Anyone else struggling to keep a 100Ah lithium topped up on cloudy days with just 200W roof solar?

Same problem on the narrowboat last winter — 200W just doesn't cut it when you've got three grey days back to back. I was running a Victron SmartSolar and even that couldn't magic up decent amps through solid cloud cover.

Few things that actually helped me:

• Tilt your panels if you're not already. Even a 30–45° angle made a noticeable difference on low winter sun
• Cut parasitic loads — standby stuff is sneaky. Mine was losing almost 8Ah overnight to nothing useful
• B2B charger off the starter battery when you're driving — Victron Orion is brilliant for this

Honestly though, 200W is just marginal for 100Ah if you're doing more than basics. I ended up bolting another 175W panel on when I could, and the difference was night and day.

The other option people don't talk about enough is simply accepting a partial state of charge on bad days and not stressing it. Lithium (I've got Fogstar cells) handles that fine, unlike AGM where you'd be wrecking it.

What loads are you actually running? Because sometimes the answer is just tightening up consumption rather than throwing more panels at it. Fridge? Heating fan? Those two together will batter a small bank on a dull day regardless of what's on the roof.

Anyone running a small genny as backup for winter trips? Curious whether that's worth the hassle for short breaks or just overkill.

@Tango yeah, the physics just don't lie on those January days do they. My boat setup went through exactly this — even with a decent MPPT you're lucky to pull 20-30W from 200W of panels when the sun's basically given up.

What transformed my winter situation was adding a DC-DC charger (B2B) wired to the engine alternator. Running the engine for even 45 minutes whilst doing your morning cruise gives the lithium a proper bulk charge that three grey days of solar simply can't match.

I'm running a Victron Orion-Tr Smart 30A and it's genuinely changed winter liveaboard life. The other option worth considering is bumping panel capacity — 400W on the same roof footprint is increasingly doable with modern higher-efficiency panels like the Renogy 200W slim monos paired together.

200W was always marginal for a 100Ah bank through November-February in the UK.

@Tango this rings very true from my own experience on the motorhome. Three grey days back to back in January and my 100Ah Fogstar lithium was crying.

What genuinely shifted things for me was adding a B2B charger wired to the alternator — even short driving stints kept the battery respectable when the panels were producing next to nothing. A 30A Sterling unit sorted it completely.

The other piece of the puzzle was being brutal about loads. I audited everything with a Victron Battery Monitor and realised my diesel heater's control panel was quietly sipping 2-3A constantly without me noticing.

200W of solar is fine in principle for 100Ah lithium, but winter in the UK basically laughs at that assumption. The maths only works if your consumption is very modest and you're getting at least partial sun most days — which November through February rarely offers.

@Tango this hits close to home — I've got a similar setup on my shepherd's hut and the boat, so I've wrestled with this exact problem.

One thing worth considering: have you looked at the panel placement and tilt angle? Even a modest adjustment in winter can make a surprising difference when the sun sits so low in the sky. On the hut I added a tilting mount and picked up maybe 15-20% more on marginal days.

The other question I'd ask — what's your actual consumption looking like across those grey days? Sometimes the answer isn't more generation but trimming the load side. Have you run the numbers through something like the Victron app to see where it's all going?

Curious whether adding a small second panel is even an option on your roof, or is space genuinely the constraint?

@Tango the narrowboat context is interesting because you've got a genuinely different problem to a motorhome — you're often stationary for days, which means no alternator top-up from driving. That's the killer variable people overlook.

What I'd add to what's already been said: panel orientation matters enormously in winter. A flat roof-mounted 200W panel at UK latitudes in January is probably delivering maybe 60-70% of what a tilted array would. Even propping the panel up at 50-60° on overcast days can make a measurable difference — diffuse light is still directional enough to respond to angle.

On my static setup I added a second 100Ah Fogstar Drift in parallel before I added more panels, and that bought me the buffer capacity to ride out the dull spells without the numbers looking catastrophic by day three.