Anyone else finding PWM controllers a total waste of time with smaller panels?

Been running a 100W panel on a Victron BlueSolar MPPT 75/15 for about eight months now, charging a 100Ah AGM on my little off-grid shed setup. Genuinely surprised how much better it performs compared to the cheap 20A PWM I had before — even on those grim overcast days we've been getting lately, it's still pulling 2–3A when the PWM would've given up entirely.

I know the standard advice is "PWM is fine for small systems," and I get the logic — the efficiency gap supposedly narrows when your panel voltage is close to battery voltage. But my 100W panel has a Vmp around 18V and the battery sits at 12–13V, so there's a fair chunk of potential just being thrown away with PWM. Swapped to the MPPT and my average daily harvest went up noticeably, probably 15–20% rough estimate, which over winter actually matters quite a bit.

Has anyone done a proper side-by-side comparison with similar-sized kit? Wondering if the ~£40–50 price difference between a decent PWM and an entry-level MPPT is genuinely worth it for a system this small, or whether I just got lucky with my particular setup. Curious what others are seeing, especially anyone running panels in the 80–160W range on 12V batteries.

Jonno25 | 47 posts

@AshHermit Completely agree - the difference really shows on overcast days doesn't it? PWM just can't harvest those lower voltage conditions efficiently, whereas the MPPT keeps finding the sweet spot regardless. I ran a PWM on a similar sized setup for ages thinking I was saving money, and when I finally switched over I was genuinely shocked at the difference in charge times during a grey November week. For anyone sitting on the fence - if your panel voltage is significantly higher than your battery voltage, the maths on MPPT just makes sense, even on smaller systems. The payback period isn't as long as people assume.

Yeah @AshHermit, the difference really becomes obvious on overcast days doesn't it? PWM is basically just a switch, so you're always leaving voltage on the table when conditions aren't perfect. The MPPT is constantly hunting for that sweet spot on the power curve, which is exactly when you need every watt you can squeeze out - particularly here in the UK where we're rarely blessed with proper full-sun conditions!

I'd say the crossover point where MPPT genuinely pays for itself is lower than most people think. Even on a modest 100W setup like yours the payback period isn't that long when you factor in how much more usable charge you're actually getting into that AGM over a typical grey British winter.

Stick with it - you won't go back to PWM once you've seen the difference on your battery state of charge across a week.

My static caravan's PWM controller was basically an expensive paperweight from October through March — MPPT on the same panel felt like I'd secretly added a second one. 🤷‍♀️

DaiBennett | 203 posts

Same story on the narrowboat — swapped out a cheap PWM unit for a Victron 100/20 last winter and the difference in low-light harvesting was genuinely embarrassing. The old controller was basically doing nothing on grey days.

Worth noting the maths too: with a 12V nominal panel actually sitting at 18-21V open circuit, PWM is just throwing that extra voltage away. MPPT pulls proper power from that headroom.

@HeatherChild the October-March point is spot on — that's when it really matters. Summer you'd barely notice, but a UK winter with 4 hours of weak sun? Every watt counts.

Only caveat — if your panel voltage is barely above battery voltage anyway, MPPT gains shrink a lot. But with a standard 100W panel on 12V? No contest.

Really useful thread, this. I'm in the middle of planning a shepherd's hut build and also a separate garden office setup, and the PWM vs MPPT question keeps coming up in my research.

@DaiBennett the narrowboat comparison is interesting because my shepherd's hut will have similar constraints — limited roof space, so I can't just throw more panels at the problem.

Can I ask a practical question: at what panel wattage does MPPT genuinely stop being worth the premium? I've seen people say PWM is "fine" up to 100W but reading threads like this I'm starting to doubt that. Also wondering whether a single Victron unit could sensibly serve both my hut and the garden office if they're reasonably close together, or whether I'd realistically need two separate controllers?

PhilFox99 | 847 posts

@NicolaKing85 For the shepherd's hut especially, I'd go MPPT without hesitation — those small spaces often mean you're working with limited panel real estate, so squeezing every last watt out matters enormously. Worth knowing that the efficiency gap between PWM and MPPT widens considerably when your panel voltage is significantly higher than your battery voltage, which is pretty much the default situation with most modern panels.

For the garden office, have a think about what panels you're planning — if you're going with a higher voltage string arrangement to keep cable runs manageable, MPPT becomes almost essential rather than just preferable.

The Victron kit @AshHermit and @DaiBennett mention integrates nicely with the app too, which I find genuinely useful rather than just a gimmick. Being able to check yesterday's harvest whilst drinking your morning brew is rather satisfying!