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

Running a small chest freezer on my narrowboat at the mo, and I'm trying to work out if my setup is actually viable before I commit to stocking it with a month's worth of sausages.

Got a 200Ah Fogstar Drift lithium sitting under the dinette, paired with a modest 200W solar panel on the roof — which, let's be honest, does approximately nothing between October and March up here on the cut.

The freezer is a Subcold Ultra 35, rated at about ~0.5 kWh per 24 hours according to the spec sheet, though I know real-world draw can vary depending on ambient temp and how often you're raiding it for said sausages.

My rough maths says:

• 200Ah @ 12.8V = ~2.56 kWh usable (assuming 80% DoD = ~2kWh safely)
• Freezer overnight (~12 hrs) = roughly 0.25 kWh
• Seems fine in theory?

But I've been around kitchens long enough to know the difference between theory and what actually lands on the plate. A few things I'm not sure about:

• Does the compressor startup surge cause any issues with a standard 12V inverter?
• Should I be running it directly on 12V DC (the Subcold has a DC option) rather than through an inverter to save losses?
• Is 200Ah genuinely enough if I have zero solar input for a few cloudy days?

Anyone actually done this long-term on a boat or static? Would love to hear from people running real setups rather than just spreadsheet warriors (no offence to the spreadsheet warriors, you know who you

@RetiredChef the sausages are the real variable here aren't they — how warm are they going in 😄

Seriously though, depends massively on the freezer's actual draw. Most small chest freezers are pulling maybe 30-60W when the compressor kicks in, but they're not running constantly — duty cycle's probably 30-50

OllieGraham91 | 47 posts

@RetiredChef good question before you commit to the sausages! The short answer is probably yes, but it depends on your freezer's actual draw.

Most small chest freezers run at around 30-60W but cycle on and off, so real-world consumption overnight (say 8 hours) is often 150-300Wh rather than the theoretical maximum. Your 200Ah battery at 12V gives you 2400Wh total, but with lithium you generally want to keep above 20% capacity, so call it ~1920Wh usable.

The freezer overnight should be well within that — the bigger question is what else you're running simultaneously on the boat. Lights, a phone charger, a pump — it all adds up.

Worth sticking a cheap energy monitor on the freezer for 24 hours before loading it up, so you know your actual figures rather than guessing. £15 on Amazon well spent!

Good question to stress-test before you fill the thing. Running a cabin off a 200Ah Fogstar pack myself, and I've had a small Klarstein chest freezer drawing roughly 30–40Wh per hour once it's settled at temperature — so overnight (say 10 hours) you're looking at around 300–400Wh total.

On a 200Ah / 12V lithium that's ~2,400Wh usable (don't go below 20%), so the freezer alone? Comfortable.

The catch with narrowboats is ambient temperature swings — the compressor works harder in summer. And @EcoFlowMaster is right that pre-loading warm sausages will spike consumption initially.

Worth fitting a Victron SmartShunt if you haven't already — watching actual draw over a few nights tells you more than any calculation will.

The narrowboat angle matters here — ambient temp in the hull will swing a lot depending on mooring and season, and chest freezers hate working hard against a warm environment.

What's the compressor duty cycle on your unit? That's the real number you need. My garden office freezer pulls around 80–90W when running but only kicks in maybe 30–40% of the time in cool weather. That works out to roughly 30–40Ah overnight — well within a 200Ah Fogstar.

Summer on a sun-baked narrowboat? Entirely different story. I've seen duty cycles creep toward 60–70% when my office bakes in July.

Worth grabbing a cheap Shelly plug or similar energy monitor and logging a 24-hour baseline before the sausages go in. Takes the guesswork out completely.

One thing worth adding — duty cycle is your friend here. A decent chest freezer won't be running the compressor constantly, more like 30-50% of the time depending on ambient. So your actual draw overnight is usually much lower than the nameplate wattage suggests.

Worth sticking a Shelly plug or similar on it for a few days to log real consumption before you commit. Saved me a lot of guesswork when I was sizing for my tiny house setup.

Also, @CrispyWanderer's point about hull temps is spot on — winter mooring could actually help you there, less work for the compressor.