Anyone else running two different panel brands in parallel? Seeing weird voltage drop issues

Finally wired up my second set of panels on the cabin roof last weekend — mixed a couple of older 200W Renogy monos with two newer 175W units I picked up second-hand. All feeding into a Victron SmartSolar 100/30.

Been noticing the array voltage is sitting lower than I'd expect, especially on cloudy days. Sunny spells it's fine, hitting around 34–36V open circuit, but the minute there's any shade or haze it drops off faster than when I just had the Renogys on their own. Wondering if the mismatched Vmp is dragging the whole string down.

I've got them wired as two parallel strings rather than series, so I thought mismatched Voc wouldn't be a massive issue. But I'm second-guessing myself now. Anyone had similar with mixed panels? Worth chucking a bypass diode setup in, or am I overcomplicating it?

@WonkySkipper been there with my boat setup. Mixed panels are a right headache if their Vmp values don't match closely — the controller essentially has to compromise, and you lose output from the stronger string dragging down to meet the weaker one.

Worth checking the spec sheets side by side. If the Vmp differs by more than a volt or two, seriously consider running them as separate strings into separate MPPTs rather than paralleling them. A second small Victron SmartSolar 75/15 is relatively cheap compared to the losses you'll bleed over a season.

Also check your bypass diodes — mismatched panels can cause one set to partially act as a load under certain conditions, especially with shading in the mix.

What are the actual Vmp figures on both panel types? That'd help narrow down whether it's a mismatch issue or something else.

Smudge_6508 | 847 posts | ⚡ Off-Grid Enthusiast

@WonkySkipper worth checking what your Victron app is showing for actual input voltage versus the panel specs. I ran a similar mixed setup on my narrowboat for about eight months and the MPPT was constantly hunting for the sweet spot — never quite settling. In my case the older panels had degraded enough that their actual Vmp had drifted noticeably from the label rating.

Quick thing to try: measure each string independently with a multimeter under decent midday sun and compare real-world figures rather than relying on spec sheets. Those second-hand 175W units could be anything performance-wise depending on age and history.

Also double-check your cable runs are equal lengths between strings — uneven resistance will compound the voltage mismatch issue. Might seem obvious but it's easily overlooked when you're rushing to get everything wired before dark! 😄

MidlandsBoater | 1,203 posts | ⚡ Solar Addict

@WonkySkipper I'd have a look at the actual Imp ratings rather than just the wattage — if your older Renogys and those second-hand 175W units have noticeably different current outputs, the weaker string effectively drags the stronger one down when wired in parallel. It's not just Vmp that matters here.

Are you running them as separate strings into the SmartSolar, or have you already joined them before the controller? If your MPPT has enough input channels or you've got a second controller spare, keeping them as independent strings makes a massive difference in my experience. I did something similar on my narrowboat last summer and separating the strings properly gave me back nearly 15% yield on sunny days. What spec are the mystery 175W panels — any markings on the back?