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

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

The freezer is a Subcold 60L chest unit, rated at around 90W running draw, but obviously it doesn't run continuously — cycles on and off. From what I can measure with my Victron BMV-712, it's pulling roughly 0.8–1.2kWh per 24 hours in mild weather, so overnight (say 10 hours) I'd expect maybe 400–600Wh of actual consumption.

My battery is a Fogstar Drift 200Ah 12V LiFePO4, which gives me a usable capacity of around 2.4kWh at 100% DoD (though I try to stay above 20% for longevity, so realistically 1.9kWh usable).

On paper that looks absolutely fine — plenty of headroom. But I'm concerned about a few things:

1. Ambient temperature — I'm moored in the Lake District and overnight temps can drop quite low, which I know affects battery performance somewhat even with LiFePO4
2. Other loads — LED lighting, phone charging, a water pump occasionally
3. Inverter efficiency losses — running the freezer off a 12V→240V inverter adds maybe 10–15% overhead

So realistically, is 200Ah enough, or am I cutting it finer than I think? Has anyone done proper overnight tests with a chest freezer in cooler UK conditions? Curious whether warmer freezer contents (e.g.,

Thommo53 | Posts: 847

@LakelandNomad the key thing you'll want to check is the freezer's actual running wattage rather than the peak figure on the label. A typical small chest freezer pulls somewhere between 30-80W when the compressor's running, but it's not running constantly — duty cycle is usually around 25-40% depending on ambient temp and how full it is.

Rough maths: say 50W average draw × 8 hours = 400Wh. Your 200Ah LiFePO4 at 12V gives you roughly 2400Wh usable (assuming you're not going below 20% SOC), so you should be absolutely fine overnight.

Narrowboats do get cold at night though, which actually helps — the freezer won't work as hard. Stick a cheap plug-in energy monitor on it for a day or two and you'll know exactly what you're dealing with.

BoxerCamper | Posts: 1,203

Right, so here's the thing nobody tells you until you're staring at a dead freezer at 2am on the Llangollen Canal (not that that's ever happened to me, ahem).

Chest freezers are actually your friend for overnight running — once they're cold, they cycle maybe 20-30% of the time. A typical small chest freezer draws around 80-100W when running, so you're realistically looking at 30-50Wh per hour averaged out.

Over 8 hours? Call it 300-400Wh. Your 200Ah LiFePO4 is sitting on roughly 2,560Wh usable — you're barely scratching it.

The real villain is ambient temperature. Warm engine room = compressor works harder = burns more capacity.

Stick a Victron SmartShunt on it and you'll see exactly what's happening in real time rather than guessing.

QuietTrekker | Posts: 412

Worth adding — chest freezers are actually pretty efficient overnight because the compressor cycles rather than runs constantly. Mine pulls about 30–40W average on a well-loaded, well-sealed unit.

200Ah LiFePO4 at 12V = 2.4kWh usable (assuming ~100% DoD on LiFePO4, realistically 80% = ~1.9kWh). A small chest freezer averaging 35W over 8 hours = ~280Wh. You're absolutely fine.

The caveats though:

• Ambient temp matters — warmer nights = more cycling
• How full is it? Thermal mass helps massively
• Inverter efficiency losses if you're running 240V

What inverter are you using? Some cheaper ones idle-draw quite a bit. I switched to a Victron unit and noticed a real difference. The idle consumption on a dodgy inverter can quietly murder your battery overnight before the freezer even gets a look-in.

HollyBaker | Posts: 634

One thing worth being precise about: 200Ah at 12V is 2.4kWh nominal, but with LiFePO4 you're realistically only drawing down to 20% SoC, so usable capacity is around 1.92kWh.

A small chest freezer typically pulls 30–60W average (compressor duty cycle included). Call it 45W average × 8hrs = 360Wh. You're fine on paper.

BUT — factor in inverter losses if you're running AC (typically 10–15% with a Victron), plus any other overnight loads (bilge pump, lighting, etc.). Those add up on a narrowboat.

I run a similar setup on mine. The freezer itself isn't the problem; it's everything else quietly nibbling away overnight that catches people out. Check your actual consumption with a battery monitor — Victron BMV-712 will show you exactly what's happening.