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

Running a small chest freezer on the narrowboat and trying to figure out if my 200Ah LiFePO4 (Fogstar Drift, so usable capacity is the full 200Ah realistically) will see me through overnight without solar input.

The freezer is a Subcold 35L — pulls about 45W when the compressor is running but obviously it's not running constantly. From what I've read, duty cycle on a well-insulated chest freezer can be as low as 20-30% once it's pre-cooled, so I'm trying to work out if my maths is right:

• Compressor runs ~30% of the time
• 45W × 0.30 = ~13.5W average draw
• Over 8 hours = ~108Wh
• At 12V that's roughly 9Ah

Which seems almost too comfortable. Even doubling that for a worst-case scenario (warm night, lid being opened, etc.) you're looking at 18Ah — barely touching a 200Ah bank.

Where I'm less sure:

1. Does duty cycle actually stay that low in summer? Ambient temps on the cut can be brutal in a steel hull.
2. Is there anything I'm missing with the inverter overhead? Running it through a Victron Phoenix 375VA and I know there's some idle draw to account for.
3. Anyone running a similar setup who can share actual figures from a battery monitor rather than theory?

Not trying to be told "just try it" — I want to understand the numbers before I commit to running the freezer overnight and waking up to a flat bank and defrosted food.

Tel1997 | ⭐ Regular

@AngliaOffGrid Short answer - almost certainly yes, but it depends on the freezer and ambient temps.

A typical small chest freezer (50-80L) draws somewhere around 30-60W when the compressor's running, but it's not running constantly - maybe 30-50% duty cycle once it's up to temperature. So realistically you're looking at 15-30W average, call it 15-25Ah over 8 hours overnight.

Your 200Ah Drift has bags of headroom for that. Even a worst-case scenario with a warm ambient and inefficient unit, you'd struggle to pull more than 50Ah overnight.

What freezer is it specifically? That'd help nail it down. Also worth sticking a plug-in energy monitor on it for a day to get actual figures rather than guessing - makes everything much simpler to plan around. 👍

VivaroNomad | ⭐ Regular

This exact question kept me up at night when I was setting up my shepherd's hut last summer! Had a tiny 60L chest freezer running off a 100Ah Fogstar Drift and honestly it sailed through warm July nights drawing maybe 15-20Ah total — chest freezers are brilliant efficiency-wise because cold air doesn't just fall out when you open them.

The killer variable nobody mentions: how full is it? A half-empty freezer works way harder cycling on and off. Chuck some water bottles in to fill the space — thermal mass is your friend overnight when there's no solar coming in.

Your 200Ah should be absolutely fine. I'd stick a cheap energy monitor inline first though (I use a Victron BMV-712) just to actually see what it's pulling rather than guessing. Knowledge is power and all that! 📊

JohnBaker | ⭐⭐ Veteran

Running a Shoreline chest freezer on my narrowboat for two years so can give you actual numbers rather than theory.

My 70L unit draws roughly 80-100W when the compressor runs, but it's only cycling maybe 25-30% of the time overnight once the contents are already frozen down — so real-world consumption lands around 150-200Wh per 8-hour stretch in a cool engine room.

Your 200Ah Fogstar at 12V gives you 2400Wh usable. Even if ambient temperatures push that cycle rate up to 40%, you're looking at ~320Wh overnight. You've got masses of headroom.

Two caveats worth flagging though:

• If the freezer contents aren't already cold when you start, compressor runtime spikes dramatically
• Summer mooring in direct sun on the hull side nearest the freezer will hurt efficiency considerably

What's the ambient location of the freezer on your boat?

BurnWalker | ⭐ Regular

What wattage is the freezer? That's the bit that actually matters and you've not said.

On my narrowboat I ran a cheap 60L chest freezer through summer — compressor was cycling maybe 30-40% of the time because chest freezers are efficient and cold air doesn't fall out when you open them. Overnight is roughly 8-9 hours, so even a fairly hungry unit shouldn't batter a 200Ah bank too badly.

Bigger concern for me was always the inverter draw if you're running pure sine — what are you running it through? Some of those smaller inverters idle at 10-15W which adds up overnight.

@JohnBaker's actual numbers will be more useful than guesswork — what freezer model is it specifically?