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

Got two 175W panels already wired to my Victron SmartSolar 100/20 on the motorhome roof. Found a cracking deal on a single 200W panel and wondering if it's worth chucking it in series with the existing two.

I know the theory says the MPPT will pull everything down to the weakest panel's output, but in practice how bad is it really? Same Voc on all three (around 21V open circuit), just different Isc — the 175s are about 9.5A and the 200W is pushing 10.8A.

Has anyone actually tried this rather than just quoting the textbook? Wondering if the real-world loss is negligible or if I'd genuinely be throwing money away vs. just selling the 200W panel on.

Mixing wattages in series is fine — the MPPT doesn't care about nameplate ratings, it just chases the voltage peak — but do check your Voc figures don't push you over the 100V input limit on a cold morning, because Victron's 100/20 will not forgive you and neither will your wallet.

HazelHermit | 847 posts | ⚡ Off-grid since 2019

@ExPostie86 is on the right track, but worth adding — the critical thing with series strings isn't the wattage mismatch, it's whether the panel specs are compatible. Check the Voc and Imp figures on all three panels. In series your voltages add up, so make sure the combined Voc stays safely under your controller's 100V input limit, especially on a cold morning when Voc peaks. The weaker panels' current rating will also cap the whole string, so that 200W panel won't perform to its potential. Honestly for a motorhome setup it'll still work fine practically speaking — you're not running a commercial array. Just run the numbers first and use the Victron MPPT calculator to double-check before you connect anything. 👍

RustyCaptain | Posts: 847

Worth flagging that whilst @ExPostie86 is right about the MPPT chasing the voltage curve, your limiting factor in series will be the current — the 200W panel almost certainly has a higher Isc than your 175W panels, and in series the weakest current rating pulls everything down to match. You'd essentially be wasting a chunk of that 200W panel's potential.

Might be worth checking the spec sheets — if the Voc values stack up sensibly and you're not going to breach the 100V input limit on that SmartSolar, electrically it'll work. But parallel wiring might actually suit you better here, current adds rather than being constrained. What's the Vmp on all three panels? That'd help work out which config gets you more usable watts in practice. 🔧

The bit nobody's mentioned yet — check your Voc figures before wiring anything in series. Three panels in series on a cold morning can spike surprisingly high, and the 100/20 has a 100V absolute max input. Blow that and your SmartSolar is a very expensive paperweight.

Done something similar on my static — mixed a couple of old 160W Renogy panels with a newer 200W unit. Worked fine once I'd done the maths on worst-case Voc. The MPPT pulled decent figures overall, though obviously the 160s dragged things down slightly in poor light.

Use the Victron MPPT calculator on their site — it'll show you exactly where you stand on voltage limits for your specific location.

CamperTel | Posts: 1,203 | ⚡ Solar nerd since 2015

Good shout from @ExSquaddie on the Voc — absolutely critical, especially on a cold winter morning when Voc spikes. The 100/20 has a 100V absolute maximum input, so do your maths carefully before committing.

On the mixing itself: your existing 175W panels likely have a lower Imp than the 200W panel. In series that's fine — current is equal throughout a series string. The MPPT will just work to the lowest common denominator effectively.

My bigger concern would be whether all three panels share similar Vmp characteristics. Mismatched Vmp figures in series means the controller's tracking point gets compromised and you'll leave performance on the table.

What's the make/model of your existing 175W panels? Worth comparing the datasheets side by side before you spend owt.

SparkyGaffer | Posts: 2,156 | ⚡ Approved Installer

Good points all round. Just to add the practical side — your 100/20 has a max input of 580W, so three panels totalling 550W keeps you comfortably within spec there. The real gotcha with mixed wattages in series is that your 175W panels will almost certainly have a lower Isc than the 200W, so the whole string's current gets capped at the weakest panel anyway. You're essentially paying for 200W and getting maybe 175W-equivalent in that configuration. Parallel would be worth considering if your Voc figures (as @ExSquaddie rightly flagged) allow it — keeps each panel closer to its own MPP. What are the actual Vmp and Imp figures off the datasheets? Post those and we can give you a proper answer rather than educated guessing.