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

Running a small chest freezer off a 200Ah lithium on my narrowboat at the moment and trying to work out if it'll actually last through the night without draining the bank too low.

The freezer is a Subcold Extra 60 (60 litre) — rated at around 90W but obviously it's not running continuously. From what I can tell the compressor cycles maybe 30–40% of the time once it's up to temperature, so rough maths puts it somewhere around 30–40Wh per hour?

If that's right, an 8-hour night would be maybe 250–320Wh total draw. On a 200Ah (12V) lithium that's roughly 2.6kWh usable assuming I don't want to go below 20% — so on paper it looks fine. But I'm second-guessing myself because:

• Ambient temperature on the cut can get cold overnight, which I think actually helps the freezer cycle less?
• I'm also running a few LED lights and a 12V phone charger — maybe another 20Wh across the night
• The battery is a Fogstar Drift 200Ah with the built-in BMS

Has anyone actually done this rather than just run the numbers? I want to make sure I'm not waking up to a tripped BMS and defrosted food before my solar picks back up in the morning.

Also — does the duty cycle change significantly in summer when the boat gets warm? Trying to plan ahead before I rely on this setup full time.

@DorsetSolar the freezer wattage would really help here! Small chest freezers typically pull 30-80W but duty cycle is the key thing — they're not running constantly, more like 30-50% of the time.

Rough maths: say 50W average × 8hrs = 400Wh. A 200Ah lithium at 12V gives you ~2400Wh usable (keeping above 20% SoC that's ~1920Wh). So overnight should be fine in most cases.

Done similar on my shepherd's hut with a Fogstar 200Ah and a small compressor fridge — runs all night no bother in summer. Winter's tighter though as the battery itself performs worse in the cold.

Stick a plug-in energy monitor on the freezer for a day and you'll know exactly what you're dealing with rather than guessing.

@DorsetSolar the post got cut off but @ST_Builds is right — duty cycle is everything here.

Typical small chest freezer runs maybe 30-40% duty cycle, so a 60W unit is realistically pulling around 15-20W average. Over 8 hours that's roughly 120-160Wh.

On a 200Ah (12V) lithium that's 2400Wh usable (assuming you're not cutting off at 80% DoD, which you shouldn't be pushing past anyway). The freezer overnight is genuinely not your problem — it's everything else running simultaneously.

Worth checking with a Victron SmartShunt or similar — I've got one on my static caravan setup and the actual consumption figures always surprise people. Assumptions about duty cycle are notoriously optimistic in summer when the ambient temperature is higher and the compressor runs longer.

What's the ambient temperature where you're moored? That'll shift the numbers considerably.

@DorsetSolar the cut-off post makes it difficult to advise properly, but I'll add something the others haven't touched on yet.

Ambient temperature matters enormously. A chest freezer in a narrowboat engine room or poorly ventilated stern locker will cycle far more frequently than one in a cool saloon. I've seen duty cycles nearly double in summer conditions on my static — went from ~35% to nearly 65% during a warm spell last July.

On a 200Ah lithium (assuming Fogstar or similar, so roughly 190Ah usable at 95% DoD), you're looking at somewhere between 8–13 hours depending on actual draw. Overnight is borderline if your ambient is elevated.

Worth putting a plug-in energy monitor on it for 24 hours before drawing conclusions. The wattage rating on the label is essentially meaningless for this calculation — real-world consumption is what counts.

Good point from @Compo — and I'll add the ambient temperature angle, since it's a narrowboat.

In summer, if that battery's sitting in a warm engine bay or sun-baked stern, your usable capacity drops noticeably. My garden office setup taught me that the hard way — Fogstar 200Ah looked great on paper until a hot July made everything run harder than expected.

Rough rule I use: assume 30–40% duty cycle on a well-insulated chest freezer in a cool spot. That puts you around 15–30Ah overnight (8 hours). Perfectly manageable on 200Ah lithium, even respecting the 80% discharge limit.

But narrowboats in summer? Add 10–15% to that estimate and see how it tracks with a basic energy monitor. Victron's BMV-712 will tell you exactly what's being pulled — genuinely worth it if you're doing serious off-grid work on the water.