Anyone know if a 200Ah lithium battery will run a small chest freezer overnight?

Running a small chest freezer overnight is something I've been trying to work out for my narrowboat setup. I've got a 200Ah lithium (Fogstar Drift) and a Victron SmartShunt keeping an eye on things, but I'm not confident I've done the maths right.

The freezer I'm looking at is a 60-litre chest freezer rated at around 90W. I know these things cycle on and off rather than run continuously — from what I've read it's maybe 30-40% duty cycle? So rough figures:

• 90W × ~8 hours (overnight) × 40% duty = roughly 29Ah at 12V?

That seems almost too good to be true. Does the startup surge matter for lithium, or is that more of a concern with lead acid?

Also wondering whether ambient temperature plays a big role — the boat can get cold overnight obviously, which I'd assume actually helps the freezer work less hard? Or am I wrong about that?

My main use case is keeping frozen food when I'm moored away from a hook-up for 2-3 nights. I've got 400W of solar on the roof but in autumn/winter that's obviously unreliable.

Has anyone actually measured their freezer draw with a proper energy monitor rather than just estimating? Would love to see some real-world figures before I commit to buying one.

KentOffGrid | 847 posts | ⚡ Solar & LiFePO4 enthusiast

Hey @Emma1996, short answer is probably yes, but it depends heavily on the freezer itself. Small chest freezers are surprisingly efficient — a decent one might only draw 30-60Ah overnight once it's settled at temperature. Your 200Ah Fogstar gives you around 180Ah usable (keeping above 10%), so you've got plenty of headroom.

Key thing to watch: the initial pull-down if the freezer's been off for a while will hammer the battery much harder than steady-state running.

Your SmartShunt is brilliant for this — run it for a few nights and check your actual consumption rather than guessing. What's the model of the freezer? That'd help give you a much more specific answer. Chest freezers vary wildly in efficiency between manufacturers.

MoorLee | 312 posts | 🔋 Battery nerd on a budget

@Emma1996 the real question is — will the freezer run overnight, or will you run overnight checking the SmartShunt every 20 minutes in a blind panic? 😄

Honestly though, a small chest freezer is one of

DevonDweller | 2,341 posts | 🔋 LiFePO4 advocate | Static van & narrowboat

The numbers actually work out reasonably well here. A typical small chest freezer (40–60L) draws around 80–150Wh per 24 hours depending on ambient temperature — canal air tends to keep things cooler than a house, which helps enormously.

On your 200Ah Fogstar Drift (assuming 12V, so ~2,400Wh usable at 80% DoD = ~1,920Wh), overnight consumption should be roughly 40–70Wh, leaving plenty of headroom.

Critical caveat though: the compressor start surge can be 3–5× running watts. Check your inverter's surge rating if you're running AC. A 12V compressor freezer like an Iceco or BougeRV bypasses this entirely and is far more efficient — well worth considering on a narrowboat where every watt-hour counts.

Your SmartShunt will show exactly what's happening. Watch the overnight Ah consumed figure rather than guessing.

Stacey72 | 156 posts | ⚡ Solar & wind hybrid | Narrowboat liveaboard

@Emma1996 I've got almost exactly this setup on my boat! One thing worth mentioning that the others haven't touched on — check whether your freezer has a decent thermostat and lid seal. A poorly sealing lid can double your consumption overnight, which would make the maths much tighter. Also, the Fogstar Drift handles partial state of charge really well, so don't panic if your SmartShunt shows it dipping into the 50s percentage-wise — it'll be fine. Pre-cooling the freezer before you rely solely on battery (while you're still plugged into shore power or the alternator's running) makes a noticeable difference too. Narrows the overnight draw considerably in my experience. 🙂