Anyone else running two different watt-peak panels in series/parallel on an MPPT — how badly does mismatch actually hurt you?

Ended up with a bit of a Frankenstein array on the van roof — two 175W panels I had sitting around from a previous job (back when I was still laying bricks and picking up surplus kit) wired in series with a single 200W panel I grabbed from Renogy. All feeding into a Victron SmartSolar 100/30. On paper the Voc and Isc figures are close but not identical, and the 175s have a slightly lower Vmp.

Victron MPPT Connect is showing around 480–490W peak on a clear south-facing day, which against a theoretical 550W maximum feels like roughly 10–12% losses — more than I'd expect purely from temperature and wiring resistance. Running a parallel config instead of series dropped the current into a safer range for the controller but didn't obviously improve yield, which surprised me. I'm now wondering how much is genuine mismatch loss versus the panels themselves just being degraded after sitting in a barn for three years.

Has anyone done a proper before/after comparison swapping a mismatched string for a matched one on a similar-sized MPPT setup? Particularly interested if anyone's used Fogstar or one of the newer bifacial panels to replace tired older stock — curious whether the efficiency gains are worth the cost when you're only pulling off a Transit roof.

@ExBrickie31 running a similar bodged setup here — got a 200W Renogy and a 160W panel I sourced secondhand wired in parallel into a Victron SmartSolar 100/30.

The mismatch does hurt but probably less than you'd fear. Main thing I've noticed is the MPPT essentially chases the lower panel's characteristics and you lose some efficiency from the stronger one. In parallel it's more forgiving than series — series mismatch is where it really bites you because the weakest panel throttles the whole string's current.

What are the Voc and Vmp figures on your two 175W panels? If they're genuinely matched on voltage even if wattage differs slightly, series can work reasonably well. Worth checking on a clear day with a clamp meter what you're actually pulling vs theoretical max.

Hey @ExBrickie31 and @WheresMeWires31 — the mismatch penalty really depends on how close the Vmp and Imp figures are between the panels, not just the watt-peak ratings. Two panels with similar voltage characteristics in series will fare better than you might expect, honestly. The MPPT will find the best compromise operating point across the combined string, so you're rarely losing as much as people fear.

Where it gets messier is parallel mismatched setups like @WheresMeWires31 has — differing Vmp means one panel can drag the other down more noticeably. Worth sticking bypass/blocking diodes in if you haven't already.

Grab a few days of logging data if your controller supports it and compare against the theoretical combined output — that'll tell you far more than any rule of thumb. I'd wager you're losing maybe 5-10% at worst in decent conditions.

Hey @ExBrickie31 — worth grabbing the actual datasheet figures for both panels rather than just going by the headline wattage. Two 175W panels from different manufacturers can have surprisingly different Vmp values, and that's what'll really bite you in series. I ran mismatched panels for about eight months on my Transit and honestly the losses weren't catastrophic — maybe 8-12% below what I'd theoretically expect on a good day. The MPPT does a decent job of finding a compromise operating point. Shading hurts far more than mismatch in my experience. If you haven't already, chuck a logging unit on your controller for a week and see what you're actually getting versus the theoretical combined output. That'll tell you more than any calculator.

Hey @ExBrickie31 — just to add a practical angle here, I've been running a 200W and a 150W in series on an EPever Tracer for about eight months now and honestly the real-world hit has been less dramatic than I expected from reading around. The MPPT does a decent job of finding a compromise operating point. Where I did notice it was on overcast days — diffuse light seems to exaggerate the mismatch more than direct sun does, presumably because the weaker panel drags things down more when irradiance is already low. Worth keeping an eye on your controller's logging if it has it. Mine showed the array sitting about 12-15% below what the theoretical combined output should've been in those conditions. Not ideal but liveable for a van setup. Shading is the bigger enemy — even a small shadow on one panel kills output far worse than the wattage mismatch itself.