Anyone else running a garden office entirely off-grid? Sizing questions inside

Been running my garden office on a standalone solar setup for about eight months now and trying to work out whether I've got the balance right. Currently on 400W of panels (two 200W Renogy mono) feeding into a Victron SmartSolar 100/30 MPPT, with a 200Ah Fogstar Drift LiFePO4 as the battery bank. Inverter is a Victron Phoenix 12/1200.

Typical loads are a laptop, two monitors, a desk lamp, and a small fan heater in winter — that last one is obviously the killer. On a decent day in summer I'm comfortably in surplus, but November through February I'm regularly ending the day under 50% SOC even without the heater running much.

Thinking about adding another 200W panel and maybe a second 100Ah Fogstar to give myself more buffer, but not sure if panel capacity or storage is the actual bottleneck in winter. The MPPT should handle the extra panel fine on paper.

Has anyone done proper winter load calculations for a UK garden office setup? What ratio of panel watts to usable battery capacity are you running, and did adding more panels actually help in deep winter or did you end up needing grid backup regardless?

@IslandExplorer Interesting one — I'm coming at this from the opposite direction, running a similar setup on our narrowboat rather than a static building, but the sizing logic is identical.

One thing that catches people out with garden offices specifically: winter shading from deciduous trees. In summer your 400W performs beautifully, then November hits and suddenly a tree you hadn't noticed is blocking half your array from 10am onwards.

Worth pulling your Victron stats for December/January specifically — not the eight-month average. That's where you'll find your real weak spot.

On the battery side, what's your minimum state of charge set to? Running Fogstar Drift cells here and keeping them above 20% made a noticeable difference to longevity. If you're bottoming out regularly in winter, more panels will serve you better than more capacity.

What does your typical daily consumption actually look like — monitors, heating, kettle?

@LisaKelly66 snap — narrowboat gang represent, though my "garden office" is technically the bit of the boat where I pretend to work while watching ducks.

HarbourSam | 847 posts

@IslandExplorer Eight months in is actually a decent point to reassess — you've got data across the seasons which is gold. The key question I'd ask is what your battery state of charge looks like at the end of a typical working day in December versus June. If you're regularly dipping below 50% SOC in winter, your panel array is probably undersized for year-round reliability rather than the battery capacity itself.

400W is workable for a modest office load but the low winter sun angles in the UK mean you're effectively getting maybe 1-1.5 peak sun hours on a grey January day. Worth logging your actual consumption with a clamp meter if you haven't already — people are often surprised how much a monitor or small heater adds up.

What's your inverter situation and what loads are you actually running?

@IslandExplorer The piece nobody's mentioned yet is winter derating — those 400W panels will be putting out considerably less than rated in December/January, both from low sun angles and the shorter day window. Worth pulling your inverter or charge controller logs for November onwards specifically and checking your actual harvest figures rather than relying on average daily totals.

For emergency backup framing (my main use case), I'd argue battery capacity matters more than panel size — you want enough stored energy to ride out 3-4 consecutive grey days without touching mains. With a typical garden office load, that often means people are undersized on storage rather than generation. What's your battery bank actually sitting at — capacity and chemistry? LiFePO4 changes the calculus significantly versus AGM given usable depth of discharge.