What's the minimum viable backup setup for a static caravan during a winter grid outage?

Been thinking about this since we had a 14-hour outage last January — pipes nearly froze and the gas combi wouldn't fire without 240V for the ignition board and controls. That was the wake-up call. The caravan sits on a residential park in North Yorkshire, so grid-tied normally, but winter storms are getting worse and I'd rather not rely on the park's ageing infrastructure.

Currently running a Victron Multiplus-II 3kVA in the motorhome with 200Ah of Fogstar Drift LiFePO4, and I've been tempted to replicate something similar for the static — but that feels like overkill for pure emergency use. The combi boiler draws about 150W running, 300W peak on ignition. A couple of LED circuits, the fridge (80W average), and the router is probably the realistic load. I've roughed it out at maybe 600–700W continuous worst case, 6–8 hours overnight being the critical window.

The question I keep circling is whether a decent inverter-charger (say a Victron Multiplus-II 1200 or even a cheaper Victron Phoenix + separate charger combo) with 100Ah LiFePO4 would actually be sufficient, or whether I'm undersizing it and would regret it at 2am in February. Anyone running a static backup setup with real winter data? Particularly interested in whether the Multiplus-II 1200's 20A passthrough is a bottleneck with some of the larger peak loads a combi can throw on startup.

@Cerbo_Geek that combi boiler issue catches a lot of people out — same thing happened to me in the van when I tried to run a Truma off a sketchy inverter. The boiler itself barely draws anything but that control board is surprisingly fussy about sine wave quality.

Genuine question for the thread though — for a static caravan specifically, what's the minimum battery capacity people reckon you'd need to run a combi through the night?

I'm guessing:

• Pure sine inverter (non-negotiable for the ignition board)
• Enough battery to cover the pump + controls cycling every 20-30 mins
• Some kind of solar or small genny top-up for extended outages

Would a single Fogstar Drift 100Ah even cut it for 14 hours, or are we realistically looking at 200Ah+ before it becomes viable? Anyone actually measured their boiler's standby draw?

@Cerbo_Geek a Victron MultiPlus-II 500VA + a single Fogstar Drift 100Ah is basically the "frozen pipes insurance policy" starter pack — boiler fires, lights stay on, dignity intact.

@Cerbo_Geek worth noting that most combi boilers only need a trickle of power to run — typically 50-150W while firing, less when just the controls are idling. So you don't need a massive inverter, but you do need something that can handle the startup surge reliably.

One thing nobody's mentioned yet: a lot of static caravan installs have electric trace heating on the pipes, which will absolutely drain a small battery overnight without you realising. Check whether yours has it and factor that into your sizing — it can easily add another 100-200Wh per hour in serious cold.

Also consider a small UPS as a stopgap if budget's tight before you build a proper system. Not ideal long-term but it'll keep the boiler ticking for several hours during a short outage.

@Cerbo_Geek one thing nobody's mentioned yet — don't overlook the startup surge on that boiler's pump and controls. Even if running draw is modest as @CrispySkipper says, you might see a brief spike on ignition that catches out undersized inverters. Worth checking your boiler's spec sheet for peak/startup current before buying anything.

Also, assuming you've got mains water supply to the caravan, a small 12V submersible pump or even just leaving a tap trickling overnight costs almost nothing from a battery and can save your pipes if the heating does fail. Belt and braces approach.

@Cerbo_Master's suggestion is solid kit, but if budget's tight, even a decent leisure battery and a basic 300W pure sine inverter would keep that boiler ticking through most overnight outages.

Great points all round. One thing I'd add — if your static is unoccupied for stretches during winter, consider pairing that setup with a simple frost thermostat wired to trigger the boiler automatically. That way you're not relying on someone being there to notice the temperature dropping. Also worth checking whether your caravan's consumer unit has a dedicated circuit you can feed from the inverter without backfeeding anything awkward — some older static installations are a bit haphazard internally. A proper changeover switch or even just a well-labelled partial setup could save you headaches. @Ken1999 makes a fair point about surge — I'd suggest testing your specific boiler's actual startup draw with a plug-in energy monitor before committing to a particular inverter size. They vary quite a bit in practice.