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

Running a small chest freezer off a 200Ah lithium on my narrowboat and genuinely not sure if I've got enough capacity to sleep soundly about it.

The freezer is a Subcold 60L — rated at around 45W but obviously it cycles, so real-world draw is probably closer to 15-20Wh per hour once you account for the compressor duty cycle. Over 8 hours that's maybe 120-160Wh, which should be well within what a 200Ah (roughly 2,400Wh usable if lithium) can handle.

But here's where I get nervous:

• My Fogstar Drift 200Ah is shared with other loads — bilge pump, lights, a 12V fan
• I'm moored somewhere shaded this week, so solar top-up overnight is obviously zero
• The freezer is only half-full, which I've read actually makes the compressor work harder

Has anyone actually logged this with a Victron BMV or similar? I keep second-guessing the theory and wondering if there's something I'm missing about compressor startup draw causing issues, or whether ambient temperature (it's been warm lately, even on the cut) meaningfully affects duty cycle enough to matter.

Tempted to just run it for a night and watch the BMV like a hawk, but wanted to sanity-check the maths with people who've actually done this before committing the freezer space to a full shop.

@TorDweller should be fine honestly. Chest freezers are decent because they cycle on/off rather than running flat-out — realistically that 45W unit is probably averaging 15-25W over time once it's up to temp.

Rough numbers:

• 200Ah @ 12v = 2400Wh usable (assuming Fogstar or similar LiFePO4, so ~100% DoD safe)
• 8hrs overnight @ say 20W average = 160Wh used

That's barely a dent. Even if it's pulling harder in warm weather you're looking at maybe 300-400Wh worst case.

My garden office setup runs a similar sized compressor fridge and it barely touches the battery overnight — always wakes up above 95%.

Only caveat — make sure your BMS can handle the compressor startup spike. Usually fine but worth checking the spec sheet.

@TorDweller A few things nobody's mentioned yet — ambient temperature matters enormously here. In summer on a narrowboat that hull absorbs heat like nobody's business, and your freezer will cycle far more aggressively than the manufacturer's duty cycle suggests. Also, what's your BMS low-voltage cutoff set to? A 200Ah lithium is nominally 200Ah but if your BMS trips at 20% you've effectively got 160Ah usable.

Do the actual maths: 45W ÷ 12V = 3.75A draw at peak, but realistic average with cycling is probably 1.5–2A. Over 8 hours that's 12–16Ah. Sounds brilliant until your alternator isn't running and you've also got a bilge pump, nav lights, and a phone charger quietly murdering your bank overnight.

One 200Ah Fogstar or similar should manage it, but I wouldn't be sleeping soundly without a decent battery monitor telling me exactly what's happening.

@TorDweller I'll add some numbers to back this up. That Subcold will likely have a duty cycle of 30-40% in typical UK ambient temps, so you're realistically looking at 15-18W average draw. Over an 8-hour night that's roughly 120-145Wh, or about 10-12Ah from a 12V system — well within a 200Ah lithium's comfortable range even accounting for a 80% DoD limit.

I run a similar setup on my narrowboat and the freezer is genuinely one of the least demanding loads overnight. What will bite you is forgetting to factor in everything else drawing simultaneously — inverter standby, bilge pump, nav lights if you're moored with them on.

Worth chucking a Victron Smart Shunt in-line if you haven't already — takes the guesswork out completely and you'll see exactly what's happening overnight.

@TorDweller ran almost identical numbers on my van build — Fogstar 200Ah with a small compressor fridge, and the maths that nobody wants to do is: at 40% duty cycle your 45W freezer is only pulling ~18W average, meaning roughly 0.75kWh over 8 hours, which against a 200Ah/12V (2.4kWh usable at 80% DoD) leaves you with plenty of headroom — assuming your BMS doesn't throw a strop at 3am like mine did.

Ran a chest freezer off a 100Ah Fogstar in my static for a whole winter before I realised I'd wired the BMS backwards. Still worked, don't ask me how.

Point being — 200Ah is plenty for a 45W freezer overnight. @PennineNomad's duty cycle numbers are spot on. You're

Really interesting thread this — I did almost exactly this on my narrowboat last year before upgrading to a 300Ah bank.

One thing nobody's mentioned yet: ambient temperature matters enormously. A narrowboat bilge in January vs July can shift that duty cycle @PennineNomad mentioned quite dramatically. My freezer was working significantly harder in summer when the boat was baking.

Also worth factoring in your inverter efficiency losses if you're running 240V — a pure sine inverter sat idle still draws 10-20W. If the Subcold has a 12V option, use it directly and you'll claw back meaningful capacity overnight.

200Ah lithium (so roughly 180Ah usable at 90%) against maybe 25-35Ah actual overnight draw? You're almost certainly fine, but I'd bung a Victron SmartShunt on there and check real-world consumption for a few nights before fully trusting the maths.

Jumping in as I'm curious about this for my motorhome setup too. Quick maths question — is the 45W rating a peak draw or an average? I'm assuming compressor freezers cycle on and off, so actual consumption over 8 hours could be significantly less than 45W × 8 = 360Wh.

Does anyone know the typical duty cycle on a unit like the Subcold 60L? If it's cycling at roughly 30-40%, you'd be looking at maybe 100-150Wh overnight, which would barely touch a 200Ah Victron or Fogstar bank.

Has anyone actually measured this with a Shunt or smart meter rather than going off the nameplate rating?