Powering a garden office — is 400W solar + 100Ah LiFePO4 actually enough?

Finally getting round to sorting proper off-grid power for my 6x4m garden office. Currently running an extension lead from the house which is frankly embarrassing. Plan is to go fully independent this year.

I'm looking at a 400W panel array (two 200W Renogy monos) feeding a Victron SmartSolar 100/20 MPPT, into a 100Ah LiFePO4 (probably Fogstar Drift). Inverter would be a Victron Phoenix 12/500. Loads are modest — laptop, monitor, a few LED lights, phone charging, and occasionally a small fan heater on the mildest setting (though I know that's the killer).

The heater is the bit I'm genuinely unsure about. Even a 750W heater running 2 hours a day in winter would chew through that 100Ah battery embarrassingly fast, and winter solar here in the North West is basically a cruel joke. Wondering if anyone's gone down the route of keeping a small propane or diesel heater for warmth and keeping the electrical side purely for low-draw kit?

Has anyone built a similar setup and actually stress-tested it through a UK winter? Would love to know if 100Ah is a realistic starting point or whether I'm kidding myself and should jump straight to 200Ah.

PanelChris | 847 posts

@LindaPrice87 depends entirely on your loads, really. 400W solar + 100Ah LiFePO4 is a decent starting point but could be tight in winter. A 6x4 office with just a laptop, monitor, and LED lighting should be fine through summer, but if you're running any form of electric heating you'll chew through that battery before lunch.

What's your typical working pattern — full days or just a few hours? And are you in the north or south of the UK? Makes a real difference to your December/January yield. Up in Scotland you might see barely 1-2 peak sun hours some days.

I'd also suggest considering a small MPPT controller with data logging so you can actually see your consumption patterns before committing to anything larger. Knowledge is power, as they say! 😄

BurnWalker | 203 posts

@LindaPrice87 What's actually running in there though? That's the question nobody's answered yet. A laptop and LED lighting is completely different from running a monitor, a printer, and a small heater.

100Ah LiFePO4 gives you roughly 1kWh usable. Fine for light loads, borderline useless if you're trying to heat the place in winter. And 400W solar in January in the UK — have you actually modelled that? You'll be lucky to average 1-2 hours peak equivalent some days.

What inverter are you planning? And have you looked at a Victron MPPT rather than whatever cheap controller comes bundled in those all-in-one kits? Makes a real difference squeezing every watt out of limited winter sun.

BodgeItAndScarper | 1,247 posts

@LindaPrice87 @BurnWalker is right — loads are everything here. But I'll add: UK winters will absolutely hammer that system. November through February you might see 1-2 usable sun hours on a good day. 400W panel sounds reasonable on paper, then December arrives and you're getting 40W if you're lucky.

I'd be looking at minimum 200Ah if you want any resilience. Fogstar Drift cells are decent value if you're going DIY.

Also — is this just lighting and a laptop, or are you running monitors, a heater, kettle? Because a 2kW fan heater will drain 100Ah in about an hour. People always forget the kettle.

What's your actual daily load estimate? Without that figure, nobody can give you a sensible answer.

Geordie | 412 posts

@LindaPrice87 ran almost identical numbers on my motorhome before scaling up — and the honest answer is: UK winter will absolutely batter you. November through February, you're realistically pulling maybe 30-50% of rated output on a good day, and "good day" is doing a lot of heavy lifting there.

The 100Ah LiFePO4 becomes your actual lifeline when the panels are doing nothing useful for three days straight. I'd seriously consider 200Ah if budget allows — Fogstar Drift cells are decent value right now.

The other thing nobody's mentioned yet: what's your inverter situation? Running a laptop and monitor is one thing. Chuck a small heater in there during a grey January afternoon and that whole system is crying.

What's the actual load list? Even rough watts would help massively here.