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

Got a 200Ah lithium (Fogstar Drift 12V) sitting in the garage that I mostly use for emergency backup when the power goes out. Recently we had a 14-hour outage and I was scrambling to keep the chest freezer going — ended up just hoping for the best, which felt daft.

So I'm trying to work out properly whether that 200Ah would actually cut it for an overnight stretch (say 8-10 hours) if it happens again.

The freezer is a fairly basic Argos chest freezer, around 100 litres. I think the running wattage is somewhere between 80-150W but honestly I've never actually measured it — it cycles on and off obviously, so the actual consumption will be lower than peak draw.

Few specific questions:

• Should I be measuring with a plug-in energy monitor to get real consumption figures before I rely on this?
• At 50% DoD (trying to keep the Fogstar healthy), I'm effectively working with 100Ah — is that going to be enough or marginal?
• Do I need to factor in inverter losses on top? Running it through a basic 1000W pure sine unit.

The battery's connected to a Victron SmartShunt so I can monitor state of charge, which helps. But I've no solar on this particular setup — it's purely grid-charge-and-store for backup purposes.

Anyone done something similar and got real-world figures rather than back-of-a-fag-packet maths?

@WheresMeWires31 depends entirely on the freezer's actual draw. Most small chest freezers (40-60L) pull around 30-45W when running, but they're not running constantly — duty cycle is typically 20-40% depending on ambient temp and how full it is.

Rough maths: say 40W × 30% duty = ~12Wh per hour, so 14 hours ≈ 170Wh. At 12V that's roughly 14Ah — well within your 200Ah Fogstar's usable capacity (assuming you're not draining below 20%).

Where it gets messier: compressor startup surge, and if it's a hot garage the duty cycle climbs sharply. I've run a 60L chest freezer off my narrowboat bank through summer nights without drama.

Grab a Shelly plug or basic energy monitor first — measure actual consumption over 24hrs before committing to anything permanent.

@WheresMeWires31 quick maths — 200Ah at 12V is 2.4kWh usable (assuming you're not hammering a lithium below 20% SOC, so call it ~1.9kWh realistically). A decent small chest freezer running at maybe 35W average over 14 hours is roughly 490Wh. So yes, comfortably, assuming nothing else is draining the bank.

What'll kill you is if the freezer's been off a while and has to work hard to pull temps back down — compressor startup draws spike. Worth sticking a plug-in energy monitor on it for a few days first so you actually know what it's consuming rather than guessing.

Also check the Fogstar's BMS specs for sustained discharge — shouldn't be an issue for freezer loads but worth knowing your limits before you're mid-crisis again.

@SOC_Nerd did the maths so I don't have to, bless him — but don't forget your inverter is also quietly sipping watts like it's at a free bar, so factor in 10-15W of idle draw from whatever you're running the freezer through, otherwise your "overnight" becomes "until 3am panic."

Good shout from @AnneButler on the inverter losses — that caught me out badly the first winter I had the boat set up. I was scratching my head wondering where my capacity was disappearing to, convinced the Fogstar was dodgy. Turned out my cheap inverter was essentially running a small heater while doing absolutely nothing useful.

Worth adding: chest freezer positioning matters more than people think. Shove it in a warm corner of the garage and it'll cycle far more aggressively than one sitting in a cool utility room. Mine runs maybe half as often in winter versus summer, which completely changes your overnight calculation.

If you've got a clamp meter, genuinely worth logging actual consumption over 24 hours before trusting any spec sheet. Manufacturers are optimistic, shall we say.