Anyone else struggling to keep a 100Ah lithium topped up with just one 100W roof panel?

Mate, one panel on a 100Ah lithium is like trying to fill a bath with a hosepipe pointed vaguely in the direction of the bath — on a cloudy Tuesday in Wolverhampton.

Been there with my old setup before I wised up. The maths looks fine on paper (100W panel, maybe 30Ah a day if you're lucky with UK skies), but the actual harvest once you factor in November, trees, the panel being at a rubbish angle on a flat roof, and the Victron MPPT deciding it fancies a rest — you're looking at maybe half that on a good week.

Couple of things that actually helped me:

• Add a second panel — even a cheap 100W Renogy chucked on a tiltable bracket made a massive difference
• Split charge relay or a B2B charger — driving to the next spot? Let the alternator earn its keep
• Check your parasitic loads — my "off" fridge was quietly sipping 8A like it owned the place

The Fogstar 100Ah I've got now handles partial charging better than my old AGM ever did, but it still needs feeding. One panel in the UK isn't a solar setup, it's more of a solar suggestion.

Anyone running a decent B2B alongside a single panel finding that's enough, or are you all just quietly adding more panels and pretending you planned it that way from the start? 👀

@ExBrickie94 the Wolverhampton analogy is genuinely quite good, I'll give you that.

Worth being pedantic about the actual numbers though — a 100W panel in the UK is realistically going to give you maybe 300-400Wh on a decent summer day, less in winter, potentially embarrassingly little in November. Your 100Ah lithium at 12V is 1200Wh usable (roughly). So you're not completely mad, but you've got basically no margin for cloudy days or heavy loads.

Had a similar situation on my static van before I doubled up the panels. The difference was night and day.

The hosepipe analogy only really holds if you're running a decent load — what are you actually powering? Because if it's just LED lights and phone charging, one panel might scrape by. If there's a compressor fridge involved... yeah, get another panel on there sharpish.

@ExBrickie94 the panel sizing issue is real, but don't overlook the controller side either. A 100W panel through a PWM controller into a lithium is leaving a meaningful chunk of potential yield on the table — MPPT will typically recover 20-30% more in UK conditions where you're spending most of your time in partial irradiance rather than peak sun.

Running a Victron SmartSolar 75/15 on my shepherd's hut build transformed what I was seeing from a similarly modest array before I expanded it. The difference between a flat February and being genuinely comfortable is often the controller choice, not just raw panel wattage.

That said, one 100W panel is genuinely undersized for a 100Ah lithium if you're running anything meaningful. The rough rule I work to is 20-25W per 10Ah as a practical minimum for touring.

@BordersExplorer makes a fair point about the controller — I switched from a cheap PWM unit to a Victron SmartSolar 75/15 on my static caravan setup and genuinely picked up usable charge on days where the old controller was basically doing nothing.

But honestly, if you're running a 100Ah lithium (Fogstar Drift in my case), one 100W panel is just undersized from the start. Lithium will happily accept charge at a rate a single panel can't deliver. You're leaving capacity on the table every decent morning.

What's your actual daily consumption? That's the bit nobody mentions — panel size is meaningless without knowing what you're pulling out overnight. Running just a 12V compressor fridge and phone charging is very different to an inverter for a laptop and a diesel heater fan running all night.

What's the van setup specifically?

@LisaStewart71 the Victron SmartSolar is hard to argue against, but the bigger issue nobody's mentioned yet is panel orientation on a motorhome roof.

Fixed flat-mount panels take a brutal efficiency hit at anything other than solar noon. My shepherd's hut install uses a tiltable frame — even a 30° tilt versus flat makes a measurable difference to morning and evening harvest, which is precisely when you want the controller working hard.

On a 100W/100Ah setup, you're also fighting a capacity-to-charge-rate mismatch. A healthy lithium wants 0.2–0.5C charge current. Your 100W panel delivers maybe 5–6A real-world — that's 0.05–0.06C on a 100Ah battery. You're barely maintaining it, let alone recovering from meaningful discharge. Two panels wired in parallel through a decent MPPT is the minimum sensible starting point.