Swapped my PWM for an MPPT on a cloudy day — the difference actually surprised me

Been running a 200W panel on a PWM controller for about 18 months in my Cornish setup and always assumed the efficiency gains from MPPT were mostly a fair-weather thing. Finally got round to swapping in a Victron SmartSolar 75/15 last weekend, and I tested both back-to-back on what I'd describe as a proper grey October day — solid overcast, maybe 15–20% of full sun.

The PWM was pulling around 2.8A into my 12V leisure battery. After fitting the MPPT and letting it settle for a few minutes, it was consistently showing 4.1–4.4A. That's a significant jump for conditions where I genuinely expected the difference to be minimal. The panel's Voc is around 22V so there's clearly headroom for the MPPT to work with even in low light.

I know the maths is well documented, but seeing it live on the Victron app was a different thing entirely. Wondering if anyone else has done a similar direct comparison, especially in low-irradiance conditions? Also curious whether the gap closes at all in summer when a 12V panel's voltage naturally drops closer to what a PWM needs anyway — I've read conflicting things on that.

Wendy1967 | 847 posts | ⚡ Off-Grid Veteran

@CornishExplorer this is actually what convinced me to make the switch years ago too! The cloudy-day performance is where MPPT really earns its keep, because the panel's voltage still sits reasonably high even in diffuse light — the PWM just can't exploit that. It's basically throwing away potential harvest precisely when you can least afford to.

Cornwall's got to be the perfect testing ground for this as well, given your typical weather! I'd be curious what figures you're actually seeing — are you logging your battery charge data? Would be brilliant to see some before/after comparisons if you kept records from the PWM days. Even rough notes would help others on the fence about upgrading. Some folks here still think MPPT is only worth it on larger arrays, so real-world cloudy-day data from a modest 200W setup is genuinely useful.

Smudge_6508 | 312 posts | 🔋 Getting There

@CornishExplorer This makes complete sense when you think about what's actually happening. In low light, your panel voltage drops but it doesn't collapse nearly as dramatically as the current does. A PWM just pulls the panel down to battery voltage regardless, so you're essentially throwing away whatever headroom remains. The MPPT hunts for that sweet spot continuously, which matters especially on grey days because the optimal point shifts around more than people realise.

Cornwall's got to be a decent real-world test case too — you're not exactly blessed with guaranteed sunshine! Curious what panels you're running and whether they're higher voltage ones, because the conversion headroom really amplifies the gains. I noticed the biggest jump on my setup when I moved from a matched 12V panel to a higher Voc panel paired with the MPPT. Night and day on overcast mornings.