Shed office nearly done but not sure about the battery size for just a laptop and a few lights

For a laptop and a few lights you're honestly in pretty safe territory with even a modest setup, but it's worth doing the maths properly rather than guessing.

A typical laptop pulls 45–65W, and if you're running LED lighting (say 3–4 fittings at 8W each), you're looking at roughly 100W continuous load. Over an 8-hour working day that's around 800Wh consumed.

The key figures you need to nail down:

• Usable capacity — lithium (LiFePO4) gives you ~95% usable, lead-acid only ~50%
• Days of autonomy — how many cloudy days do you want covered without solar input?
• Your actual usage pattern — are you in there daily or just weekends?

For my tiny house build I went with a Fogstar Drift 200Ah 12V LiFePO4 and it's genuinely overkill for light loads, but future-proofing matters when you start adding monitors, a printer, a phone charger, a small fan in summer...

Realistically for a shed office with the loads you've described, a 100Ah LiFePO4 at 12V (giving roughly 1.2kWh usable) should cover a full working day comfortably with a bit of headroom. Pair it with a Victron SmartSolar MPPT controller and even a modest 200W panel and you'll rarely touch grid backup.

What inverter are you planning? And is the laptop running directly off 12V or via a 230V inverter? That changes efficiency losses significantly and might affect your sizing.

Would be good to hear what solar array you've got planned as well — battery sizing in isolation

@KangooNomad don't forget standby loads. People always calculate their laptop and lights then wonder why the battery's half dead by morning because they left a router, phone charger and some random wall wart plugged in.

Measure your actual idle draw with a clamp meter or a Shelly EM before you size anything. I did this with my cabin and found nearly 40W of phantom load I hadn't accounted for. Would've undersized the battery badly.

Also what inverter are you running? A cheap one can add 10-20W overhead just sitting there. Victron Multiplus is efficient but obviously costs accordingly. If it's purely DC loads, ditch the inverter entirely and run 12V direct — saves you a chunk.

Good point from @GrumpyBuilder — standby loads are sneaky little vampires 🧛

For what it's worth, I run my garden office on a 200Ah Fogstar Drift LiFePO4 and it's absolute overkill for laptop + lights. You could genuinely get away with 100Ah if the sun shows up occasionally.

One thing nobody mentions: a decent BMS saves your bacon when you inevitably leave something on overnight. Victron's kit makes it dead easy to monitor too — the app becomes weirdly addictive.

Don't overthink it for your use case — you're not running a data centre! Start sensible, see how you get on. Easy enough to add capacity later if needed. 💪

Had almost exactly this dilemma when I wired up my shepherd's hut last spring. Ended up going bigger than I thought I needed — a 100Ah Fogstar Drift LiFePO4 — and honestly it's been the right call every time a grey week rolls in.

The bit nobody tells you: a router or monitor on standby adds up surprisingly fast over 8 hours sat there doing nothing useful. I logged mine with a basic energy monitor before sizing the battery and it changed my whole calculation.

For just a laptop and LED strips, @KangooNomad's numbers are solid, but I'd say treat 100Ah lithium as your floor, not your ceiling. The difference in cost between 100Ah and 200Ah is rarely enough to justify the anxiety of watching your state-of-charge creep down on a Tuesday in November.

Went through exactly this with my van conversion a couple of years back — thought I'd nailed the load calc, then realised I'd completely forgotten the router, the phone charger ticking away, and a little USB fan I'd bodged in.

Ended up doubling my original estimate and I'm still glad I did.

For a shed office I'd honestly say a 100Ah lithium (Fogstar do decent value ones) gets you through a comfortable working day, but 200Ah means you're not watching the Victron app like a hawk every afternoon in January when the panels are doing absolutely nothing useful.

The lights matter less than people think — swap to decent LED strips and the whole lot probably pulls less than 20W combined. It's always the stuff you forgot to count that bites you.

Laptop + LEDs is maybe 100W peak — a 100Ah LiFePO4 gives you roughly 1kWh usable, so you're looking at 8–10 hours before you're sweating. Fogstar Drift 100Ah is what I'd stick in a shed without bankrupting yourself, chuck a Victron SmartShunt on it and you'll actually know what's happening rather than guessing like @JohnDixon presumably was 😄 Don't forget a proper BMS and a decent charger — a Victron IP65 15A is tidy for shed use. One thing nobody's mentioned: inverter standby draw will quietly murder your battery overnight if you leave it live, seen it drain 10–15Ah by morning on my motorhome setup doing absolutely nothing useful.