Anyone know if a 200Ah lithium battery is enough to run a small chest freezer overnight?

Running a small chest freezer overnight is something I've been mulling over for my narrowboat setup and I'd love to hear from people who've actually done it.

Context: I've got a 200Ah lithium (Fogstar Drift 12V) paired with 400W of solar on the roof. During summer I'm obviously fine keeping things topped up, but I'm thinking about whether the battery alone could handle a chest freezer through the night without solar input — so roughly 10–12 hours.

From what I've read, a small chest freezer (something like a 60–100 litre unit) draws maybe 30–60W when the compressor kicks in, but the duty cycle is maybe 30–50%? So real-world consumption might be somewhere in the region of 0.5–1.5 kWh overnight?

On a 200Ah/12V bank that's roughly 2.4 kWh usable (assuming I don't want to drop below 20% SOC), so on paper it should be fine. But I'm wondering if there are gotchas I'm missing:

• Does ambient temperature affect compressor duty cycle significantly on a boat?
• Are there any 12V chest freezers that are notably more efficient than others? I've seen the Brass Monkey and Dometic ones mentioned a lot
• Would it be worth adding a basic current clamp meter to actually measure what my freezer pulls before committing?

Has anyone got real data from running something similar? Especially interested in narrowboat or marine setups where the insulation situation might be a bit different to a static off-grid cabin.

Depends entirely on the freezer — my cabin chest freezer pulls about 30-35W running average once it's cold-soaked, so overnight (say 10hrs) you're looking at ~300-350Wh, which is well within a 200Ah lithium's usable capacity.

The killer is the compressor start surge and how often it cycles — a warm boat hold in summer will hammer it far harder than a well-insulated shepherd's hut in February (ask me how I know).

@SmartSolar_Master what's the actual wattage on the freezer label? Because "small chest freezer" could mean anything from a Snomaster to some sketchy Amazon special that lies through its teeth on the spec sheet.

Done this on my narrowboat for the past two winters so can offer some real-world numbers.

A decent small chest freezer (I run a Sealey 35L) averages around 25-40W depending on ambient temp. Over 12 hours that's roughly 0.3-0.5 kWh, so maybe 25-40Ah at 12V — well within what a 200Ah lithium can handle, even respecting the 80% usable capacity limit.

The bigger concern is start-up surge — compressors can spike 3-4x running wattage momentarily. Make sure your inverter handles that without nuisance tripping.

Also worth noting: if your boat is moored somewhere cold overnight, the freezer barely cycles at all. Mine hardly touched the batteries last January.

What freezer are you actually looking at? Brand and stated wattage would help give you a more precise answer.

The angle nobody's mentioned yet — what are you charging from?

I ran a similar setup last summer whilst touring the South Downs in my van. Got obsessed with the EV side of things and started logging everything. The freezer maths worked fine on paper, but my 200Ah Fogstar Drift was regularly getting hammered because I'd prioritised charge cycles for the car over the freezer's overnight needs.

The battery capacity question is almost secondary to your charge replenishment rate. A 200Ah bank that's only sitting at 60% by dusk because solar's been poor is a very different proposition to one that's topped up properly.

What's your solar array looking like, @SmartSolar_Master? And are you on shore power at any point? That changes the whole conversation.

Ran into this exact situation getting my shepherd's hut sorted — the freezer question kept me up at night before the battery did.

One thing worth flagging: ambient temperature matters enormously. My chest freezer in the hut during a warm August was cycling far more frequently than the same unit in October. Effectively doubled the overnight draw during a heatwave.

@SussexDweller raises the charging question, which is the real crux. I pair a Victron SmartSolar with a Fogstar 200Ah and found the freezer is actually fine — it's when I started factoring in EV charging top-ups from the same bank that things got interesting and required proper load-scheduling.

At 200Ah lithium you've got roughly 100Ah usable overnight if you're being sensible about depth of discharge. A well-insulated chest freezer should sit comfortably within that, but don't assume — measure first with a plug-in energy monitor for a few nights.