Anyone else running two different wattage panels in series on a Victron MPPT — any gotchas?

So here's the situation. I've got a 175W Renogy panel that's been bolted to the roof of my Transit conversion for about two years, doing sterling work. Just picked up a second-hand 200W panel (same 36-cell, 12V nominal config) at a van meet for £40 — couldn't say no. Plan is to wire them in series into my Victron SmartSolar 100/30.

The voltage side stacks up fine — combined Voc sits around 45V which the controller handles no bother. What I'm less sure about is the current mismatch. The 175W pulls about 9.7A at Pmax, the 200W is closer to 10.8A. Series strings are limited by the weakest panel, right, so I'm essentially throttling that 200W down to match the smaller one. I've run the numbers and even in that scenario I should be pulling a fair bit more harvest than running the 175W solo — but I want to know if anyone's seen anything weird happen in practice.

Has the Victron thrown any faults or behaved oddly when panels are mismatched like this? Wondering if shading on the smaller panel causes the MPPT to hunt about more than usual. Real-world experience would be brilliant here.

@WildHermit done exactly this on the narrowboat — mixed a 180W and a 200W Victron panel in series on a SmartSolar 100/30.

Key gotcha: the string will only ever produce current matching the lower-rated panel. So your 175W effectively caps the pair. You won't get the full 200W's potential current output.

What actually matters more than wattage is matching the Imp values (current at max power) as closely as possible — check the spec sheets. If they're within ~0.5A of each other you're largely fine.

Also double-check your combined Voc doesn't exceed the MPPT's input limit, especially on a cold morning — voltage spikes when panels are cold.

Which Victron controller are you running? The SmartSolar app will show you exactly what you're harvesting so you can judge whether the mismatch is genuinely hurting you.

Running mismatched panels in series, the key gotcha is current matching — the MPPT will be limited to whichever panel has the lower Isc. So your 175W and 200W will effectively both behave like the lower-current panel. Worth checking the datasheets to see how close the Isc values actually are before assuming you're losing masses of output.

Also check your combined Voc doesn't exceed your controller's input limit — add both Voc figures together and make sure you've got headroom, especially on a cold morning when voltage spikes.

In my tiny house setup I run a similar mixed arrangement and the real-world losses were smaller than I expected, maybe 5-8%. The Victron MPPT handles it fine and the VictronConnect app lets you monitor what you're actually harvesting vs what you'd theoretically expect.

Great thread this. @MoorHermit is right about current limiting, but worth flagging the Voc side too — add both panels' open-circuit voltages together and make sure you've got enough headroom below your controller's maximum input voltage, especially on a cold morning when Voc climbs. A frosty January start can push things surprisingly close to the limit.

Also worth checking whether your two panels have similar temperature coefficients — if they behave differently across temperature ranges, you can get some odd MPPT hunting behaviour, though in practice on a Transit roof seeing similar conditions it's rarely a dealbreaker.

Victron's MPPT calculator tool on their website will let you plug in both panels' specs and check you're within safe limits before you commit to wiring anything up. Takes five minutes and saves potential controller grief. What's the Voc on that second-hand 200W?

@WattAMess and @MoorHermit have covered the electrical side nicely — the one thing nobody's mentioned yet is shading behaviour: in series, one bird landing on your lower-rated panel drags the whole string down like it owes it money, so if your Transit roof gets any partial shade (trees, carpark structures, that lorry you always end up behind on the M6), you'll really feel the mismatch pain there more than anywhere else. Had exactly this grief on my van conversion before I shuffled things around. Worth chucking the Victron app's history graphs at it for a week before assuming your yield is "normal" — the VRM portal will show you exactly where it's sulking. 🐦