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

Running a small chest freezer on my narrowboat at the moment and trying to work out if a single 200Ah LiFePO4 is going to cut it overnight without solar input.

The freezer is a Brass Monkey 30L — pulls around 45W when the compressor kicks in, duty cycle seems to be roughly 30-40% in a cool environment (it's in the cabin, not outside). So I'm estimating maybe 150-200Wh overnight over say 10-12 hours?

A 200Ah battery at 12V is theoretically 2400Wh, but I'm only comfortable pulling it down to 80% DoD on a LiFePO4 — so that's around 1920Wh usable. On paper that's masses of headroom.

But I keep second-guessing myself:

• Am I underestimating the freezer's consumption? It's ambient-dependent obviously
• Should I be accounting for inverter losses if I run it on AC? (I've got a Victron Phoenix 12/375 — not sure what efficiency that runs at under light load)
• Does a full chest freezer run more or less efficiently than a half-empty one?

Battery is a Fogstar Drift 200Ah for reference, so the capacity figures should be fairly reliable.

Anyone actually done this long-term rather than just theorising? Would feel better hearing from someone running a similar setup before I commit to keeping frozen food aboard permanently.

@HollyBaker done this in my motorhome with a similar sized compressor freezer. Rough rule of thumb — a 30L chest freezer pulls maybe 30-45W average once it's up to temp (compressor cycles on/off). So overnight say 8-10 hours, you're looking at roughly 30-45Ah from a 12V bank.

200Ah LiFePO4 you've got loads of headroom tbh, even accounting for not wanting to drop below 20% SOC you've got ~160Ah usable.

Main thing I'd watch is the startup surge — compressor kick-on can spike pretty hard so make sure your inverter/12V setup can handle it.

Also worth checking ambient temp on the boat — colder nights actually help the freezer work less hard.

Should be absolutely fine overnight with zero solar input. I'd not worry about it.

@HollyBaker running a Fogstar 200Ah in my cabin for backup — chest freezer is one of the main loads.

Key thing people miss: ambient temp matters massively. Freezer on a narrowboat in summer vs winter is a completely different story. Mine draws way more in warm weather.

Also check if yours has a fast-freeze cycle — some kick in randomly and spike consumption.

Rough experience: my small compressor fridge/freezer pulls maybe 15-25Ah per 24hrs in cool conditions, more like 35-40Ah when it's warm. So 200Ah should be fine overnight, but I wouldn't want to go multiple cloudy days without solar input.

What's your overnight window — 10 hours or more like 14-15?

@HollyBaker — narrowboat life, nice! 🚤

Short answer: probably yes, but it depends how often someone's raiding it for ice lollies at 2am 😄

The compressor cycle is the key thing — a well-insulated 30L in cool ambient (narrowboat bilge air helps) might only pull

Brass Monkey 30L typically pulls around 30–45W when the compressor kicks in, but average consumption over 24hrs is usually closer to 0.3–0.5kWh depending on ambient temp — narrowboat bilge in summer vs winter makes a massive difference.

200Ah LiFePO4 at 12V = 2.4kWh usable (assuming you're not murdering it below 80% DoD like a savage), so overnight you're looking at maybe 150–250Wh draw — you've got headroom.

The sneaky killer nobody mentions: lid gasket condition and how often you're opening it. Worn seal = compressor cycling every 8 minutes = your maths goes straight in the canal.

Stick a Victron SmartShunt on it and actually measure the thing for a few nights before trusting anyone's estimates, including mine.

Ran almost exactly this setup last winter on my boat — Fogstar Drift 200Ah paired with a Victron SmartShunt so I could actually see what was happening overnight.

The numbers @FormerCop mentions line up with what I was seeing. Where it gets interesting on a narrowboat is ambient temperature — my engine room gets surprisingly cold in January, which actually helped the freezer cycle less.

One thing worth watching: your BMS cutoff voltage. If you're only using 80% usable capacity (sensible for longevity), you're working with 160Ah. A well-insulated chest freezer in cool conditions should sit comfortably within that, but I'd run the SmartShunt for a few nights before trusting it completely.

Don't open the lid more than necessary before bed — sounds obvious but it makes a real difference to compressor runtime.

@LisaKelly66 has the right idea with the SmartShunt — know your actual consumption, not a guess.

Quick maths: 200Ah × 12V = 2.4kWh usable (assuming you're not flogging it below 20% SOC), and a chest freezer averaging ~15–20Wh will chew through roughly 180–240Wh overnight (12hrs) — so you're looking at using perhaps 10–15% of capacity. Perfectly manageable.

Caveat: ambient temperature matters enormously on a narrowboat — a freezer working harder in a warm engine room versus a cold well deck is a very different animal. Also check your BMS continuous discharge rating against compressor startup surge; most decent cells handle it fine but worth confirming.

Fogstar or Victron-monitored setup = you'll actually know rather than speculate, which is rather the point of telemetry.

Good point from @OldSailor — the maths stacks up reasonably well on paper, but ambient temperature is the variable nobody mentions. A chest freezer in a cold engine room or below-waterline locker in January will cycle far less frequently than one sitting in a warm saloon. That directly affects overnight draw.

Worth also considering your minimum usable voltage — if you're on a BMS that cuts at 2.8V/cell, you're not getting the full 200Ah anyway. A Victron BMV-712 or SmartShunt will show you real state-of-charge rather than voltage guesswork.

One 200Ah should probably cope, but I wouldn't run it below 20% SOC routinely.

Worth adding from a static caravan angle — chest freezers are surprisingly efficient once they reach temperature. The compressor duty cycle drops significantly in cooler ambient temps (which overnight usually means), so your real consumption could be noticeably lower than the nameplate suggests.

That said, a single 200Ah cell is cutting it fine if you're also running any other loads overnight. Worth checking whether your LiFePO4 has a low-temperature cutoff too — some cheaper cells won't discharge properly below 5°C, which could catch you out on a cold night even before the capacity question matters.