Does shading one panel tank the whole array or just that string?

Running two 200W Renogy panels on my Sprinter roof, wired in series feeding a Victron MPPT 100/30. Noticed yesterday that when a shadow from a roof vent hit maybe 20% of one panel in the afternoon, the whole system seemed to drop way more than I'd expect — looked like I was barely pulling 60-70W total instead of the usual 180-190W on a clear day.

Is that normal behaviour for a series setup? I understand the weakest link theory in series strings, but losing over 60% of output from a partial shadow on one panel seems extreme. Would parallel wiring fix this, or am I likely looking at a bypass diode issue on that panel?

Has anyone actually measured the difference between series vs parallel on a two-panel setup with partial shading? Wondering if it's worth rewiring before summer.

MPPTFan | 847 posts

@SprinterWanderer With series wiring, the shaded panel acts like a bottleneck — current through the whole string drops to match the weakest panel, so yes, that partial shadow can hit your output harder than you'd expect. 20% shading on one panel can tank the entire string by 40-50% in some cases.

A few things worth knowing: Renogy panels should have bypass diodes built in, which helps limit the damage somewhat. Also worth checking your MPPT controller's history in the Victron app — the graphs will show you exactly when the drop happened and by how much.

If shading is a regular occurrence on your route, it might be worth considering rewiring to parallel instead. You'd lose some voltage headroom but shading becomes far less catastrophic. What's your battery bank voltage?

@SprinterWanderer series wiring is your enemy here. When one panel gets shaded, the whole string output tanks — the shaded panel becomes the bottleneck. It's basically physics.

Parallel would limit the damage to just the shaded panel, but then you'd need to check your Victron can handle the lower voltage.

My motorhome setup runs panels in parallel for exactly this reason — roof vents, bike rack shadow, trees at campsites. Real-world shading is unavoidable.

Worth checking if your Renogy panels have bypass diodes built in (most do) — helps a bit but doesn't fully solve series shading issues.

If you're rewiring, factor in the voltage requirements of the 100/30 first. Minimum input needs to exceed your battery voltage by a reasonable margin or the MPPT won't do anything useful.

Tommo | 2,341 posts

@SprinterWanderer Worth adding some nuance here — your Renogy panels almost certainly have bypass diodes built into the junction box (usually three per panel, protecting cell groups). So it's not always a total wipeout. What typically happens is the shaded cell group gets bypassed, meaning you lose roughly a third of that panel's contribution rather than the entire string grinding to zero.

That said, @MPPTFan and @DownsCruiser are right that series strings are vulnerable. On my own Transit build I eventually rewired to parallel specifically because I'm dealing with partial shading from a Webasto vent and a Maxfan. Parallel means your unshaded panel carries on regardless.

One practical test — check your Victron app history. If your voltage was dropping significantly rather than just current, that points to bypass diodes not triggering cleanly, which can happen on cheaper panels.