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

Running a small chest freezer off-grid on the narrowboat and trying to work out if my 200Ah LiFePO4 (Fogstar Drift 12V) will actually cope overnight without flattening itself.

The freezer is a Lec CF100, rated at 100W but obviously it doesn't run continuously — duty cycle is probably 30–40% depending on ambient temperature and how full it is.

My rough maths:

• Assume 35W average draw over 8 hours = 280Wh
• At 12V that's roughly 23Ah from the battery
• 200Ah usable to 80% DoD = 160Ah available

So on paper it looks very comfortable, but I'm second-guessing myself because:

1. The compressor startup surge — does this cause any BMS protection issues worth worrying about?
2. Summer vs winter — the freezer will work harder moored somewhere warm
3. I've read conflicting things about whether chest freezers are actually more efficient than uprights in practice, or whether that's just marketing

Has anyone actually measured this rather than estimated? I've got a Victron SmartShunt fitted so I can log real consumption data once I get the freezer aboard, but before I commit to buying it I'd rather sanity-check the theory.

Also worth noting my solar input overnight is zero obviously, so the battery needs to carry it fully until the alternator or shore power tops it back up in the morning. Is 200Ah genuinely sufficient or should I be looking at expanding to 300Ah before adding this load?

@DevonDweller the key figure you're missing is the freezer's actual duty cycle — most small chest freezers only run the compressor 30–50% of the time once at temperature. So a freezer rated at 60W isn't pulling 60W continuously.

Rough maths for an 8-hour overnight:

• 60W × 40% duty cycle = 24W average
• 24W ÷ 12V = 2A draw
• 8 hours = ~16Ah consumed

Your 200Ah Fogstar Drift (usable ~180Ah at 90% DoD) will barely notice that. Even worst-case 100% duty cycle you're looking at ~40Ah — still comfortable.

The caveat: ambient temperature matters enormously. A poorly insulated freezer in a warm engine bay will hammer that duty cycle upward. Stick a plug-in energy monitor on it for 24 hours first — the actual data beats any estimate.

@PennineVanLifer is right about duty cycle being the key — worth actually measuring it with a simple plug-in energy monitor before committing to anything.

From my motorhome experience, a small chest freezer in decent ambient temps typically draws 30-50Wh per hour averaged out. Overnight (say 10 hours) that's 300-500Wh.

Your Fogstar 200Ah at 12V gives you 2400Wh total, but realistically you want to stay above 20% SOC with LiFePO4, so working capacity is roughly 1920Wh.

Overnight freezer load = easily manageable, assuming your solar is recharging it properly during the day. The narrowboat angle concerns me slightly — how much panel do you have? Trees and bridges can murder your harvest.

Worst case scenario, stick a Victron SmartShunt on there and watch real consumption data for a few days rather than guessing.

@DevonDweller ran a similar setup at my cabin last winter — 200Ah Fogstar with a small chest freezer. Overnight was fine, typically only pulled 30-40Ah depending on ambient temp. The colder the space it's sitting in, the less it cycles. Where it gets dicey is summer when the compressor's working harder.

One thing worth doing — pre-freeze the contents well before switching to battery-only overnight. Thermal mass does a surprising amount of work for you.

Also keep an eye on your low voltage cutoff settings. LiFePO4 down to 20% is fine but don't let it creep lower if you're unsure when solar will top it back up.

Should be manageable on 200Ah honestly.

@DanFisher89

Good timing on this thread — I've got a nearly identical setup on my own narrowboat. One thing nobody's mentioned yet: ambient temperature makes a massive difference to how hard your freezer has to work. In a boat engine bay or poorly insulated space, duty cycle can creep up significantly compared to manufacturer specs. Also worth checking whether your BMS has low-temperature cutoff — if you're moored somewhere cold overnight, some LiFePO4 batteries won't discharge below freezing. The Fogstar Drift should be fine on that front from memory, but double-check the spec sheet. @MarineSophie is right that overnight is generally doable, but I'd suggest keeping an eye on your State of Charge for the first few nights rather than just assuming it's all fine. Give yourself a baseline before you rely on it fully.