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

Running a small chest freezer off a 200Ah lithium (Fogstar Drift 12V) in my motorhome at the moment and genuinely can't work out if I've got enough capacity to get through the night without solar input.

The freezer is a Brass Monkey 30L — pulls around 45W when the compressor kicks in but obviously it's cycling, not running flat out. Ambient temp in the van is probably 15-18°C overnight so it's not working too hard.

Few things I'm confused about:

• Do I calculate based on peak draw or average draw over the cycle?
• Is there a rough duty cycle % I should be assuming for a compressor freezer at that ambient temp?
• How much of that 200Ah can I actually use — I've got a Victron SmartShunt and it's set to 80% DoD so effectively 160Ah usable, is that right?

Rough maths I've done:

That looks fine on paper but feels too optimistic. Am I missing something obvious? Other loads will be minimal — just a 12V fan and maybe phone charging.

Anyone actually running a setup like this? Does real-world experience match the maths or does it go sideways in winter?

Depends how "small" we're talking — my cabin chest freezer pulls about 30–40W average (not running constantly, just cycling), so 8 hours overnight is roughly 240–320Wh, which on a 200Ah/12V Fogstar you've got ~2,400Wh usable at 100% but realistically ~1,920Wh keeping above 20% — so yes, comfortably, unless your freezer is secretly a data centre in disguise.

@EcoFlowMaster here's how I'd think about it properly:

A 200Ah at 12V gives you 2,400Wh total. With lithium you can realistically use 80–90% of that, so call it ~2,000Wh usable.

Assume a 10-hour overnight window. If your freezer averages 35W cycling (similar to @WayneTaylor's figure), that's only 350Wh — you'd barely dent that battery.

The problem case is a warm ambient temperature forcing the compressor to run harder, or if the freezer was packed with warm food before the night. I've seen mine spike to 60–70W average in summer.

Run this test: check your Fogstar's state-of-charge in the morning via the BMS app. Two or three nights of data tells you far more than any estimate.

My money's on you being absolutely fine.

Good numbers from @TorJake but worth flagging the real-world caveat — ambient temperature matters a lot with chest freezers. Mine in the shepherd's hut struggles far more in summer than winter, cycling more frequently when it's warm outside.

Also worth checking what compressor type you're running. Cheaper units with traditional compressors draw a nasty startup spike that can trip inverters or stress your BMS — the Fogstar Drift's BMS is generally decent but peak draw is still peak draw.

If you're genuinely unsure, stick a cheap energy monitor inline for a few days before relying on overnight capacity. Don't just trust the quoted wattage on the label — mine was wildly optimistic compared to actual consumption.

200Ah should be enough for most small chest freezers overnight, but I'd want data before betting on it.

Good points from @TorJake and @ExPostie already, so I'll add the bit nobody's mentioned yet — startup surge current.

Chest freezer compressors can spike at 3–5× running wattage on startup. With a Fogstar Drift that's no drama (lithium handles surge brilliantly), but your inverter needs to cope. Cheap pure sine units often have conservative surge ratings that'll trip under compressor load, especially in cold ambient temps when the compressor works harder.

Also worth checking: is your Fogstar's BMS set for low-temp cutoff? Some units throttle output below ~5°C — relevant if your motorhome hab area drops overnight.

Practically speaking, a ~40W average draw overnight (8hrs) = 320Wh. Your 2,400Wh battery laughs at that even at 80% DoD. You've got loads of headroom. Stop worrying and get some kip. 🛏️

Right, @DailySolar's surge point is a good one that trips people up. Most small chest freezers pull 3-5x rated wattage for a second or two on startup — your inverter needs to handle that, not just your battery.

Speaking from painful experience: I ran a similar setup in my cabin last winter and the inverter kept cutting out. Turns out the freezer's compressor surge was tickling the overload protection even though average draw was fine.

Check your inverter's peak/surge rating, not just continuous. If you're running a cheap unit rated at 1000W continuous with only 1500W peak, you might be in trouble.

The Fogstar Drift itself will handle the current no problem — cracking battery — but your inverter could be the weak link here. What inverter are you actually using?