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

Running a small chest freezer overnight is something I've been trying to work out for my narrowboat setup, so I'd love to hear from anyone who's done this.

I've got a 200Ah lithium (LiFePO4) battery — Fogstar Drift if it matters — paired with a modest solar array. The freezer I'm looking at is a 45-litre chest freezer, rated at around 85W but obviously it cycles on and off rather than running continuously.

My rough maths says something like:

• Assume 30–40% duty cycle overnight (no solar gain, ambient around 10–15°C in the UK)
• That's maybe 85W × 8 hours × 0.35 = ~238Wh
• At 12V that's roughly 20Ah, which sounds manageable

But I'm worried I'm being naïve here. A few specific questions:

1. Does the compressor startup surge cause any real-world issues with a standard 12V inverter, or does LiFePO4 handle that comfortably?
2. Is my duty cycle estimate realistic, or does a freezer work harder than I'm assuming when it's not well-insulated (the boat isn't exactly a temperature-stable environment)?
3. Would I be better off looking at a 12V compressor freezer (like a Brass Monkey or similar) rather than running a 240V unit through an inverter?

I'm also wondering whether 200Ah gives me enough headroom if I want to keep 50% depth of discharge as a minimum — that leaves me only 100Ah usable (roughly 1,200Wh at 12V), so in theory there's plenty of room, but real-world

@RustySpanner done this on my boat setup. Short answer: yes, comfortably — but depends on the freezer.

A decent small chest freezer (60-80L) typically draws 30-50Wh per hour once it's up to temp. Overnight ~10hrs = 300-500Wh.

Your 200Ah LiFePO4 at 12V = 2400Wh usable (assuming you're not going below 20% SOC). So you've got headroom to spare.

Few caveats though:

• Ambient temp matters a lot — colder outside = freezer works less
• Startup surge can catch cheaper inverters out
• If it's a compressor fridge/freezer combo, draw creeps up

I'd honestly stick a Victron SmartShunt on it and monitor actual consumption for a week before relying on it. Guesswork bites you eventually.

What freezer model are you looking at? Some are genuinely awful on efficiency.

@RustySpanner I've got a similar setup on my narrowboat — 200Ah Fogstar Drift LiFePO4 running a small Shoreline chest freezer overnight. Works fine as long as you're not opening it constantly and the ambient temp isn't roasting.

One thing worth factoring in: chest freezers are significantly more efficient than uprights because cold air doesn't fall out when you open the lid. That makes a real difference to your overnight draw.

As @MarineGaz says, the freezer model matters enormously. Check the actual watt-hour rating rather than just wattage — a freezer might be 60W peak but only cycling 30-40% of the time, so your real overnight consumption could be 150-200Wh rather than the scary headline figure.

With 200Ah LiFePO4 at 12V that's ~2400Wh usable (to 80% DoD), so you've got plenty of headroom.

Something worth adding from my shepherd's hut experience — ambient temperature makes a huge difference that people rarely mention.

My little chest freezer draws maybe 30–40% less during a cold British night compared to a warm August evening. I've watched this play out on my Victron BMV-712 over dozens of nights. Summer was the only time I ever felt nervous about my 200Ah getting chewed through.

Also worth checking: is the freezer positioned away from any heat sources? Mine was initially too close to my gas hob area and the compressor was working overtime. Moved it six inches, noticed the difference immediately on the shunt readings.

@BoatPete makes a good point about the Fogstar Drift — solid bit of kit. The usable capacity headroom on LiFePO4 versus old lead-acid is what really makes overnight freezer running viable in the first place.

@BreezyMechanic raises exactly what I was going to add — my shepherd's hut is in a reasonably exposed spot and I've tracked this obsessively through winter.

One thing nobody's mentioned yet: how full the freezer is matters enormously. A well-packed freezer holds its temperature far more efficiently than a half-empty one. I fill the gaps with old water bottles, frozen solid. Cuts my compressor run-time noticeably on the overnight logs I pull from my Victron Cerbo.

Also worth checking your freezer's thermostat setting — most people run them far colder than necessary, which hammers consumption for no real benefit. Mine was set to -22°C when -18°C is perfectly adequate. Made a measurable difference on the battery state of charge come morning.

For EV charging context, 200Ah lithium is modest — but for a freezer overnight, it's genuinely comfortable with sensible management.