compromise point — you won't extract full theoretical output from every panel simultaneously, but in practice the losses are small. My Victron app data shows maybe 4-5% below calculated maximum on clear days.

Worth running your numbers through Victron's MPPT calculator first though.

BoatSteve | 📍 Array | ⚡ Tiny House / Shepherd's Hut / Solar

The voltage matching is what catches people out more than wattage. Your 100/20 will just harvest whatever the lowest-voltage string dictates if you parallel mismatched panels — worth running the numbers before you bolt anything down.

On my shepherd's hut I've got a 175W and a 160W Renogy running in parallel into a Victron SmartSolar 75/15. Different vintage panels, slightly different Voc. Been absolutely fine for 14 months, the MPPT just finds the curve and gets on with it.

Main thing I'd flag: make sure your new panel's Voc doesn't push the combined series voltage beyond the controller's input limit. Seen people fry a perfectly good Victron that way. Check the datasheet temperatures too — cold mornings in the UK can spike Voc noticeably higher than rated.

Mel1980 | 📍 Array | ⚡ Cabin / Solar

Been running a similarly chaotic roof situation for three years — the MPPT doesn't care about your mixed wattage feelings, it just finds the best compromise voltage and gets on with it, like a very stoic British electrician.

Shaun1970 | 📍 Derbyshire | ⚡ Posts: 312

@BoxerProject I've been running a mixed array on my workshop for nearly three years — 2x 250W panels paired with a stray 180W I picked up secondhand. All wired in series into a Victron 100/30.

The key thing nobody mentioned yet: keep an eye on your Vmp figures across the panels, not just the wattage. Mine were close enough (around 30-31V each) that it's worked a treat. Where people come unstuck is wildly mismatched Vmp pulling the whole string down to the weakest panel's operating point.

The SmartSolar's VictronConnect app is brilliant for spotting if something's underperforming. I'd say just go for it, but do your homework on the spec sheets first before committing to any particular panel. 👍

HazelSoul | 📍 Array | ⚡ Off-Grid / Tiny House / Narrowboat

Running mixed wattage on my narrowboat setup — 2x 175W older Renogy panels alongside a 200W — all into a Victron SmartSolar 75/15. Been about 18 months now, no dramas.

Main thing to watch is your Vmp values when stringing in series. If they're close enough, you won't lose much. Mine are within about 2V of each other so the MPPT finds a reasonable compromise.

Where it gets messier is parallel strings of mismatched panels — the lower Voc panel can drag things down more noticeably. Series is generally the tidier option with mixed kit.

Victron's Connect app is handy for spotting if something looks off in the yield data. Just keep an eye on it the first few weeks and you'll know quickly enough if it's causing issues.

LakelandNomad | 📍 Array | ⚡ Emergency Backup / Solar / Boat

Worth flagging something nobody's mentioned yet — temperature coefficients between mismatched panels can bite you harder than wattage differences alone. If your new panels have a significantly different Voc temp coefficient, your series string voltage on a cold Lakeland winter morning could creep uncomfortably close to your Victron's 100V input ceiling. The SmartSolar 100/20 has no margin for error there.

I've run mixed wattages on my boat installation (two 175W Renogy panels paired with a salvaged 100W panel of unknown provenance) for about 18 months — the MPPT handles it fine operationally, but I always calculated worst-case Voc at -10°C before wiring anything up.

Check the datasheets on both panel generations and run the cold Voc calculation. Victron's MPPT calculator tool makes this straightforward. That's your real limiting factor, not the wattage mismatch itself.

HeathOllie | 📍 Array | ⚡ Posts: Garden Office Enthusiast

The critical thing nobody's spelling out clearly: mixing wattages is fine, mixing Isc values is where you get into trouble on a series string. On my garden office setup I've got two mismatched panels in series feeding a Victron SmartSolar 75/15 — different wattages but crucially similar short-circuit currents, so the lower-current panel doesn't become a dead load choking the string.

Check your panels' Isc figures on the datasheets, not just headline wattage. If they're within ~5% of each other you'll lose very little in practice. The MPPT handles voltage mismatch far better than current mismatch.

LesKnight | 📍 Yorkshire | ⚡ Off-Grid Cabin / Solar / DIY

Been running a mixed array for about three years now — 2x 250W and 1x 180W all feeding a Victron 100/30. The main thing I'd add that nobody's touched on yet is to keep an eye on your temperature coefficients when mixing panels from different manufacturers. If they're responding differently to heat, your strings can get a bit argumentative on hot summer days. Not catastrophic, just leaving performance on the table. Victron's VRM portal is genuinely useful for spotting these inefficiencies over time. Worth logging a full summer before committing to any permanent wiring decisions.

ThistleTel | 📍 Array | ⚡ EV Charging / Solar / Off-Grid

Something worth adding to what @LesKnight's long-term experience confirms — the Victron SmartSolar handles mismatched strings better than most because its perturb-and-observe MPPT algorithm is reasonably robust at finding a compromised global peak. What actually kills efficiency in mixed wattage setups isn't the controller struggling, it's when you mix different Vmp values in series. Wattage differences matter far less than voltage mismatch. If your new panel's Vmp sits within roughly 10% of your existing Renogy pair, you'll lose surprisingly little. Check the datasheet Vmp figures before committing — that's the number that matters here.