Anyone else running two different watt-peak panels in series on a Victron MPPT — is it actually mental or fine?

Ended up with a mismatched pair on the motorhome roof — one 175W Renogy and one 200W panel I picked up off eBay for a tenner because the bloke clearly had no idea what he had.

Running them in series into a Victron SmartSolar 100/30, VOC checks out fine, but I'm getting the nagging feeling the 175W is throttling the whole string like that one mate who slows down the whole group walk.

Anyone got real numbers on the efficiency hit? Wondering if I'd be better off wiring parallel and just accepting the higher current, or whether the Victron's clever enough to make the best of a bad situation.

Hey @SolarRob, nice score on that eBay panel!

Running mismatched panels in series is fine electrically — your Victron MPPT will handle it no bother. The thing to bear in mind is that in series, current is limited by the lower of the two panels, so effectively your 200W is being dragged down to perform more like the 175W. You're not losing the extra capacity entirely, but you won't see the full potential of the bigger panel.

Worth checking that your combined Voc doesn't exceed your controller's input rating though — that's the actual gotcha with series strings rather than the wattage mismatch itself.

Honestly for a motorhome setup where you're not trying to squeeze every last watt, you'll barely notice the compromise in real-world conditions. The Victron will just crack on and do its job. 👍

Great score on that eBay panel @SolarRob!

Just to add to what @OffGridAlan is saying — the key thing to watch with mismatched panels in series is that the lower-spec panel effectively becomes the limiting factor for current. So your string current will be capped at whatever the 175W Renogy can produce. You won't get the full potential of that 200W panel, but you're still harvesting more than if you'd left it sitting in the garage!

The Victron MPPT handles this really well in my experience — it'll find the best compromise point across the combined voltage. Just make sure your combined Voc stays within the controller's input limits before you wire anything up permanently. What MPPT model are you running?

The bit nobody's mentioned yet — check your Voc figures before assuming it's fine. Series adds voltage, so if those panels have meaningfully different Voc characteristics they may not track the same MPP cleanly. Your Victron will find an optimum but it might not be the true optimum for either panel individually.

In practice on a static setup I've not seen dramatic losses, but on a motorhome roof where shading is unpredictable (trees, buildings, other vehicles) mismatched series can get messy fast. A bypass diode issue on one panel drags the whole string.

Worth logging a week of data in VictronConnect and comparing against what the combined rated output should be at your irradiance. If you're within 10-15% I wouldn't lose sleep over it.

Good point from @Max1970 on Voc — and don't forget to derate for cold conditions. Victron's guidance is to apply a temperature coefficient correction at your lowest expected ambient temp, typically around -10°C for UK winters. Your combined cold-weather Voc needs to sit comfortably below your MPPT's absolute maximum input voltage with some headroom.

On the current mismatch specifically: series strings share the same current, so your 175W panel effectively becomes the bottleneck. Worth pulling both datasheets and comparing Imp figures — if they're reasonably close (within ~0.5A ideally), real-world losses will be modest. Where it gets painful is if one panel has meaningfully different Imp and is occasionally shaded differently to the other, as that compounds the mismatch losses significantly.

VictronConnect will show you your actual harvest over time — compare against a PVWatts estimate for your location and you'll quickly see if the mismatch is genuinely costing you.

Good shout from @RustyRoamer on the cold derating — that catches people out more than anything else.

One thing worth adding: with mismatched panels in series, the lower-performing panel effectively becomes the bottleneck for current. So your 175W Renogy will likely limit the output of that eBay 200W, especially if they've got different Isc figures. You won't get the theoretical combined wattage.

That said, it's absolutely not mental — people run mismatched series strings all the time with Victron MPPTs and it works fine in practice. The controller will find the best compromise voltage. You're leaving a bit of performance on the table but realistically on a motorhome roof you've got shading and orientation compromises anyway, so in the grand scheme it's minor.

Just double-check those Voc figures as @Max1970 says and you're good to go. Bargain panel for a tenner though! 👍