Anyone else using a leisure battery as a short-term UPS for their router and NAS during power cuts?

So we've been getting a fair few grid outages lately — nothing catastrophic, maybe 20–30 minutes at a time, but enough to corrupt a backup job or knock the family off a video call at the worst possible moment. I've rigged up a basic setup using a 100Ah AGM leisure battery, a Victron IP65 15A charger keeping it topped off from the mains, and a 300W Victron Phoenix inverter feeding a standard 4-gang extension lead. Router, NAS, and a small network switch hang off it. Total draw is around 40–50W, so in theory I've got well over 12 hours of runtime if needed.

The bit I'm still fussing over is the switchover. Right now there's no automatic transfer — I literally just unplug the extension lead from the wall and plug it into the inverter output when the power goes. Takes about 10 seconds, which is fine for the NAS since it's set to handle brief interruptions, but the router reboots and you lose connectivity for a minute or so. I've been looking at the Victron MultiPlus 12/500 as a proper solution since it has an almost instant transfer relay, but that feels like overkill budget-wise for what is essentially a cheap insurance job.

Has anyone found a decent middle-ground solution — maybe a proper UPS unit that plays nicely with a leisure battery rather than its own sealed internal pack? I've seen some threads mention modifying cheaper UPS units to accept external batteries, but the ones I've found seem to be US-focused and I'm not sure how well that translates to our 230V setup here. Keen to hear what others have cobbled together.

@Chopper72 this is almost exactly what I started doing about eighteen months ago, except my motivation was keeping the EV charger's management system alive during cuts — the OCPP controller would lose its session and need a full reset every time the grid blinked.

Ended up with a 100Ah Fogstar Drift lithium sat under the stairs, a Victron SmartShunt keeping an eye on state of charge, and a small pure sine inverter feeding a dedicated socket for the router, NAS, and the charger's brains.

The key bit people overlook is automatic transfer switching — without it you're still scrambling for the manual plug swap while everything's already gone dark. A proper ATS relay sorted that completely for me.

Twenty-minute outages became genuinely invisible to the household. Battery barely notices.

Been running a Fogstar 100Ah lithium under my desk for two years specifically for this — router, NAS, and a small mesh node — and honestly the hardest part wasn't the wiring, it was convincing myself the grid was actually unreliable enough to justify it before the third corrupted backup made the decision for me.

That lot runs comfortably for hours off a single battery, so the 20–30 minute outages @Chopper72 is describing are basically a rounding error — just make sure your Victron MPPT is actually topping it back up between cuts rather than letting it sit at 80% like mine mysteriously preferred to do for six months.

Been doing something similar for about a year now — started with an old 100Ah AGM I had spare from a caravan project, though I've since swapped to a 90Ah lithium which is far better for partial state of charge cycling.

One thing worth mentioning that I don't think's been covered yet: check the idle draw of your NAS carefully before sizing everything up. Mine was pulling nearly 30W just ticking over, which surprised me. Some of the older units are real vampires compared to modern ones.

Also worth investing in a proper low-voltage cutoff to protect the battery — I use a simple BMS relay setup. Learned that the hard way after running an AGM down too far during a longer-than-expected outage. @AnneButler your Fogstar setup sounds solid, what inverter are you running with it if you don't mind me asking?

Really useful thread this — I've got a 120Ah LiFePO4 under the stairs doing exactly this job. One thing worth mentioning that I don't see covered yet: invest in a decent low-voltage disconnect if you're not using a purpose-built lithium BMS setup. I learned the hard way that letting a battery sag too low trying to keep a NAS spinning through a longer-than-expected outage isn't great for longevity. Also worth calculating your actual load properly before sizing — my router, NAS, and a small switch together pull barely 35W, so even a modest battery gives you hours of runtime rather than minutes. @AnneButler curious what inverter/charger combo you're running with yours? I went with a Victron IP22 for charging and it's been rock solid.

Similar setup here — been running a 120Ah lithium through a Victron IP65 charger for about eight months now. One thing I'd add that nobody's mentioned yet: worth putting a small DC-DC converter inline if your router and NAS have different voltage requirements rather than running everything off a cheap inverter. You'll lose a lot less to conversion inefficiency that way, especially if your gear accepts 12V direct. Also keep an eye on your idle draw — I was surprised how much my NAS was pulling just sitting there. Ended up setting aggressive sleep schedules which stretched my runtime considerably. @AnneButler curious what charge controller you're using with your Fogstar? I've been debating whether my current setup is the most sensible approach or if I should be looking at something more integrated.

Running something similar in my shepherd's hut actually, though the principle's identical for a house setup.

One thing nobody's mentioned — standby power draw matters more than you'd think over weeks and months. My router, NAS and a small switch idle around 35W continuous. On a 100Ah LiFePO4 (Fogstar Drift) that's still 20+ hours of runtime, but the slow trickle drain means your charger is cycling constantly.

Worth setting your charger's absorption voltage correctly for the battery chemistry rather than leaving it on defaults — I fried an AGM doing exactly that before I switched to lithium.

Also: test your cutover time. Some cheaper inverter/charger combos have a 20-30ms switchover delay that's enough to reboot routers depending on their PSU capacitors. Victron MultiPlus is near-instant; budget units less so.

Good point from @DodgyHermit about the shepherd's hut crossover — van and off-grid builds are basically the same problem domain.

One thing nobody's mentioned yet: standby power draw of the inverter itself can quietly drain your battery over days/weeks if the UPS isn't being used. Either pick a pure sine inverter with a low idle draw (some Victron Multiplus units are decent here), or wire your router/NAS directly to 12V using a DC-DC buck converter if the devices support it. Cuts out the inverter entirely and massively improves efficiency for small continuous loads.

My van setup runs the router at 12V direct — draws maybe 8W constant with zero inverter losses.