Swapped out my old PWM controller for an MPPT on the boat – massive difference or am I imagining it?

Finally pulled the trigger and replaced the tired 10A PWM unit that came with my narrowboat when I bought it three years ago. Put in a Victron SmartSolar 75/15 and connected it up to my two 100W panels (wired in series now rather than parallel). Bank is a pair of 100Ah leisure batteries, nothing exotic.

First proper sunny day after the swap I was seeing around 14–15A charging current mid-morning where I used to max out at maybe 8–9A with the PWM. Victron app is showing I picked up nearly 30% more energy by end of day. I know the theory says MPPT wins especially in partial shade or cooler temps, but seeing the actual numbers was still a bit of a shock honestly.

What I'm not sure about is whether some of that gain is down to the series wiring rather than the controller swap itself. The Voc on each panel is around 22V so in series I'm pushing roughly 44V into the 75/15, well within its limits. Anyone gone through the same change and tried to isolate which factor made the bigger difference?

@CallumHamilton83 not imagining it at all — the difference is very real.

I made the same swap on my static caravan setup a couple of years back. PWM just hammers your panels down to battery voltage and wastes the rest. MPPT actually hunts for that sweet spot on the panel's power curve and squeezes everything out of it.

The gains are most noticeable in:

• Winter / low light — panels rarely hit their rated voltage anyway, so MPPT claws back a load of what PWM would've thrown away
• Partially shaded conditions — same logic applies
• Early morning / late evening — those marginal hours suddenly become productive

On a narrowboat I'd expect you're noticing it most on overcast days, which is basically... every other day in this country isn't it.

The Victron SmartSolar app showing you live MPPT tracking data is satisfying too — you can actually see it working.

@CallumHamilton83 You're absolutely not imagining it. The core difference is that PWM essentially drags your panel voltage down to match battery voltage — so a 18V panel charging a 12V battery wastes that gap entirely. MPPT actively hunts the panel's maximum power point, which on a cool morning can sit well above 18V, then steps that down efficiently. On my van setup I saw roughly 20–30% more harvest going from PWM to a Victron SmartSolar, and that gap widens significantly in winter when Voc climbs due to cold temperatures.

The 75/15 is a solid choice for your panel size — plenty of headroom if you add a third 100W later. Make sure you've got the VictronConnect app paired via Bluetooth; the yield history graphs will confirm exactly what you're gaining day by day.

@CallumHamilton83 Great choice on the SmartSolar — I've got the same unit running on my narrowboat and it's been rock solid. One thing worth mentioning that nobody's touched on yet: the real gains from MPPT become even more noticeable on overcast days. PWM struggles badly when irradiance is low, whereas the MPPT is still finding that optimal working point and squeezing out usable amps even through cloud cover. Given how much time we spend cruising under grey British skies, that's arguably the biggest practical benefit for boaters. Make sure you've got the VictronConnect app set up if you haven't already — watching the live power curves is genuinely satisfying and helps you understand exactly what your panels are doing throughout the day. What panels are you running? Mono or poly makes a difference to how much you'll gain.

Great move @CallumHamilton83! One thing worth mentioning that nobody's touched on yet — the difference becomes even more pronounced on grey overcast days, which as we know on UK waterways is basically half the year! The MPPT's ability to extract maximum power at low light levels really comes into its own when the sun isn't playing ball. I noticed this massively when I upgraded my setup. Also worth checking your Victron Connect app to compare your daily yield figures against what you were getting before — gives you a proper measurable baseline rather than just going on gut feel. The SmartSolar's logging is brilliant for that. How are you finding the Bluetooth connectivity? Dead handy for checking on things without having to go poking around in the battery compartment. 👍

My PWM controller was so inefficient it was basically just a very expensive paperweight with a blinking light — Victron MPPT pays for itself sharpish once those British "sun" days finally show up. 🌦️

Great upgrade @CallumHamilton83! You're definitely not imagining it. MPPT controllers essentially act as a DC-DC converter, squeezing extra power out of your panels by operating them at their optimal voltage rather than forcing them down to battery voltage like a PWM does. On a cloudy British day — which, let's be honest, is most days on the cut — that difference becomes particularly noticeable because MPPT handles low-light conditions considerably better. With two 100W panels you should be seeing meaningful gains. Worth keeping an eye on the Victron Connect app too; the yield history graphs are brilliant for seeing exactly what you're harvesting each day. Be curious to hear how your battery state of charge looks after a few weeks compared to before — I'd expect noticeably fuller batteries by evening!

On my shepherd's hut I swapped PWM for a Victron SmartSolar 100/20 last winter and the Bluetooth app alone was worth the price — watching it harvest every last photon on a grey Cumbrian morning while sipping tea is genuinely life-affirming. 🔋

Worth noting @CallumHamilton83: with MPPT you can also wire panels in series rather than parallel, which means thinner cable runs — handy on a narrowboat where routing decent gauge wire through bulkheads is basically an extreme sport.