Does anyone know if a 200W panel can realistically charge a 100Ah lithium battery on a cloudy UK day?

So I've finally taken the plunge and fitted a 200W mono panel on the roof of my Transit conversion, wired up to a Renogy 40A MPPT controller and a 100Ah lithium (LiFePO4) battery from Fogstar. Everything seems to be working fine on sunny days — I'm getting around 8–10A into the battery most of the afternoon, which feels about right.

The problem is I'm planning a trip to Scotland in October and I'm genuinely worried about keeping up with my usage on those grey, overcast days we all know and love. I'm running a 12V compressor fridge (reckons it draws about 4–5A on average), a few USB chargers, and occasionally a 12V laptop adaptor. So probably 60–70Ah per day if I'm honest.

Has anyone got real-world figures for what a 200W panel actually produces on a proper cloudy UK day? I've seen people quote things like 10–20% of rated output, which would mean I'm barely scraping 20–40W on a bad day — that feels pretty grim. Is that roughly what people are seeing?

Also wondering whether it's worth adding a second panel if there's space, or whether I should just budget for shore power hookups on the worst days. Any experience with Scotland in autumn specifically would be dead helpful.

@Debbie1999 great setup choice — Fogstar Drift is a cracking battery for the money.

Realistic expectation on a proper overcast UK day: your 200W panel will likely deliver 15–40W in heavy cloud, maybe 60–80W in light overcast. So you're looking at perhaps 1–4 Ah per hour rather than the theoretical 10–11Ah.

The silver lining (pun intended) is that LiFePO4 accepts whatever current arrives without complaint, unlike AGM which sulphates from partial charging.

Practical numbers from my own boat setup:

• Heavy overcast: ~0.5–1.5kWh generated daily from a 400W array
• Scale that down proportionally for your 200W

Your 100Ah Fogstar at 80% usable depth means you need roughly 80Ah to fully replenish. On a grey February day? Possibly 2–3 days of cloudy weather to recover from deep discharge.

Worth enabling the Renogy's equalization-free LiFePO4

On the boat I typically get 1-2 peak sun hours on a grim overcast day in the UK, sometimes less in winter. So realistically your 200W panel might only push out 40-80Wh on a proper grey day — that's not nothing but won't fully charge a 100Ah (roughly 1280Wh usable) from low state.

Worth checking what your Renogy controller is actually seeing via its app — the real numbers tell you more than any estimate.

Good news is LiFePO4 takes whatever charge is available efficiently, no faff with absorption stages dragging on like with AGM. So every scrap of sun counts.

If you're static for days at a stretch you'll probably be fine with moderate usage. Running hard off-grid in November though? Might want a backup plan — hook-up, alternator, or a second panel down the line.

On a proper grey UK "summer" day you're looking at maybe 2-3 hours of useful sun equivalent, so realistically 40-60Wh if you're lucky — enough to top up but don't expect miracles if you've been running a compressor fridge overnight. 🌧️

Hey @Debbie1999, great setup! Just to add to what @BazBurns and @LutonBuild have said — the good news with LiFePO4 is that you're not fighting internal resistance the same way you would with AGM, so whatever energy you do generate is going into the battery far more efficiently. Your Renogy MPPT will also squeeze out every last watt compared to a PWM controller, which makes a real difference on those murky days. If you're sitting at say 50% SoC, even a modest overcast day should claw back a meaningful chunk. Where it gets tricky is consecutive gloomy days with heavy usage — that's when a small backup like a B2B from your alternator earns its keep. How much power are you typically drawing daily?