What's the minimum viable battery + inverter setup for keeping a garden office alive during a power cut?

After the two outages we had in Norfolk last winter (longest was about 6 hours), I've been thinking seriously about a dedicated backup system for my garden office rather than relying on extension leads from the house. The office runs a laptop, a monitor, a small fan heater on its lowest setting (about 750W), and a few USB chargers. Rough load estimate is maybe 900–1,000W peak, but sustained draw is probably closer to 400–500W for most of the working day.

I've been looking at a Fogstar Drift 100Ah LiFePO4 as the battery (£299 at the moment) paired with something like a Victron Phoenix 1200VA inverter. The maths suggests 100Ah at 12V gives me roughly 1.2kWh usable, which at 400W draw would last maybe 2.5–3 hours — not quite enough for a full outage, so I'm wondering whether a second 100Ah in parallel is the sensible move straight away rather than upgrading later.

The tricky bit for me is the changeover — I don't want to be scrambling to plug things in when the lights go out. Has anyone integrated a proper automatic transfer switch (ATS) with a setup this size? Is there a cost-effective option for a single-circuit garden office, or am I overcomplicating it?

Kingy74 | Posts: 847

Good shout @NorfolkExplorer - garden offices are brilliant for this because the loads are usually pretty predictable.

Work out your actual consumption first - typically a laptop, monitor, router/mesh node, and maybe a small lamp. You're probably looking at 150-300W running load depending on your setup.

For 6 hours I'd suggest a minimum of 1.5-2kWh of usable battery (so a 24V 100Ah LiFePO4 gets you there comfortably) paired with a 1000W pure sine inverter. Pure sine is non-negotiable if you're running sensitive electronics.

Victron stuff is pricey but bombproof - their Phoenix inverters are well regarded here. EG4 and Fogstar batteries are worth a look for the budget-conscious.

Don't forget a small solar panel on the office roof to keep things topped up between outages rather than relying solely on mains charging. 👍

DorsetCruiser | 847 posts

Good shout @NorfolkExplorer. Worth thinking carefully about your actual load first — most garden offices are surprisingly manageable if you're disciplined about it.

A decent starting point would be a 200Ah lithium battery (LiFePO4, not lead-acid) paired with a 1000W pure sine inverter. That'll comfortably handle LED lighting, laptop, router, and a monitor for well beyond 6 hours without breaking a sweat.

Key thing people overlook: pure sine wave inverter is non-negotiable if you're running sensitive electronics. Modified sine can cause havoc with laptops and monitors over time.

Budget roughly £400-600 for a solid entry-level setup. Brands like Fogstar or Fogstar Drift for the battery, Victron for the inverter if you want reliability that'll last years rather than months.

What's your typical load — do you run anything power-hungry like a heater or kettle?

AshWalker | Posts: 1,203

The UPS-style switchover time matters more than most people realise here. A standard inverter can have a 20-50ms transfer delay — desktop computers and some NAS units will reboot during that gap. Look specifically at inverter-chargers with sub-20ms transfer (Victron MultiPlus is the benchmark), not just a standalone inverter bolted to a battery.

For sizing: catalogue every load in watts, multiply by your worst-case outage hours, add 20% for inverter inefficiency, then double it for battery longevity (you never want to drain LiFePO4 below ~20%).

Running a shepherd's hut office myself on a Victron MultiPlus-II 24/3000 with 200Ah of Fogstar Drift cells — six-hour outages become genuinely invisible. The grid-tied charging keeps it topped between events.

@DorsetCruiser is right that load assessment comes first — don't size the battery until you've actually metered the office for a working week.

PanelSteve | Posts: 2,341

@NorfolkExplorer six hours in Norfolk in winter — so basically just a regular Tuesday then 😄

Seriously though, the boat has schooled me well here. I ran a 200Ah Fogstar lithium paired with a Victron MultiPlus 500VA for two

ExpertWanderer | Posts: 2,156

Jumping in to add something nobody's mentioned yet — if this is purely backup rather than day-to-day off-grid use, a proper automatic transfer switch (ATS) built into something like a Victron MultiPlus is worth the extra outlay. It'll switch over in around 20ms, which most computers and routers won't even notice.

For a typical garden office — laptop, monitor, router, a lamp or two — you're realistically looking at 300-500W continuous draw. A 100Ah LiFePO4 at 12V gives you roughly 1.2kWh usable, so comfortably 4-6 hours at that load.

@NorfolkExplorer given your 6-hour worst case, I'd honestly size for two 100Ah batteries from the off rather than upgrading later. The cabling and labour costs make retrofitting painful. Budget around £800-1,200 all in depending on inverter quality.

RhysLewis77 | Posts: 847

Ran almost this exact setup on my narrowboat before I upgraded — a Victron MultiPlus 500VA paired with a Fogstar Drift 100Ah lithium did the job beautifully for laptop, router, a couple of LED strips and phone charging. Six hours is genuinely comfortable on that. The MultiPlus handles the switchover in milliseconds so your computer never even notices the grid has gone. Only thing I'd add to what's already been said — make sure your inverter's idle draw isn't quietly murdering your battery while you're waiting for an outage that might never come.

GlenSimon | Posts: 847

My garden office runs a Victron MultiPlus paired with a 100Ah Fogstar Drift LiFePO4 — overkill for six hours but I sleep considerably better than I did with the "extension lead from the house and a prayer" approach I'd been heroically committed to for three years.