Mixing panel wattages on a single MPPT — actually fine or am I asking for trouble?

Been scratching my head over this for a few weeks now. Currently running a 200W Renogy mono panel on my van roof feeding a Victron SmartSolar 100/20, and I've got a spare 100W panel sitting in the garage doing nothing. Tempted to wire them in parallel and just... see what happens. But I keep reading conflicting things online.

From what I understand, as long as the Voc of both panels is within the controller's input limits, parallel wiring means each panel operates somewhat independently — the 200W isn't going to drag the 100W down in the same brutal way series wiring would. In practice though, if the panels have different Isc values (which these do — roughly 10A vs 5A), is the MPPT actually going to find a sensible combined MPP, or is it just going to compromise and leave watts on the table?

My specific concern is partial shading. The 100W panel would be mounted at a slightly different angle due to roof geometry on my Transit, so there'll be times it's shaded when the 200W isn't. In a parallel setup that presumably just reduces total current without tanking the whole array — but I'd love to hear from anyone who's actually done this rather than just theorised about it.

Has anyone run mismatched panels in parallel on a Victron MPPT and bothered to log the VRM data to see what it actually does to harvest efficiency? Curious whether the real-world loss is negligible or worth caring about.

@VanAmy I'm in a similar situation on my static caravan — ended up wiring a 175W and a 100W together into a Victron SmartSolar 75/15.

The key thing I'd want to know before you wire them in parallel is whether the Voc ratings are reasonably close. If they're wildly different you can get odd behaviour, but if they're both standard 12V nominal panels the Voc is usually similar enough.

Main concern is the smaller panel will effectively cap performance in series, or in parallel the MPPT just sees the combined current anyway and tracks whatever point makes sense — it won't damage anything but you might not squeeze full efficiency out of either panel.

Has anyone actually measured real-world output doing exactly this? I've been meaning to put my clamp meter on mine properly but keep putting it off.

Hey @VanAmy, the key thing to check before anything else is whether the panels have compatible Voc and Vmp figures. If you're wiring them in parallel, the voltages need to be reasonably close — mismatched voltages mean the lower-voltage panel effectively drags the other one down to its level, and you lose efficiency on the stronger panel. Different wattages in parallel is generally fine as long as the voltage figures align. Chuck the specs up here and we can have a proper look. Also worth confirming your 100/20 can handle the combined current — 300W at 12V is pushing toward its limit depending on your battery voltage. Worth running the numbers before you commit!

Hey @VanAmy! Building on what @Compo27 is getting at — once you've confirmed the voltage compatibility, also pay attention to the current ratings. In a parallel setup, your currents add together, so make sure your 100/20 can handle the combined output (spoiler: 300W at 12V is pushing 25A, which exceeds the 20A limit, so you'd want to keep an eye on that). Series wiring dodges that issue but doubles your voltage instead. Also worth knowing that mismatched panels will generally drag performance down to the weaker panel's characteristics under partial shading. That said, plenty of van builders run mixed wattages without major headaches in good open-sky conditions. The Victron VictronConnect app will show you exactly what's happening once it's all hooked up, which is handy for spotting any oddities. What's the Vmp on that spare 100W panel?

Really useful thread — I went through exactly this when setting up my tiny house system last year.

One thing nobody's mentioned yet: shading becomes a much bigger headache when mixing wattages. If your 100W panel gets even partial shade, the MPPT has to compromise across the whole string. On a van roof where things like roof bars or vents cast shadows at certain times of day, this can quietly eat into your gains more than the mismatched wattage itself does.

Worth thinking about whether parallel or series wiring suits your specific shade situation before you commit to the setup. I'd honestly run the Victron MPPT Calculator (free online tool) with both configurations before touching any wiring.

@RussWebb did your caravan setup hold up over winter with the mixed panels? Curious whether the lower sun angles caused any issues with that compromise.