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

Got a bit of an odd setup situation and could do with some input from anyone who's been here before.

Running a small off-grid build — basically a converted timber-frame tiny house — and I'm trying to figure out if a single 200Ah LiFePO4 (Fogstar Drift, 12V) will realistically carry a combi boiler through a full night without solar input.

The boiler itself is a small Ideal Logic, draws around 80-100W when the pump and controls are running. Not the burner obviously, that's gas — just the electrical parasitic load. Heating would be on a timer, maybe 6-8 hours overnight.

So rough maths:

• 100W × 8hrs = 800Wh
• 200Ah × 12V = 2,400Wh usable (if 100%)
• At 80% DoD that's ~1,920Wh

On paper it looks fine but I'm not accounting for inverter losses, BMS overhead, or whether the boiler pulls more on startup. Anyone actually measured their boiler draw with a clamp meter or energy monitor?

Also wondering if it's worth sticking a Victron SmartShunt in to actually track what's going out overnight rather than guessing. Already running a Victron MPPT so it'd integrate nicely.

Any real-world experience would be dead useful before I commit to whether one battery is enough or whether I need to look at a second unit.

Reply by PeteGreen84

@GoldenMaker Combi boilers are tricky because the power draw varies quite a bit depending on the model. Most modern combis sit somewhere between 100-200W during normal operation, but can spike to 250W+ during ignition and pump cycles.

Over an 8-hour night you're potentially looking at 1-1.5kWh minimum, and that's assuming fairly modest usage.

A 200Ah lithium at 12V gives you roughly 2.4kWh usable (factoring in a sensible discharge limit), so it's theoretically possible, but you'd be cutting it fine with very little headroom for anything else running overnight.

What boiler model are you considering? That'd help massively in giving you a more accurate answer. Some of the smaller Worcester Boschs are surprisingly efficient. Also worth checking whether your inverter can handle those ignition spikes cleanly.

Depends entirely on the boiler — ignition spike can hit 200W+ but running draw is typically 80-150W; a 200Ah LiFePO4 at 12V gives you roughly 2.4kWh usable, so maths might work if you're not also running lights, phone chargers and whatever else your tiny house is quietly eating.

Real problem is the boiler pump cycling on and off all night — that repetitive inrush is what kills batteries faster than sustained load, ask me how I know (hint: two dead AGMs in my static caravan before I switched to Fogstar Drift cells).

Get a Victron BMV-712 logging overnight first before committing — flying blind on battery sizing is how this hobby gets expensive quickly.

Good point from @Rob1963 on the ignition spike — worth adding that most combis also have a standby draw people forget about, typically 5-10W constantly just keeping the controls live.

On a 200Ah LiFePO4 (say a Fogstar Drift or similar at 12V) you've realistically got around 2kWh usable. If the boiler's firing intermittently overnight rather than flat-out, you might scrape through, but it'll be marginal.

Bigger concern for me would be the inverter — combi boilers can be fussy about sine wave quality. Make sure you're running a pure sine inverter, not modified. Victron Multiplus would handle it no problem but obviously that's a different budget conversation.

What boiler model is it? That'd help massively with actual numbers.

Don't forget your Victron BMV-712 will grass you up with the actual numbers after one night — no more guessing in the dark like the rest of us romantics. 🕯️