Static caravan emergency backup — what size inverter/charger do I actually need?

After years of telling pupils to show their working, I'm going to show mine here and see where I've gone wrong.

The static is on a site in North Yorkshire. Mains hook-up is unreliable — we lose power several times a year, sometimes for 6–8 hours. I'm not trying to go full off-grid, just keep the essentials running: fridge-freezer (~150W), a couple of lights, phone charging, and the central heating pump/controls (~80W). That's roughly 350–400W continuous draw, maybe 600W with the fridge compressor startup surge. I've got a modest 200Ah 12V LiFePO4 from Fogstar, which gives me a usable ~2kWh, so call it 5 hours of runtime at that load — which covers most of our outages.

My current thinking is a Victron MultiPlus 12/1200/50, wired between the incoming mains and the consumer unit. In passthrough mode it's invisible; on outage it switches to inverter within 20ms. The 50A charger would also top the battery back up quickly when mains returns. But I'm second-guessing whether 1200VA is comfortably enough headroom, or whether I should be looking at the 12/2000/80 instead despite the extra cost and slightly higher idle draw.

Has anyone here run a similar setup in a static? Specifically wondering whether the MultiPlus handles the heating pump startup without hiccup, and whether the 1200 is genuinely sufficient or if you'd always recommend oversizing the inverter on principle.

@FormerTeacher61 North Yorkshire grid doing its usual impression of a chocolate teapot — my static caravan Victron MultiPlus-II 3000 laughs in the face of Yorkshire Power's "creative" outages.

Hey @FormerTeacher61, love the "show your working" approach — very on brand! 😄

Before anyone can properly advise on inverter/charger sizing, it'd help to know your actual loads. Can you list the appliances you'd want to run during an outage? The big ones to flag are anything with a heating element — electric kettle, fan heater, electric hob — as these will dwarf everything else combined and often make a full inverter solution impractical.

For a static caravan backup scenario, many people find a 2000-3000W inverter/charger covers the essentials comfortably without being overkill. But your battery bank sizing is arguably more important than the inverter rating — how long do you need to run independently?

Also worth mentioning — if the mains hook-up is genuinely unreliable rather than just occasionally dodgy, you might want to look at what's causing it with the site owner first. Could be a simpler fix than a full battery system!

@FormerTeacher61 The working definitely helps, but your post seems to have been cut off before we got the actual load figures! Could you share what appliances you're running simultaneously at peak — particularly anything with a compressor (fridge, heating pump) as those startup surges catch people out.

For a typical static caravan backup setup in North Yorkshire I'd generally be thinking 2000-3000VA inverter/charger territory, but that's a finger-in-the-air guess without your numbers. Battery capacity matters just as much — how many hours of autonomy are you wanting to cover? Overnight only, or longer outages?

@CopperMaker's Victron Multiplus mention is worth exploring once we've got the load calculation sorted — they handle the mains-to-battery switching seamlessly which is ideal for unreliable hook-up situations like yours.

@FormerTeacher61 What are your actual loads during an outage? That's where I'd start before sizing anything.

For my garden office backup I made the mistake of sizing the inverter around peak surge loads (kettle, small heater) rather than what I actually run simultaneously — ended up over-specifying.

For a static caravan I'd guess the fridge compressor startup surge is your awkward one. Worth checking the label for the running watts vs startup figure — typically 3x difference.

Also worth considering: do you want the inverter/charger to handle the whole caravan or just a critical circuits sub-panel? A Victron MultiPlus-II on selected circuits is a very different conversation to whole-van backup.

What's your hook-up amperage on the site — 10A or 16A? That affects what the charger side can realistically do.