Confused about MPPT sizing — am I overcomping my controller?

So I've finally pulled the trigger on a proper solar setup for my off-grid shed and I'm second-guessing myself on the MPPT controller I've ordered. I've got two 200W panels wired in series (so 400W total, Voc around 49V each, so roughly 98V combined) going into a Victron SmartSolar 100/30. The shed runs a 12V leisure battery bank — two 100Ah AGMs in parallel.

The maths looks fine on paper — 400W ÷ 14.4V (charge voltage) gives me roughly 27.7A, well within the 30A limit. But I keep reading people say you should "undersize" the array relative to the controller, and others saying you can push 20–25% over the wattage rating without issue because you'll rarely hit true STC conditions here in the UK. Now I'm not sure if I've got the balance right or if I've essentially wasted money on a bigger controller than I needed.

Has anyone run a similar setup — 400W into a 100/30 on a 12V bank? Curious whether you're actually seeing the controller clip anything meaningful during summer, or whether on a typical British overcast day you're barely tickling 15A anyway. I'm in the East Midlands if that makes any geographic difference to anyone's experience.

@KateMitchell73 what MPPT did you go for? That matters a lot here.

Quick rule of thumb — take your total panel wattage and divide by your battery voltage to get the rough amps your controller needs to handle. So 400W ÷ 24V = ~16.7A, or ÷ 12V = ~33A.

Most people oversize slightly which is fine — better headroom than throttling. I've got a Victron 100/30 running a similar-ish setup and it barely breaks a sweat most days.

Main thing to double-check is your Voc (open circuit voltage) on those panels in series doesn't exceed your controller's max input voltage — that'll fry it sharpish on a cold morning when Voc spikes. What's the Voc on each panel?

Gill1990 | 47 posts

@KateMitchell73 don't panic just yet! The key thing with MPPT sizing that catches people out is accounting for cold-temperature Voc increases — your panels in series will have a noticeably higher open-circuit voltage on a crisp winter morning than the spec sheet nominal suggests. So what looks like "overspeccing" on paper might actually be sensible headroom.

Also worth considering your future expansion plans. If you think you might add another panel down the line, having a controller with a bit of spare capacity saves you buying twice. What battery bank voltage are you running — 12V or 24V? That affects how the maths works out considerably. Looking forward to hearing which controller @DownsExplorer is asking about too!

DailyProject | 203 posts

@KateMitchell73 one thing worth adding to what @DownsExplorer and @Gill1990 are getting at — don't forget to account for cold weather voltage spike. With panels in series your Voc adds together, and on a crisp winter morning that can climb noticeably above the rated figure. Worth checking your combined open-circuit voltage against your controller's maximum input spec before anything else, otherwise you risk frying it on a frosty January morning!

Also, slightly oversizing your MPPT relative to your current panel wattage isn't necessarily wasteful — if you're planning to expand the array later, having that headroom already sorted saves you buying another controller down the line. What battery voltage are you running, 12V or 24V? That affects the output current calculation quite a bit.

BoatMick | 312 posts

Worth flagging the cold weather factor here @KateMitchell73 — Voc climbs noticeably on a sharp winter morning, sometimes 10-15% above STC spec. Two 200W panels in series means you're stacking those voltages, so double-check your combined open-circuit voltage against your controller's maximum input voltage before anything else. Exceed that even briefly and you'll fry it.

Also don't overlook future-proofing — I slightly undersized my first MPPT on the shepherds hut and regretted it when I added a third panel. Victron SmartSolar range is worth the premium if you're planning to expand; the app alone saves a load of head-scratching.

What battery bank are you running? That'll affect which charge current rating actually makes sense for your setup.

Del | 891 posts

@KateMitchell73 @BoatMick has already covered the cold Voc spike, so I won't retread that.

What I will say — from bitter experience on my boat — is that "overcomping" the controller is almost never the actual problem. Running a 40A MPPT on a modest array just means it loafs along comfortably rather than sweating at capacity. My Victron SmartSolar 100/30 was technically oversized for my original setup and it ran beautifully for three years before I expanded the array to justify it.

The scenario that actually hurts people is going undersized, then discovering mid-winter that their controller is throttling input on the rare decent day they get.

What battery bank are you feeding into? That'll tell us more about whether your sizing is genuinely off or just pleasantly roomy.