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

Had a 20A PWM controller on my 32ft narrowboat for about three years, paired with two 100W panels wired in series and a 110Ah leisure battery. Charging was always a bit hit and miss, especially on overcast days, and I was lucky to see 10–11A going in on a decent afternoon.

Picked up a Victron SmartSolar 75/15 last month and installed it over a weekend. First sunny day after, I was seeing 14–15A into the battery and the thing was hitting absorption stage by early afternoon — something the old PWM never really managed. Even on a grey morning I'm pulling 6–8A, which feels like a proper improvement.

My question is whether anyone else has noticed a similarly big jump, or if I just happened to have a particularly dodgy PWM controller before. I've read the theory about MPPT being 20–30% more efficient but that gap feels larger than I expected. Also wondering if the Victron app logging is making it seem better because I can actually see what's happening now, rather than just guessing.

WillowGazer | 847 posts | ⚓ Midlands Canals

@Owen1991 You're definitely not imagining it! The gains are most noticeable exactly where you're describing — overcast days and low-light conditions. MPPT controllers are constantly hunting for the optimal voltage/current combination from your panels, whereas PWM essentially just clips the panel voltage down to match the battery. On a narrowboat where you're often dealing with trees, bridges, and grey British skies, that efficiency difference really adds up over a day's cruising.

Worth mentioning — with your panels wired in series, the MPPT is now actually able to use that higher string voltage properly, which a PWM controller couldn't do efficiently at all. That alone probably accounts for much of what you're noticing. What controller did you go with? Some of the Victron units give you lovely monitoring data so you can see exactly what you're harvesting throughout the day. 📊

LiFePO4_King | 1,203 posts | ⚡ Scottish Highlands

@Owen1991 Worth mentioning that your series wiring is actually the key reason you're seeing such dramatic gains now. With two 100W panels in series, your array voltage is roughly double the panel Vmp — probably around 36-38V — which a PWM controller was essentially throttling back to battery voltage and wasting. Your MPPT can actually harvest that higher voltage and convert it into usable charge current. On overcast days especially, that voltage headroom is what keeps current flowing when irradiance is low. Have you noticed what your actual peak charge amps are now? I'd be curious whether you're hitting the controller's rated output. Also, with only 110Ah on board, you might find you're hitting absorption stage surprisingly quickly on decent days — no bad thing, but worth watching your battery temperature if it's a sealed unit.

SamReid | 412 posts | ⚓ Yorkshire Waterways

@Owen1991 Great upgrade choice! One thing worth adding that nobody's mentioned yet — on overcast days specifically, MPPT really earns its keep because it can work with the lower, more variable voltages that diffuse light produces, whereas PWM essentially just wastes that potential. Your 110Ah battery is also on the smaller side for two 100W panels, so you'll likely find it reaches full charge earlier in the day now, which is actually a good sign the system's working properly rather than trickling away inefficiently like before. If you're not already, keep an eye on your controller's daily harvest figures — comparing those logs across similar weather days will give you concrete numbers rather than gut feeling. Most people see 15-30% gains, sometimes more in winter months.

PeakCamper | 203 posts | 🚐 Somewhere with dodgy signal

@Owen1991 I made the same swap on my van build last year — going from PWM to an MPPT felt like upgrading from a garden hosepipe to actual mains pressure.

Quick question though: what MPPT did you

Thommo40 | 847 posts | ⛵ Midlands Canals

@Owen1991 You're definitely not imagining it! The real magic with MPPT on a narrowboat is how it handles those patchy, cloudy British days — which let's face it is most of them. PWM basically needs near-perfect conditions to perform well, whereas MPPT is constantly hunting for the optimal operating point even when the sun's playing hide and seek behind clouds. On a 200W setup you're probably seeing anywhere from 15-30% more harvest depending on conditions.

One thing I'd check is your battery temperature compensation if your MPPT controller supports it — narrowboats can get surprisingly cold overnight and the charging voltage needs adjusting accordingly, otherwise you're either undercharging or potentially stressing the battery. Most decent MPPT units have a temp sensor cable available as an add-on if yours didn't come with one.