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

Running a Fogstar Drift 200Ah on my narrowboat and eyeing up a small chest freezer for the summer — anyone actually done the maths on whether that's a realistic overnight setup?

The freezer I'm looking at pulls about 45W running but obviously cycles on and off, so I've seen estimates ranging from 0.5–1.5kWh per 24 hours depending on ambient temp and how full you keep it. On a warm summer night on the cut, I'd expect it to work harder than it would sat in someone's garage.

My concern is that by the time I've also got a 12V fan, phone charging, a few LED strips, and the inverter parasitic draw ticking away, I'm wondering if 200Ah (so roughly ~2kWh usable at 80% DoD) is going to leave me anything sensible by morning — especially on a cloudy day when the solar isn't topping things back up as quickly as I'd like.

• Running 200W of Renogy panels into a Victron MPPT
• Shore power isn't always available on moorings
• Mostly stationary overnight, so no alternator input

Is there a rule of thumb people use for chest freezer sizing against battery capacity, or is it genuinely just "try it and watch the Victron app in a panic at 2am"?

@ThistleKen welcome to the forum, great first question! 🎉

Running a chest freezer on a Fogstar 200Ah is very doable — narrowboat life here and I've had a small chest freezer ticking away for two summers now.

Quick rough maths:

• Small chest freezer ≈ 30–50W average draw (they cycle, not constant)
• 8hr overnight ≈ 3–4Ah per hour realistically
• That's maybe 25–35Ah overnight

Your 200Ah gives you ~100Ah usable (keeping above 50%) — so you've got loads of headroom, even accounting for other loads.

Key thing: chest freezers are brilliant vs uprights for efficiency, so you've already picked the right style. Just make sure you've got decent solar or alternator charging to top back up during the day.

What size freezer are you looking at exactly? That'll help nail the numbers down properly.

@ThistleKen depends entirely on the freezer. A decent chest freezer runs maybe 30-80W but only cycles on ~30-40% of the time, so real-world draw is often 15-25W average. Over 12 hours that's 180-300Wh.

Your 200Ah Drift at 12V is 2400Wh usable (don't go below 20%). So yes, mathematically you're fine — on paper.

Problem is ambient temp. Narrowboat in summer = hot engine bay or stern deck, freezer works harder, duty cycle climbs. I've seen "efficient" chest freezers absolutely hammer a battery bank in a warm space.

Get the actual model number and look up measured consumption from a site like CLASP before committing. Don't trust the manufacturer's figures.

@ThistleKen done exactly this on my setup — had a 12V Alpicool chest freezer running off a 200Ah LiFePO4 last summer.

Key thing @GrumpyBuilder is hinting at: ambient temp matters massively. On a warm narrowboat in July, duty cycle creeps up noticeably vs a cooler space.

Rough rule of thumb I use:

• Small chest freezer ≈ 0.3–0.5kWh per 24hrs in moderate temps
• 200Ah @ 12V = 2.4kWh usable (assuming 80% DoD)

So overnight (8hrs) you're looking at maybe 0.15–0.2kWh — totally manageable, leaving plenty headroom for other loads.

Where it gets tricky is cloudy days when you can't top up properly. I'd make sure you've got decent solar to recover, even 200W minimum.

Great question @ThistleKen — I ran a small chest freezer in my motorhome for a whole summer season and the thing nobody mentions is ambient temperature matters massively. On a hot July night the compressor cycles far more frequently than the manufacturers quote.

One thing worth checking: does the Fogstar Drift have a low voltage cutoff set sensibly? Mine was factory set quite aggressively and the freezer's startup surge kept triggering it until I adjusted the BMS settings.

Also — what's your charging situation during the day? Solar panels, engine alternator, shoreline? A 200Ah bank overnight is fine in theory but you need to be able to replenish it reliably the next morning, especially on a narrowboat where you might be moored in shade under trees.

What freezer model are you actually looking at? The wattage rating is almost meaningless — the duty cycle is what kills batteries.