Anyone else finding their MPPT controller undersized after adding more panels?

So I finally caved and added two more 200W panels to my roof last weekend, taking me up to four in total — 800W of Renogy monos wired in two parallel strings of 2S. The problem is my Victron SmartSolar 100/30 is now clearly the bottleneck. On a decent sunny afternoon I'm seeing it clamp at around 30A output while my battery voltage is still low enough that it should be pulling more. Basically leaving free energy on the table.

I'm torn between stepping up to the 100/50 or jumping straight to the 150/70 to give myself some headroom. The 150V input rating on the latter is tempting because I could potentially rewire to 3S1P and keep the string voltage higher, which I know helps with early morning and overcast performance. Budget isn't massive though — picked up the panels second-hand so I've got maybe £150-180 left to play with, which rules out a lot of options new.

Has anyone made this kind of jump mid-build and regretted (or loved) a particular controller? Especially interested if anyone's running the 150/70 in a van where space and heat dissipation are a concern — mine lives in a wooden cabinet above the wheel arch and summers can get a bit toasty in there.

Linda1972 | 847 posts

@TransitDream Oh, this is such a familiar story! I went through exactly the same thing last spring with my Sprinter. The temptation to add "just a couple more panels" is real, isn't it! 😄

One thing worth considering before you splash out on a bigger MPPT — have you checked your actual peak output versus theoretical? With UK weather being what it is, I rarely see my 600W array producing more than 400W realistically. You might find your current controller handles the real-world figures just fine, even if the numbers on paper look scary.

That said, if you're planning to stay in sunnier climates or do European trips, upgrading does make sense. What's the model of your current SmartSolar? The 100/30 and 100/50 have quite different headroom for expansion.

CaddyDream | 312 posts

@TransitDream classic mistake tbh — been there on the narrowboat. Worth checking your actual Voc in cold morning temps too, not just the rated figures. Two 2S strings can spike higher than you'd expect on a frosty day and that's when you risk frying the input on a undersized controller.

If you're staying Victron, the 100/50 is the obvious step up and the app makes migrating your settings dead easy. Just export your config before you swap it out.

Alternatively — do you actually need all four panels running simultaneously? On the boat I sometimes just disconnect a string in summer when the batteries are topped off by 10am anyway. Not a proper fix but buys you time before spending.

WillStevens | 1,203 posts

@TransitDream The Victron SmartSolar range has pretty strict input current limits worth double-checking — it's not just about wattage. With your 2S2P setup you'll want to look at both the max PV open-circuit voltage and the max charge current your controller can handle. If you're already hitting the current ceiling on sunny days you'll just be leaving power on the table rather than damaging anything, but it's still frustrating. Might be worth logging a few days' data through the VictorConnect app if you haven't already — you can see pretty clearly if you're clipping. Depending on which SmartSolar model you've got, sometimes rewiring to 4S series rather than parallel can work around current limits, though obviously you'd need to check your Voc doesn't exceed the input max first.

MasterWanderer | 156 posts

My static van had the same existential crisis when I bolted on extra panels — the Victron SmartSolar 100/30 basically threw its hands up and said "I didn't sign up for this." Ended up daisy-chaining a second MPPT rather than replacing the first, which sounds mad but actually works a treat and Victron's VE.Smart networking means they play nicely together. Two controllers sharing the load beats buying a whole new unit when you've already got one perfectly good one sat there.