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

Got a Fogstar Drift 200Ah sitting in my shepherd's hut and I'm trying to work out if it'll keep a small chest freezer ticking over through the night without me waking up to a puddle of sad ice cream.

The freezer is a Subcold Pro 35 (about 35 litres), which I think pulls somewhere around 60-80W when the compressor kicks in, but obviously it's cycling rather than running flat out the whole time.

My rough maths says if it's drawing maybe an average of 30-40W continuously (accounting for the duty cycle), that's roughly 300-400Wh overnight across 10 hours — which should be well within the 200Ah (around 2,400Wh usable on lithium) without even breaking a sweat. But I'm not accounting for:

• Ambient temperature in the hut (can get properly cold in winter, which I know affects compressor behaviour)
• Inverter inefficiency — running it through a cheap 300W pure sine unit at the moment
• Any weird startup surge nonsense

Has anyone actually done this rather than just done the maths on a napkin like me? Particularly curious whether the inverter inefficiency is going to be a meaningful dent in practice or if I'm overthinking it.

Also debating whether it's even worth running the freezer off the battery at all versus just... accepting frozen things are a luxury when you're off-grid in a shepherd's hut in rural nowhere.

RayTaylor | 847 posts

@TorFinn Ha, "puddle of sad ice cream" - we've all been there mate 😄

Short answer: almost certainly yes, but it depends on the freezer. A decent small chest freezer (think 60-100L) typically draws somewhere between 100-150Wh per 24 hours once it's already cold - they're surprisingly efficient because the cold air doesn't spill out when you open them like an upright does.

Your Fogstar 200Ah at 12V gives you roughly 2.4kWh usable (assuming you're not dropping below 20% SOC), so overnight you're only using maybe 50-75Wh. Plenty of headroom.

Key thing - make sure it's pre-chilled before you rely on the battery. Pulling a warm freezer down to temperature is where the real draw happens. Do that while you've still got solar input during the day.

What's the freezer model out of curiosity?

OldSailor | 2,341 posts

@TorFinn Chest freezers typically pull 30–80W average (compressor cycles, doesn't run flat-out), so over 8 hours you're looking at ~240–640Wh — and your Fogstar Drift's usable capacity at 12V is roughly 2,400Wh, so maths says you're laughing.

The gotcha nobody mentions: surge current at compressor startup can hit 3–5x running watts, so make sure your inverter isn't undersized or you'll get a lovely shutdown at 2am instead of a puddle.

Run a Victron SmartShunt on that bank if you haven't already — watching actual consumption overnight will tell you more than any back-of-envelope calculation, including mine.

RetiredChef | 1,204 posts

@TorFinn On my narrowboat I run a small chest freezer off a Victron 200Ah setup — the trick nobody mentions is pre-cooling it thoroughly before nightfall so the compressor barely kicks in overnight, turning your 200Ah from "probably fine" into "absolutely fine."

Also worth checking your Fogstar's low-voltage cutoff settings; most lithiums won't let you pull below 20% anyway, so you've effectively got ~160Ah usable — that's still enough to run a modest freezer and have a smug breakfast.

Stick a Victron BMV battery monitor on it if you haven't already — nothing worse than guessing at 3am whether you're about to sacrifice the fish fingers. 🐟

At 50% DoD on that Fogstar Drift you've got roughly 100Ah usable, so if your freezer's averaging even 40W you're looking at maybe 30Wh/hour — that's well within budget for an 8-hour night, assuming the lid stays shut and you're not in a heatwave running it in direct sun like some kind of optimist.