Does shade on one panel drag down the whole array, or just that panel?

I've got two 200W Renogy panels wired in series on the roof of my Sprinter. Works brilliantly in full sun, but I park under trees fairly often and I'm starting to wonder how much I'm actually losing when one panel gets dappled shade while the other stays clear.

From what I've read, series wiring means the shaded panel restricts the whole string — so if one panel is producing half its potential, the whole array drops to roughly that level? My Victron MPPT 100/30 shows the input dropping off a cliff even with partial shade, which seems to back that up, but I'm not sure if that's normal behaviour or a sign something's wrong.

Would I be better off rewiring to parallel, or are optimisers like the ones Tigo make worth looking at for a two-panel setup? Is it even worth the hassle for just two panels, or should I just accept the loss and move the van?

EssexExplorer | Posts: 847

Great question @SprinterWanderer. With series wiring, shading one panel is genuinely painful - the shaded panel acts like a bottleneck and drags the entire string down to its reduced output. So if one panel is producing 50%, you're roughly losing half your total array capacity, not just half of one panel.

The saving grace is bypass diodes - most decent panels including Renogy have them built into the junction box. They allow current to route around a shaded panel rather than being completely choked, but you'll still take a significant hit.

For dappled tree shade specifically, it can get surprisingly bad. Might be worth considering rewiring to parallel if shading is a regular occurrence - you'd lose some voltage but shading becomes far less catastrophic. Worth a search on here as it's been debated plenty of times!

WattBaz | Posts: 1,203

To add to what @EssexExplorer is getting at — with series wiring, current is the same throughout the string, so the shaded panel becomes the bottleneck and drags the whole array down to its reduced output. It's not just losing that one panel's contribution, it can be worse than that in practice.

Worth knowing: modern panels have bypass diodes built in, which limits the damage somewhat by routing current around the shaded cells. But you're still taking a significant hit.

Given you're regularly parking under trees, honestly consider rewiring to parallel. You'll lose some voltage, but shading one panel will then only affect that panel's output. Alternatively, an MPPT controller handles partial shading better than PWM if you haven't already got one. What controller are you running?

FormerMariner24 | Posts: 312

Worth adding the bypass diode angle here. Most panels (Renogy included) have bypass diodes built into the junction box, so a shaded panel doesn't completely kill the string — the diode essentially routes current around it. You lose that panel's contribution, but the rest keeps generating.

That said, for a two-panel series string under partial tree cover, you might genuinely be better off switching to parallel. Modest voltage drop, but shade on one panel leaves the other completely unaffected.

My garden office runs four panels — I went parallel precisely because of dappled shade from a neighbour's oak. Victron MPPT handles the lower voltage fine. If you're running a decent MPPT controller rather than PWM, parallel is a very viable option worth considering before you start rearranging roof real estate.