'UPS' function only, MP2 + 5kWh LFP battery, no PV

Running a Multiplus II purely as a UPS/inverter with no PV input is something I've been mulling over for my static caravan setup, and I suspect a few narrowboat owners here are in the same boat (no pun intended).

The specific use case I keep coming back to: grid-tied most of the time, battery purely for ride-through and motor-starting grunt. No solar panels complicating the picture, just a 5kWh Fogstar Drift LFP pack sitting behind a MP2 48/3000 or 5000.

A few things I'm still working through:

• Transfer time — Victron quotes sub-20ms for the UPS function. Anyone tested this against sensitive equipment like a NAS or a pump controller? Does it actually hold up in practice?
• Battery sizing — 5kWh feels adequate for overnight ride-through on low loads, but I'm less confident about repeated motor-starting cycles on a well pump without a proper calculation. What are people actually using as a rule of thumb?
• ESS vs plain inverter mode — Is there any advantage to configuring ESS without PV, or does that just add unnecessary complexity? I've seen conflicting opinions on the Victron community forum.
• Grid charging rate — setting an appropriate charge current to avoid hammering the supply on a 13A caravan hookup is straightforward enough, but curious whether others have found the MP2 5000 particularly aggressive on reconnect.

This isn't a solar project at all really — it's closer to a decent domestic UPS with proper LFP chemistry instead of sealed lead acid. Wondering if anyone's running exactly this configuration and whether the simplicity holds up or whether Victron's ecosystem keeps nudging you towards adding PV anyway.

@PN_Camper doing almost exactly this at my static caravan. No panels yet — just a Multiplus II 3000 with a 5kWh Fogstar Drift LFP sitting behind it.

Few things worth knowing:

• The grid input current limit matters a lot for charging speed — what's your site hookup rated at?
• Are you keeping the battery as genuine emergency backup, or running loads off-invert daily?
• Worth checking what your transfer time actually is — some sensitive kit still trips even on the MP2's near-instant switchover

Main question I've been puzzling over: does your static have a decent earthing setup? The MP2 has that MEN switching behaviour and it can get complicated depending on how the site's wired.

What load are you primarily trying to protect — whole caravan or just specific circuits?

@SmartSolarNerd interested to hear how you've got on with that Fogstar pack — I'm considering something similar for my garden office.

Quick question for the thread: without any PV, is the battery essentially just cycling between mains-charged and discharge during outages? Or do people deliberately run it in a partial state of charge to extend cycle life?

Also wondering about the transfer time on the MP2 in UPS mode — my garden office has a NAS and a couple of mini PCs that I'd rather not lose mid-write. Does Victron's near-zero transfer actually hold up in practice, or does it depend heavily on the connected load?

Has anyone used ESS Assistant vs just straight UPS mode without it? Not sure which makes more sense if there's no solar in the picture at all.

@PN_Camper the MP2 in UPS mode with no PV is basically just an expensive extension lead until the grid wobbles — which is precisely when it earns its keep.

Done this myself with a Fogstar Drift 5.12kWh and a Multiplus II 5000 — grid goes down, my fridge and router don't even blink. Key thing nobody mentions: set your input current limit sensibly so you're not hammering the grid when it comes back online and the battery wants a full charge in 20 minutes flat.

Also worth checking your ESS vs non-ESS assistant config — for pure UPS use with no PV ever, you might not even need ESS at all, just the basic UPS assistant keeps it dead simple.

@MelKing83 Fogstar pack has been rock solid for me, no dramas.

@PN_Camper this is pretty much exactly how my boat setup started — Multiplus II with a battery bank, no panels, just using shore power to keep the batteries topped up and the inverter handling everything onboard.

One thing worth thinking about: without PV, your 5kWh will drain faster than you expect if the grid drops for any extended period. What loads are you planning to run through it? On the narrowboat I found it was fine for essentials but anything with a heating element (kettle, fan heater) chews through capacity alarmingly quickly.

Also worth checking your transfer switch settings in VEConfig — the default tolerances can be quite tight and you might find the MP2 switching to inverter mode more than you'd like if your static caravan supply is slightly wobbly.

@ExBrickie94 lol "expensive extension lead" is a bit harsh but not entirely wrong 😄

Done something similar for the shepherd's hut — no PV, just a Victron MP2 and a Fogstar 5kWh pack acting as a buffer/UPS. Main thing I'd flag is keeping an eye on your charge cycles if you're grid-tied constantly — the battery will just sit at 100% which LFP doesn't love long-term.

Worth setting a charge limit of around 90% in VictronConnect, takes about two minutes to configure. Also make sure your transfer switch time is set low if you've got anything sensitive running — the MP2 can do sub-20ms which is genuinely impressive.

Not having PV does simplify the setup massively though, fewer things to go wrong.