Anyone else finding their MPPT controller undersized after adding a second panel?

Started with a single 200W panel running through a Victron SmartSolar 75/15 — worked fine for ages, just topping up a leisure battery as emergency backup. Recently bolted on a second 200W Renogy panel in parallel and now I'm wondering if I've pushed the controller too far. Combined Isc is around 20A which is technically over the 15A limit.

Hasn't tripped or thrown any errors yet, but the Victron app is showing charge current maxing out at 15A and I suspect I'm leaving power on the table on decent days. UK sun being what it is, maybe it barely matters, but it's nagging at me.

Is it worth swapping up to a 75/20 or even a 100/20 for a bit of headroom, or am I overthinking it for a setup that's only really meant to kick in during a power cut? Curious what others have done in similar situations.

@WheresMeWires31 did you check whether you've exceeded the 75/15's input current rating rather than just the wattage? Two 200W panels wired in parallel could easily push you past 15A depending on your panel specs — the controller will just clip anything above that rather than trip out completely.

Have you noticed whether the MPPT is running hot or just not harvesting what you'd expect on good days? Mine did something similar when I added a third panel and I couldn't work out why the numbers looked low until I actually sat down and calculated the Isc on each panel.

What's the Voc and short-circuit current on those Renogys? That'd help figure out whether you actually need to upsize to something like a 100/20 or if there's something else going on.

@WonkySparky raises the important point about current — but also worth checking your open-circuit voltage from the two panels in series. The 75/15 only handles up to 75V Voc, and two 200W panels in series could easily push you over that depending on the spec sheet. Cold mornings make it worse — Voc rises as temperature drops.

If you're running them in parallel instead, you stay within the voltage limit but you're likely hitting that 15A input ceiling @WonkySparky mentioned.

Honestly the 75/15 was a bit marginal for a single 200W panel anyway. With 400W total you really want to be looking at a 100/20 minimum, or ideally a 100/30 if you think there's any chance of adding more later. Victron's wiring unlimited app will size it properly for your exact panel specs.

Good point from @WonkySparky about the current side of things. Worth also double-checking how you've wired the second panel — series vs parallel makes a big difference here. Two 200W panels in series could push your Voc well past the 75V input limit on that controller, which would be a problem regardless of the current rating. Parallel keeps voltage the same but doubles your current, which at roughly 10-11A per panel could still breach the 15A limit depending on conditions. Either way, a 75/15 really is stretched thin for 400W total. Victron's own sizing tool would likely suggest stepping up to at least a 100/30 for your setup. Might be worth grabbing one before you do any damage to what you've already got — they hold their value reasonably well if you ever want to sell on.

Great points from everyone above. One thing nobody's mentioned yet — even if your combined figures technically scrape under the 75/15's limits, you're running with virtually no headroom. Cold mornings can push Voc noticeably higher than the rated figure, and on a bright winter's day with fresh panels you might be nudging into territory that'll stress or trip the controller.

With 400W of panels you're really looking at stepping up to something like the SmartSolar 100/20 as a minimum, ideally a 100/30 if your battery bank might grow. The good news is Victron's VictronConnect app will show you exactly what your current controller has been seeing — worth pulling the history data before you decide anything. 👍

Great thread, and good points already made above. One thing I'd add — the 75/15 designation tells you everything you need to know: 75V max input and 15A max output. With two 200W panels, even wired in parallel, you're potentially asking that controller to handle output current that could exceed what your battery voltage allows it to deliver safely. Do a quick calc: divide your combined panel wattage by your battery voltage and see what charge current you're actually expecting. If you're running a 12V system, 400W ÷ 12V = ~33A theoretical max. The controller will simply clip anything above 15A, meaning you're wasting a significant chunk of your new panel's capacity. Might be worth looking at the SmartSolar 100/30 as a reasonably priced upgrade — still Victron quality but gives you proper headroom. 😊

Been in almost exactly this situation myself last spring — added a second 200W panel and wondered why my controller seemed to be throttling output on bright days.

What caught me out was the Voc temperature correction. @CraftyGrafter is right that the 75V limit is your ceiling, but on a cold clear January morning your open-circuit voltage can spike well above the nominal figure. Two 200W panels in series could genuinely push you uncomfortably close to that 75V limit, or even breach it briefly.

If you've wired them in parallel you've got more headroom on voltage, but then you're potentially hammering the 15A current limit instead.

Honestly, for two 200W panels I'd be looking at stepping up to something like the SmartSolar 100/20 at minimum. The price difference is modest and your panels will thank you for it.

Yeah been through this exact headache. The 75/15 is rated 15A output — two 200W panels in parallel at 12V system is potentially pushing ~33A of panel current before the controller even does its thing. Something's gotta give.

Ended up swapping mine out for a Victron SmartSolar 100/30 when I added a third panel. Night and day difference, and the Victron app actually shows you where you were clipping before.

Don't bother trying to squeeze it — just upsize the controller. They're not that dear compared to leaving performance on the table all summer.

@WheresMeWires31 worth doing the actual maths before ordering anything. Two 200W panels in series could push your Voc well past the 75V input limit depending on your panel spec — that's a controller-killer waiting to happen, not just a capacity issue. Check your panel's open-circuit voltage on the datasheet (often around 24–25V each), double it, then add a temperature derating buffer for cold UK mornings. If you're staying parallel, you're likely fine on voltage but you'll definitely be clipping amps through that 15A limit. A 100/20 is a modest step-up; personally I jumped straight to a 100/30 on my Ducato build and haven't regretted the headroom.