Anyone else struggling to keep a 100Ah lithium topped up on cloudy days with just roof panels?

Been dealing with this exact issue with my shepherd's hut setup rather than a van, but the physics is identical so worth chipping in.

The short answer is yes, and it's worse than most people expect. A "100W panel" on a genuinely overcast UK winter day might be pushing 10-15W if you're lucky. You're basically trying to fill a bath with a garden hose that's got a kink in it.

A few things that helped me:

• MPPT over PWM — if you're still running a PWM controller, swap it out. Victron SmartSolar made a noticeable difference for me in low-light conditions
• Panel angle matters more in winter — flatter panels on a van roof are losing significant harvest when the sun is low
• Add capacity, not just panels — sometimes the honest answer is a second 100Ah battery rather than fighting the solar. Fogstar do decent value cells if you're going DIY

The real trap people fall into is buying more panels thinking it solves everything. On a proper grey November day in the UK, you could have 400W on the roof and still be disappointed.

What's your current controller and panel wattage? Also worth knowing if you're running 12V or 24V — 24V systems tend to extract more from marginal light conditions in my experience.

Anyone else found particular panel brands that perform noticeably better in diffuse light? I've heard good things about half-cut cell technology but haven't tested it personally yet.

@24VGeek interesting to hear the shepherd's hut angle — I'm running a similar setup in mine and have been wrestling with exactly this.

Can I ask what panels you're running and what your actual measured output looks like on a proper grey UK day? I've got a Victron SmartSolar logging everything and the numbers are genuinely sobering compared to what the specs suggest.

Specifically wondering:

• Are you seeing the panel voltage drop or mainly the current collapse?
• What's your baseline daily consumption?

I'm skeptical of the "just add more panels" advice that gets thrown around — there's presumably a point where roof space becomes the hard constraint regardless. Curious whether you've done the maths on whether a secondary charging source (shore power, generator) is actually unavoidable for UK winters rather than just optional.

Really feel this one. I've got 200W on the roof of my Transit and even that struggles to keep pace with just the fridge and a bit of phone charging on a proper grey November day up here in Yorkshire. The panels might be putting out 20-30W realistically when it's thick cloud cover.

One thing that's helped me massively is being much more disciplined about when I use heavy loads — run the kettle or charge laptops in whatever passes for the brightest part of the day, even if it's still gloomy. Sounds obvious but I wasn't doing it systematically before.

@MarineAlan curious what you're running in the shepherd's hut — would love to know if you've got any wind input supplementing the solar, because that's where I reckon the real answer lies for fixed installations.

@RussMitchell75 200W and a fridge is basically a financial arrangement where the sun takes your money and gives you slightly-cold cheese — I went through exactly this with my garden office before I stopped pretending panels were magic and just threw a Fogstar 200Ah in and wired a small MPPT properly via a Victron SmartSolar, which at least tells you how badly you're failing rather than just silently draining. The real issue nobody mentions is that UK "partly cloudy" on the tin label means your 200W panel is doing maybe 40-60W for six hours if you're lucky, so the maths simply doesn't work without either more panels or accepting you'll be running off stored capacity most of the time.

@DuctTapeDave that last line is going in my notes, cheers — brilliant.

Living aboard a boat so I know this pain intimately. The problem nobody talks about enough is that a 100Ah lithium isn't really 100Ah in practice — your Victron BMV will show you just how savage the voltage sag is under load on a grey February afternoon when your panels are pushing maybe 15-20% of rated output.

What actually saved my sanity was adding a small DC-DC charger wired to the engine alternator. Even a 20-minute pootle along the canal or — in your case — moving the hut's tow vehicle — gives the Fogstar pack a meaningful top-up that three hours of weak winter sun simply cannot match.

Solar alone in the UK through autumn and winter is optimism, not a strategy.

@DuctTapeDave honestly the sun's running a subscription service and you're on the free tier from October through March.

On the narrowboat I pair 240W of Renogy panels with a Victron MPPT and even then a proper grey week in November has me reaching for the shore power like a man with no pride. The maths that nobody mentions: a 100Ah LiFePO4 at 50% DoD needs roughly 55Ah back in, and at 10A realistic harvest on a cloudy day you're looking at five-plus hours of actual charging current — not just the panels sitting there having a think.

Two things that genuinely helped me:

• Victron MPPT over a cheap PWM (squeezes maybe 20-30% more out of flat light)
• Accepting that alternator charging via a Fogstar-compatible DC-DC charger is your real friend in winter