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

Running a small chest freezer off a 200Ah LiFePO4 on my narrowboat at the moment, and I'm genuinely unsure whether I've got enough capacity to get through a full night without the alternator or solar topping things up.

The freezer is a Shoreline 35L chest freezer — one of the cheaper compressor units. Rated at about 45W but obviously it cycles, so real-world draw is more like 0.5–0.8 kWh per 24 hours depending on ambient temp and how full it is.

My battery is a Fogstar Drift 200Ah 12V, so roughly 2.4 kWh usable if I'm keeping it above 20% SOC.

On paper that looks comfortable — maybe 8–10 hours of overnight use eating through 0.4–0.5 kWh, leaving plenty of headroom. But I've read that canal tunnels and poorly-ventilated engine bays can push freezer consumption up significantly, and I'm moored somewhere shaded this week so the 200W Renogy panel isn't doing much before 9am.

A few things I'm unclear on:

• Does a chest freezer's compressor inrush current cause any issues with a 12V inverter sized around 300W continuous? I've got a cheap unit and I'm a bit nervous about it.
• Anyone found real-world overnight consumption notably worse than the rated figures suggest?
• Is it worth running the freezer during the day to "pre-cool" it and reduce overnight cycling?

Would love to hear from anyone who's done similar — especially if you're on a boat or van where ambient temps and insulation vary wildly. Trying to avoid w

@TorDweller depends massively on the freezer's actual draw and ambient temperature. A "small" chest freezer might pull anywhere from 30-80Wh per hour depending on how well insulated it is and how warm the engine room gets.

Rough maths: even at 50Wh average, that's 400Wh over 8 hours — totally doable from a 200Ah (roughly 2.56kWh usable at 80% DoD on LiFePO4).

What I'd actually do before assuming anything:

• Grab a plug-in energy monitor (£10-15 on Amazon) and measure real-world consumption over 24hrs
• Check the freezer's duty cycle — they don't run continuously

On my van build I ran a small freezer off a 100Ah Fogstar Drift and it was fine through summer nights. Narrowboat engine rooms can get warm though, which hammers efficiency. That's your wildcard really.

@TorDweller done this on my static van. Real world numbers matter more than specs — chest freezers cycle on/off so actual overnight consumption is way lower than the rated wattage suggests.

Stick a plug-in energy monitor on it for 24hrs first. Mine came out around 200-300Wh overnight in summer, easily managed on a 100Ah Fogstar.

Cold ambient temps actually help the freezer too, less cycling needed.

The bit that'll catch you out is if you're only running at 80-90% state of charge going into the night. That 200Ah is more like 160Ah usable if you're being sensible with LiFePO4 discharge limits.

Should be fine tbh but monitor it rather than guessing.

Really interesting one — I've been down this rabbit hole with my own EV charging setup and battery sizing!

Have you actually measured what your freezer is drawing with a clamp meter or plug-in energy monitor? That's the bit I'd want to nail down first before assuming the 200Ah is enough (or not enough).

@SmartSolar_Geek and @ZFS_OffGrid are both pointing at the right thing — the duty cycle is everything here. But also worth checking: what's your usable capacity actually set to? A lot of LiFePO4 setups (Fogstar, Victron-managed, etc.) are configured to only use 80% to protect the cells, so your real headroom might be closer to 160Ah.

What BMS are you running? That might affect how aggressively it cuts out overnight if voltage dips.

@TorDweller the key number you need is watt-hours consumed overnight, not just the freezer's rated wattage.

A typical small chest freezer (say 60–100L) might pull 80–150W when running, but with a 30–40% duty cycle, you're looking at roughly 30–60Wh per hour average. Over a 10-hour night that's 300–600Wh total.

Your 200Ah LiFePO4 at 12V gives you 2,400Wh usable (assuming ~100% DoD, though I'd keep it above 20% for longevity — so realistically ~1,900Wh).

You're almost certainly fine, but ambient temperature is critical — @ZFS_OffGrid is right. A poorly-insulated narrowboat in summer with warm bilge air will hammer that duty cycle upward significantly.

Stick a Shelly plug or similar energy monitor on it for a few days. Know your actual numbers before trusting assumptions. I've been caught out before doing exactly

Got a shepherd's hut setup and ran very similar numbers on this. One thing nobody's mentioned yet — ambient temperature matters a lot. A chest freezer on a narrowboat in winter will cycle far less than in summer, so your overnight draw could vary by 30-40% depending on season.

Also worth checking: is your 200Ah pack a Fogstar or similar quality cell? Some budget LiFePO4 packs don't actually deliver full rated capacity at higher draw rates.

Rough guide from my experience:

• Small chest freezer (~50-80L): 150-250Wh overnight
• 200Ah @ 12V = 2400Wh usable (assuming 80% DoD = ~1920Wh)

You're almost certainly fine, but stick a Victron SmartShunt on it and log actual consumption for a few nights — stops the guesswork entirely.