Anyone else struggling to keep a 100Ah lithium topped up just from solar on a cloudy Scottish trip?

Spent three weeks on the cut in Scotland last autumn and genuinely felt like I was rationing power for the first time in years. Running 200W of Renogy panels feeding a 100Ah Fogstar Drift and even on the "good" days I was barely seeing 30-40% of rated output. The haar and low sun angle absolutely murders your harvest up there.

A few things that made a real difference:

• Tilt your panels if you can. Flat-mounted on a roof is brutal when the sun's barely clearing the hills at 25 degrees elevation
• Victron MPPT over a cheap PWM controller — squeezing every last watt matters when there's not much to squeeze
• Be honest about your loads. I audited mine properly and found a cheap 12V compressor fridge was the main villain

The narrowboat crowd have this dialled in because we have to — no hookup, no genny (well, some do, but it feels like cheating). The difference is we're used to treating a cloudy week as normal, not an exception.

Scotland in summer theoretically gives you long days, but "long" doesn't mean "bright." I'd genuinely suggest anyone heading north of Inverness in anything other than June/July treats their 100Ah as effectively 60Ah for planning purposes.

Anyone running a second battery or a small wind turbine alongside their solar up there? Curious whether the extra complexity is worth it given how gusty the Highlands can be — seems like it might actually complement solar better than anywhere else in the UK.

SolarGeorge | Posts: 847

@HalfAJob Scotland in autumn is genuinely brutal for solar - you're looking at maybe 1-2 peak sun hours on a grey October day, so realistically pulling 200-400Wh from your 200W array when everything cooperates. Your 100Ah Fogstar at 12V is only 1.2kWh usable anyway, so the maths gets uncomfortable quickly.

Few things worth considering: are your panels tilted at all? Even propping them up to 45° can make a meaningful difference at higher latitudes when the sun barely clears the horizon. Also worth checking your MPPT is properly sized and not clipping.

Honestly though, Scotland in October is where solar earns its limitations. A small shore power hookup or even a 12V alternator feed whilst driving between locations might be the more practical solution rather than chasing more panels.

PanelEwan | Posts: 312

@HalfAJob Three weeks on the cut in Scotland in autumn — yeah, that's genuinely one of the harder solar scenarios you can throw at a system. 200W into 100Ah sounds reasonable on paper but the maths falls apart fast when you're dealing with tree-lined canal banks and low sun angles and persistent overcast.

The bit people underestimate is panel orientation on a narrowboat — you're often pointing the wrong direction half the day depending on your mooring. I've got 400W on my roof and still saw miserable yields on the Caledonian last October.

Worth considering a small Victron MPPT if you're not already on one — the tracking efficiency genuinely matters more in marginal light than people realise. Also, what's your actual consumption? 100Ah lithium disappears surprisingly quickly if you're running a compressor fridge.

RustySkipper | Posts: 64

@HalfAJob I've had almost identical on my narrowboat — 200W felt plenty during summer but come October it's a different story entirely. Curious whether you're running MPPT or PWM? I swapped to a Victron SmartSolar 100/20 last year and the difference in harvesting those weak, low-angle winter panels was genuinely surprising. Also worth looking at whether your panels are getting any shading from masts, aerials, or even just morning dew sitting longer in cold temps. Have you considered adding even a small wind element? I keep meaning to look into a Rutland 504 or similar for those grey Scottish days when the wind's howling but the sun's nowhere. What were you typically seeing as a peak input on your worst days — like single-digit amps?

ExPostie82 | Posts: 1,203

Worth flagging something nobody's mentioned yet — panel angle matters enormously in Scotland's autumn. The sun barely climbs above 20° elevation at that latitude, so panels lying flat on a roof are losing a substantial chunk of potential harvest. Even tilting to 45–50° can recover 30–40% more energy on low-sun days.

I've got a static setup in a shepherd's hut (fixed panels, obviously) and I ran the numbers through PVWatts one winter — the difference between flat and optimally angled was genuinely startling.

Also, @HalfAJob — what's your Victron MPPT set to for absorption voltage? Some Fogstar Drifts are fussier than people realise about hitting that 14.6V properly, especially if your charge controller is undersized for low-irradiance conditions where current stays weak all day.