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

Running a narrowboat and trying to figure out if a single 200Ah lithium (Fogstar Drift 12v) is going to cut it for keeping a small chest freezer going overnight without the engine running.

The freezer is a Inventor 35L — pulls about 45W when the compressor kicks in, duty cycle seems to be roughly 30-40% depending on ambient temp. So I'm guesstimating maybe 200-250Wh over an 8-hour night?

On paper that sounds fine — 200Ah at 12v is 2.4kWh usable (well, ~2kWh if I'm keeping it above 20% SoC). But I've got other loads obviously. Lighting, phone charging, maybe the water pump. Probably another 100-150Wh realistically.

So total maybe 350-400Wh overnight? Still looks doable on paper.

But I'm a bit wary because:

• Ambient temps on the cut can be cold in autumn/winter which helps the freezer but can affect battery performance
• I'm not sure my duty cycle estimate is right
• No solar overnight obviously, relying purely on what's in the bank from daytime running

Has anyone actually done this rather than just maths'd it? Wondering if one 200Ah is genuinely enough or if I should be thinking about a second battery. Already got a Victron SmartShunt so I can actually monitor what's being pulled.

Not sure if this has been covered already, but have you factored in the freezer's duty cycle? Most small chest freezers don't run continuously — they cycle on and off, so the actual draw is lower than the rated wattage suggests.

That said, I'm running a garden office setup (not a narrowboat) and found ambient temperature makes a massive difference to how hard the compressor works. On a warm summer night vs winter it's quite different.

Has anyone done actual measurements with a clamp meter or a Victron BMV to see real consumption overnight rather than just going off the spec sheet? I've been wondering about chest freezer efficiency myself for a potential tiny house project — do the smaller ones (like 60-80L) draw significantly less than a typical 100L+ model?

Would love to see if @LochDweller can share the specific model of the freezer too.

@Liz1979 already sniffed out the duty cycle point, so I'll add the maths — a typical small chest freezer pulls ~80-150W but only runs 30-50% of the time, so realistically you're looking at 40-75Wh per hour, meaning 8hrs overnight = roughly 320-600Wh, which your 200Ah Fogstar at 12V gives you ~2,400Wh usable (to 80% DoD) — so yes, you'll be fine, with headroom to spare for the bilge pump and a brew. 🍵

Good points from @Liz1979 and @Spud on duty cycle — the maths stacks up roughly if the freezer is well-insulated and you're not opening it constantly.

Real-world caveat though: what's ambient temp on your boat? I've got a static caravan setup and summer heat absolutely hammers freezer run time — duty cycle creeps up significantly when it's warm.

Also worth checking — does your Fogstar Drift have a low-temp cutoff? Some lithiums won't discharge properly below certain temps, which could catch you out moored somewhere cold overnight.

One 200Ah 12v gives you ~2.4kWh usable at 100% DoD, but I'd not push it past 80% regularly. Tight margin for overnight depending on your freezer model. What make/model is it? That'll tell us a lot more than guessing.

Great thread — narrowboat life throws up all the fun edge cases doesn't it.

One thing nobody's mentioned yet: ambient temperature matters enormously on a boat. If your freezer is tucked in a cool engine bay or below-waterline locker, it'll cycle far less than if it's sitting in a sun-baked stern cabin in August.

I ran a similar setup in my shepherd's hut last summer — Fogstar Drift 100Ah (half your capacity) — and a small chest freezer genuinely surprised me with how little it consumed once the contents were already frozen solid. Pre-freezing everything before you cut the engine is a proper game-changer.

Your 200Ah gives you roughly 160Ah usable at Fogstar's recommended depth of discharge, so you've got decent headroom. Just make sure your BMS can handle the freezer's compressor startup spike — that brief inrush current catches people out.