Swapped out my old PWM controller for an MPPT on the boat — massive difference in cloudy weather

I've been running a modest 200W panel setup on my 26ft sloop for the last couple of seasons, originally paired with a cheap PWM controller I grabbed off eBay. Honestly thought it was doing the job fine until I borrowed a mate's Victron SmartSolar 75/15 for a fortnight while he was away. The difference in charging performance on grey overcast days — which, let's face it, is most of the British sailing season — was genuinely eye-opening. I was seeing 20–30% more energy actually making it into the battery on dull days compared to what the PWM was managing.

The panels are two 100W units wired in series giving me roughly 40V Voc into the controller, feeding a pair of 110Ah AGMs under the cockpit. With the PWM I was struggling to keep the batteries properly topped up on longer passages with the chartplotter, VHF, and nav lights running overnight. Since switching to MPPT I've barely needed to run the outboard to top up, even on a week-long trip up the East Coast last month.

I'm now seriously considering upgrading the AGMs to a 200Ah lithium setup over winter, which would mean the MPPT would really come into its own. Has anyone gone down that route on a similar-sized boat? Curious whether the weight saving alone (lithium vs AGM) made a noticeable difference to how the boat sat and handled, as well as the obvious capacity benefits.

GeorgeSmith97 | 47 posts

@CrispySkipper Great write-up, and yeah the difference on overcast days is genuinely remarkable isn't it. The MPPT's ability to hunt for that optimal voltage point matters so much more when irradiance is low and the panel's IV curve is shifting constantly. On a bright summer day the gap between PWM and MPPT is noticeable but on a typical grey British morning it can be the difference between actually charging and barely trickling.

What controller did you go with in the end? I've had good experiences with Victron's SmartSolar range on my brother's narrowboat — the Bluetooth monitoring alone is worth it for keeping an eye on things from down below. Curious whether you noticed any improvement in early morning harvest too, that's usually where I see the biggest gains with MPPT over PWM.

SmartSolar_Geek | 312 posts

@CrispySkipper This matches exactly what I found when I upgraded my van setup. PWM is basically throttling your panel voltage down to battery voltage — you're leaving serious energy on the table, especially on grey days when the panel's operating voltage drops anyway.

On a boat I'd specifically look at the Victron SmartSolar range — the Bluetooth monitoring via the VictronConnect app is genuinely useful when your controller is tucked away in a locker somewhere. Mine's mounted in an awkward spot in the van and being able to check yield data from my phone without crawling around is worth the premium alone.

Worth noting: make sure your cable runs are sized properly for MPPT — you can run higher voltage from the panels and thinner cable, then let the controller do the conversion. Some people miss that opportunity when retrofitting.

NeilBurns91 | 89 posts

Not a boat setup here but running a Victron SmartSolar 100/30 on my shepherd's hut and the MPPT advantage in low light is very real — I'm still pulling decent amps at 8am on a grey November morning when the old PWM would've barely registered anything.

One thing worth mentioning for marine specifically: the Victron app lets you check historical yield data, so you can actually see what you were losing before. Useful if you're trying to justify the cost to yourself after the fact 😄

Also worth checking your panel Voc against whatever MPPT you've gone for — boats can get cold overnight and Voc creeps up in low temps. Caught a few people out who sized their controller a bit too tight. What did you end up going for, @CrispySkipper?

QuietMaker | 203 posts

@CrispySkipper This mirrors my shepherd's hut experience almost exactly. Running a Victron SmartSolar 75/15 paired with a Fogstar 100Ah lithium, and the MPPT's ability to squeeze usable current out of that classic British "bright overcast" sky is genuinely impressive — sometimes pulling decent amps when you'd swear there was nothing worth harvesting.

What clinched it for me was watching the Victron Connect app during a patchy November day. The MPPT was constantly hunting and adjusting, never just giving up. A PWM controller in those conditions is basically just sitting there shrugging.

Worth noting: make sure your panel voltage is well-matched to the MPPT's input range, especially on a boat where salt air and temperature swings can affect things. The Victron documentation on this is actually quite thorough if you dig into it.

WheresMeWires | 847 posts

Worth adding something nobody's mentioned yet — the voltage mismatch issue is a big part of why PWM suffers so badly in low light. Your panel's Vmp drops closer to battery voltage as irradiance falls, and a PWM controller has basically nothing left to work with. MPPT is actively hunting for that peak power point regardless.

@CrispySkipper on a marine setup I'd also keep an eye on your panel's Voc vs the controller's input ceiling — saltwater corrosion on connections can cause weird voltage spikes that'll kill cheaper MPPT units. Had a Renogy unit give up the ghost on a mate's narrowboat for exactly that reason. Victron SmartSolar range is worth the premium if you're in a harsh environment, the build quality difference is genuinely noticeable when you crack one open.