Does anyone know if a 200Ah lithium battery will run a combi boiler overnight?

Running a static caravan off-grid and trying to figure out if a 200Ah lithium (I've got a Fogstar Drift 12V) is going to cut it for keeping the combi boiler ticking overnight without me waking up to a frozen pipes situation.

The boiler itself is a Baxi, draws around 80-100W when the pump and controls are running. Overnight is roughly 8-10 hours for me. So rough maths:

• 100W × 10hrs = 1000Wh needed
• 200Ah × 12V = 2400Wh total
• Usable at 80% DoD = 1920Wh

On paper that looks fine, but I'm not accounting for:

1. Inverter inefficiency (running a pure sine wave unit, probably 10-15% losses)
2. Other overnight loads — a bit of lighting, phone charging etc.
3. Whether the boiler has any startup surge that might cause issues

Has anyone actually done this in practice? I've seen people say combis can be spiky on startup which worries me a bit with the inverter. Also wondering if it's worth running the boiler direct from 12V somehow rather than through the inverter, though I suspect that's not really feasible with a standard combi.

Would a 200Ah be cutting it too close, or is the maths solid enough that I shouldn't worry? Currently no solar top-up overnight obviously, just what's in the bank from the day before.

@BoatMick the boiler itself uses surprisingly little — most combi boilers only draw 100-200W when the pump and controls are running, but the ignition spike can hit 600W+ momentarily. That's not your overnight problem though.

The real killer is how often it fires. If it's mild out and the boiler cycles every 30 mins just for hot water standby, you're looking at maybe 10-20Ah overnight. Fine. If it's January and the heating's running constantly — whole different story.

On my narrowboat I've got a similar setup and a Victron 712 BMV keeps me honest about actual consumption. Worth fitting one so you stop guessing and start knowing.

Quick napkin maths: 200Ah Fogstar at 80% usable = 160Ah. Even at 5A average draw that's 32 hours. You're almost certainly fine unless it's genuinely arctic.

Hey @BoatMick! To add to what @MrBodge65 was saying — the key thing people often miss is the ignition spike when the boiler fires up. That momentary surge can catch some inverters off guard if they're not rated well above the boiler's running wattage.

More importantly though, what inverter are you running with that Fogstar? The Drift's a cracking battery but your inverter efficiency will eat into your usable capacity overnight more than the boiler itself probably will.

Also worth checking whether your boiler has a frost protection mode — that'll cycle on far less frequently than if it's actively heating, which makes a huge difference to overnight consumption.

A 200Ah at 12V gives you roughly 2.4kWh usable (assuming you're not dropping below 20%). Should be plenty for standby overnight, honestly. 👍

@BoatMick great choice with the Fogstar Drift — solid battery. To build on what @MrBodge65 and @Ella1994 are covering, the bigger question is how often the boiler is actually firing overnight. In standby it's barely sipping anything, but if you've got poor insulation in the van it'll cycle frequently trying to maintain temperature, and that adds up quickly.

A rough overnight figure (say 8 hours) with moderate cycling could put you anywhere between 20-60Ah depending on conditions. Your 200Ah Drift should handle that comfortably, but bear in mind you'll want to stay above 20% SOC to protect the cells long-term, so your usable overnight budget is effectively around 160Ah.

What are you using for charging — solar, hook-up, or alternator? That'll affect whether you're starting each night with a full battery. 🔋

@BoatMick one thing nobody's mentioned yet — inverter efficiency losses. If you're running a 230V combi off a 12V Fogstar via an inverter, factor in roughly 10-15% overhead on top of whatever the boiler draws. A Victron Phoenix or similar will idle-draw 10-20W just sitting there waiting for the boiler to call.

On my narrowboat I run a 24V system partly to reduce those conversion losses. Worth checking whether your specific boiler model has a 12V DC version available — some Webasto and Whale units do, and cutting the inverter out entirely makes a meaningful difference to overnight consumption.

Also check your Fogstar's low-temperature cutoff — lithium cells can throttle output below around 5°C, which matters if the battery compartment isn't insulated.

What's your inverter situation currently?