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

Got a 200Ah Fogstar Drift lithium sat in the van doing sterling work on the usual suspects — lights, phone charging, the odd bit of 12V mischief — but I'm eyeing up a small chest freezer for the shepherd's hut conversion I'm cracking on with.

The freezer I'm looking at is a Subcold Ultra 35, pulls about 35W running but obviously spikes on the compressor kick-in. Reckon it cycles maybe 30–40% of the time once it's settled at temperature.

Rough back-of-a-fag-packet maths says something like:

• 35W × 12hrs × 40% duty cycle = ~168Wh overnight
• That's only ~14Ah at 12V, which sounds almost too good to be true

So either I'm doing the maths wrong (very possible), or a 200Ah bank should handle a freezer overnight without even breaking a sweat — before solar tops it back up in the morning?

Main worries are:

1. Is the inrush current on compressor startup going to cause grief with my Victron SmartShunt or trip anything?
2. Am I being naive about real-world consumption vs. the rated wattage?

The hut isn't going anywhere near the grid, so I want to be reasonably confident before I lug a freezer up a muddy field and realise I've been an optimist. Anyone actually running a chest freezer off a similar sized battery who can share real numbers rather than spec sheet fairy tales?

@Chalky the maths is tighter than it looks. A small chest freezer (say a 60L Shoreline or similar) pulls maybe 30–45W when running but the compressor duty cycle is the killer variable — could be 30% in a cool environment, way more in a hot van in summer.

Rough worst case: 45W × 50% duty × 10hrs = 225Wh. Your 200Ah battery is ~2400Wh usable (assuming Fogstar's 80% DoD recommendation), so capacity isn't really the issue overnight.

The gotcha is your inverter draw and inefficiency if you're running 240V, plus what else is on the battery simultaneously.

I ran a similar setup on my narrowboat before upgrading — worked fine until August hit and the duty cycle went mental. Worth fitting a battery monitor (Victron BMV-712 is what I use) so you can actually see what's happening rather than guessing.

@PanelEwan has covered the watt-hour side reasonably well, but nobody's mentioned the startup surge yet. Compressors kick in at 3–6x running wattage for a fraction of a second — your inverter needs to handle that without browning out, and your Fogstar's BMS needs to not trip on the current spike.

More importantly: ambient temperature. A freezer in a shepherd's hut in summer is working considerably harder than the manufacturer's stated consumption figures, which are typically measured at 25°C ambient. Double your estimated draw if it's a hot July.

Practically speaking — 200Ah at 12V is 2.4kWh usable (call it 1.9kWh being sensible with Fogstar's actual discharge curve). Overnight is doable, but you'll want decent solar recovery the following morning or you're in a cycle of deficit that catches up with you by day three.

@Chalky this is almost exactly the decision I wrestled with before adding a small chest freezer to the shepherd's hut. What nobody warns you about is ambient temperature. A freezer working hard in a hot July van is a very different beast from one sitting in a cool timber-clad hut — duty cycle swings wildly between the two.

Also worth knowing: a well-packed, pre-frozen freezer recovers far faster after compressor cycles than a half-empty one. So if you're loading it warm from the shops, that first night is genuinely your worst-case scenario.

@SolarNotSure is right about the surge — my Victron SmartShunt revealed some eye-watering spikes on startup that I'd never have guessed from the nameplate.

Run it a trial night on mains first if you can, logging actual consumption. Takes all the guesswork out.

@Chalky done exactly this on the narrowboat — ambient temperature is your silent battery thief, a freezer working hard in a sun-baked van is a completely different beast to one sat in a cool shepherd's hut, so don't just trust the theoretical numbers.

Also worth knowing: lid discipline matters enormously — every time you rummage around in there the cold air dumps out and the compressor has to earn its keep again.

If the 200Ah Fogstar is your only bank with no solar topping it up overnight, I'd honestly be nervous; you're flirting with that 50% depth-of-discharge floor that keeps lithium happy long-term. A second 200Ah slapped in parallel buys you proper headroom and peace of mind.