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

Running a small chest freezer off a 200Ah lithium (Fogstar Drift, for the record) on my narrowboat and in the garden office at various points — curious whether anyone's actually done the maths or real-world testing here before I commit to a setup.

The freezer in question is a 60-litre chest unit, rated at 90W but obviously that's peak draw, not continuous. From what I can gather, these things cycle on and off rather than running flat — probably somewhere between 30-50% duty cycle depending on ambient temperature and how stuffed full it is.

My rough back-of-envelope calculation:

• 200Ah at 12V = 2.4kWh usable (assuming 80% DoD on lithium)
• Overnight = roughly 8-10 hours
• If freezer averages, say, 45W actual draw: 45W × 10h = 450Wh
• That's less than 20% of available capacity, which sounds fine

But I'm second-guessing myself because:

1. Inverter losses — running it through a Victron Phoenix 12/375, which isn't the most efficient beast at low loads
2. Summer vs winter ambient temperatures on a boat are wildly different
3. What happens if the battery is already partially depleted from daytime loads?

Has anyone actually logged real consumption data on a similar setup? I'm half-expecting someone to tell me I've missed something obvious, which, given my track record with these things, is entirely probable.

Specific numbers from actual monitoring kit (Victron BMV, Cerbo, whatever you're running) would be genuinely useful rather than "yeah it'll be fine mate."

@SomersetNomad done exactly this with a similar setup. The key figure you need is your freezer's actual duty cycle — most small chest freezers (50–70L range) pull around 80–150W when the compressor runs, but only cycle maybe 25–40% of the time depending on ambient temp.

Rough maths: 120W × 30% duty cycle = ~36Wh/hr, so 12 hours overnight ≈ 430Wh. On a 200Ah/12V bank that's roughly 180Wh usable at 75% DoD — you'll be short.

Two options:

• Upgrade to a 200Ah 24V bank (doubles your usable capacity)
• Accept the freezer drains ~40–50% overnight and solar tops it up by midday

On my narrowboat the solar recovery side is the critical variable. What's your panel capacity? That changes the answer considerably.

Done this exact thing in my static caravan setup. The duty cycle point @Ozzy raises is the critical one, but what caught me out was ambient temperature — my chest freezer works considerably harder in summer when it's sitting in an uninsulated space. Went from roughly 20Ah overnight in winter to nearly 45Ah in a warm July.

The Fogstar Drift is a solid unit for this — decent continuous discharge rate. Just make sure you're not also running anything else that spikes overnight (water pump, lighting left on, that sort of thing).

Real-world tip: stick a plug-in energy monitor on the freezer for 48 hours before you commit. The manufacturers' quoted figures are basically useless for actual planning.

What nobody's mentioned yet — ambient temperature matters massively. My narrowboat sits through some grim winters and the freezer barely works at all compared to summer when it's cycling constantly trying to keep up. You could have identical kit and get wildly different results depending on whether you're moored somewhere shaded or baking in a marina in August.

Also, what's your inverter efficiency? Running a compressor freezer through a cheap inverter will eat into that 200Ah more than people realise. Victron kit is efficient, some no-name stuff less so.

Have you actually measured the draw with a clamp meter or are you just guessing off the spec sheet? Spec sheets are useless — real-world figures are what matters. What's the actual wattage you're seeing?

Good points all round, but one thing worth adding for the narrowboat context specifically: compressor start surge. Small chest freezers can draw 3–5× their rated running wattage for a fraction of a second on startup. With a 200Ah Fogstar Drift you've got a decent lithium bank, but check your inverter's peak surge rating — some budget units trip on this repeatedly, which is worse for longevity than the steady draw.

Also worth checking whether your Fogstar's BMS has a high-current cutoff that sits awkwardly close to that surge figure. I've seen setups where everything looks fine on paper but the BMS logs show it's tripping and reconnecting on every compressor cycle.

A clamp meter on the supply cable over 24 hours will tell you everything you actually need to know rather than relying on spec sheet figures.

Really depends on the freezer. My shepherd's hut setup runs a small Subcold chest freezer — pulls around 80-90W when the compressor's on but duty cycle in a well-insulated unit is maybe 25-30% overnight once it's already cold. So realistically you're looking at 20-25Ah across 8 hours, nowhere near the 200Ah capacity.

The critical bit @BurnWalker touches on is pre-cooling. If you're running mains beforehand and the freezer's already at temp, overnight draw is minimal. Starting from warm is a different story entirely.

On the Fogstar Drift specifically — decent BMS but worth checking your low-voltage cutoff settings if the freezer's the only load. Some people set it too conservatively and wonder why things shut off at 2am.