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

Running a small chest freezer (the Subcold Pro 35L if anyone's familiar) off a 100Ah Fogstar Drift in my tiny house setup at the moment, and honestly it's making me question every life choice I've ever made.

The maths looks fine on paper — freezer pulls roughly 30–40W average once it's up to temp, so overnight (say 8 hours) you're looking at maybe 240–320Wh, which a 200Ah at 12V should handle with headroom to spare. But real life has a nasty habit of laughing at my spreadsheets.

Few things I'm not sure about that I'd love input on:

• Ambient temperature — my array is in a fairly exposed spot and winter nights drop the efficiency of everything noticeably. Does the battery's usable capacity drop enough to matter?
• Compressor start surge — I've read these little chest freezers can spike 3–4x rated wattage on startup. Does that actually stress a lithium BMS or is it a non-issue?
• Depth of discharge — I'm trying to keep above 20% for longevity, so in practice I'd only want to draw maybe 160Ah worth anyway

Currently eyeing up a second Fogstar to pair with my existing one, or potentially jumping to a single proper 200Ah unit. Victron SmartShunt would obviously let me monitor this properly but that's another rabbit hole entirely.

Anyone actually running a chest freezer overnight on a similar sized bank who can tell me whether the theory matches reality — or whether I'm about to spend a miserable winter defrosting things I really wanted kept frozen?

PanelTina | 847 posts

@ROW_OffGrid Ha, the existential crisis is real when your freezer starts draining your bank faster than expected! 😄

The good news is a 200Ah lithium should be much more comfortable. The Subcold 35L typically draws around 30-45W when compressing, but with decent ambient temperature and a full load (thermal mass helps massively), you're realistically looking at maybe 150-200Wh overnight depending on how warm your space gets.

That leaves you with plenty of headroom on a 200Ah battery at 12V, which gives you roughly 2.4kWh usable at 100% DoD (though I'd stay above 20% personally).

The jump from 100Ah to 200Ah will make a noticeable difference. What's your solar input looking like? That'll really determine whether you can sustain it long-term rather than just surviving one night.

RelayNomad | 312 posts

@ROW_OffGrid The Subcold 35L pulls around 30-45W when the compressor's running but duty cycle is the key variable people miss. Depending on ambient temp and how full it is, you're looking at maybe 150-250Wh overnight realistically.

A 200Ah lithium (assuming 12V, so 2400Wh usable at ~80% DoD = ~1920Wh) should handle it fine on its own. The 100Ah Fogstar Drift being tight makes sense if you've got other loads competing for it.

Had similar sums with a chest freezer on my cabin setup — the compressor surge at startup is worth checking against your inverter/BMS limits too, not just capacity. Worth logging actual consumption with a decent battery monitor for a few nights before assuming the 200Ah solves it entirely. Real data beats specs every time.

SmartSolarGuy | 1,203 posts

Running a similar setup in my motorhome — had a Subcold-style compressor fridge and the overnight maths genuinely surprised me the first time I sat down with a clamp meter.

The bit nobody mentions: ambient temperature matters enormously. My consumption nearly doubled one summer when the unit was tucked against a warm wall. Add food load and a poorly sealed lid and your 200Ah could look very different in July versus January.

@RelayNomad's duty cycle point is the key variable here. I'd honestly grab a Victron SmartShunt before upgrading your battery — log a full 24 hours and you'll know your actual consumption rather than guessing. Saved me from buying unnecessary capacity when the real fix turned out to be better ventilation around the unit.

200Ah should be comfortable, but verify first.

DevonDweller | 2,156 posts

@RelayNomad has the duty cycle point spot on — that's the critical figure people miss. To actually answer the question: yes, a 200Ah lithium will comfortably run a 35L chest freezer overnight, assuming you're not drawing it below 20% SoC.

Rough maths: if the compressor averages ~15-18Wh/hr actual consumption (duty cycle included), you're looking at 120-145Wh across an 8-hour night. On a 12V 200Ah bank that's roughly 10-12Ah — barely scratching the surface.

The real concern with your current 100Ah Fogstar Drift is whether the BMS is throttling under sustained load. Worth checking the Fogstar app if you've got the Bluetooth version — I've seen them behave oddly below 20% on cold nights in my narrowboat setup.

Upgrading to 200Ah gives you comfortable headroom. Fogstar's own 200Ah Drift is solid value and pairs well if you want to keep things consistent.

DalesOffGrid | 847 posts

My Fogstar Drift 200Ah keeps the garden office mini-fridge and charges the EV overnight — the chest freezer in the shepherd's hut on a 100Ah is basically asking a Shetland pony to pull a lorry. 🐴