Anyone else find their garden office gets absolutely freezing by 4pm in winter?

Not strictly off-grid but I'm running my garden office from a 560Ah Fogstar Drift LiFePO4 bank with a pair of Victron MPPT controllers, so heating is always front of mind for me.

The 4pm wall is real. What's happening is that your thermal mass (if you even have any in a typical garden office) has been slowly losing heat all day, and by late afternoon the sun angle drops so low that any passive solar gain disappears entirely. You're basically starting from a deficit.

A few things I've found help:

• Insulating the floor makes a surprisingly big difference — most garden offices lose a huge amount upward from a cold slab or air gap underneath
• A small oil-filled radiator on a timer set to kick in around 2:30pm rather than waiting until you feel cold
• Thermal lined curtains even on small windows — cheap and effective

My main question for the group though: has anyone actually monitored their office temperature overnight and tracked how quickly it drops once you stop heating? I'm wondering whether a 60mm kingspan retrofit to the walls would justify the cost and hassle, or whether I'd just be throwing money at it.

Also curious whether anyone is running an ASHP setup scaled down to garden office size — seen a few products marketed at this but no real-world feedback from UK users in a damp climate.

@BevJackson64 I get this in my shepherd's hut — the 4pm drop is brutal in December/January when the solar contribution basically dies and you're running purely from stored capacity.

What heating are you actually running from the battery bank? I've been looking at low-wattage panel heaters on a thermostat versus just biting the bullet and adding a small propane heater to take the edge off without hammering the Fogstar bank.

Also wondering whether 560Ah is genuinely enough if you're heating and running office equipment through those dark afternoon hours — what's your typical SOC looking like at 4pm this time of year?

OddJobBob79 | 47 posts

@BevJackson64 I had exactly this problem with my timber office. What made a real difference was adding decent thermal mass inside — I put a couple of old slate tiles under my desk as a footrest and they hold the afternoon warmth surprisingly well. Also worth checking your door seals, I was losing a shocking amount of heat there. On the electrical side, I switched from a panel heater to a small oil-filled radiator on a timer — comes on around 2pm before the temperature dips, so it's already warm when the 4pm wall hits rather than playing catch-up. Costs me noticeably less from the battery too. @SolarDoug is right about winter solar being brutal — pre-heating while you've still got a bit of generation left makes a lot of sense.

Insulation's doing the heavy lifting your batteries can't — once the sun dips below the roofline at 4pm it's purely a heat-retention battle, not a generation one.

@BevJackson64 Same issue in my shepherd's hut — the thermal mass problem is real. What actually helped me was shifting my heating earlier, running the panel harder from 11am-2pm when solar yield is decent, so the structure itself absorbs heat. A cheap DS18B20 temperature logger showed me the hut was already losing 2°C/hour by 3pm regardless of what the battery was doing.

Also worth looking at a radiant panel rather than convection heating — warms you directly rather than the air volume, so the 4pm drop feels less brutal even as ambient temps fall.

@MoorKev is spot on about insulation doing the heavy lifting. I added 50mm Kingspan to my hut floor last autumn and the difference was genuinely shocking — floor was the biggest heat loss I hadn't considered.

RetiredSquaddie | 312 posts

@BevJackson64 With a 560Ah Fogstar bank you've got decent capacity, but the 4pm dip is a consumption and generation cliff simultaneously — double trouble.

Worth logging your actual load over 24hrs if you haven't already. A Victron Cerbo GX or even just the VRM portal on your existing MPPTs will show you exactly where the watts are going. Nine times out of ten people underestimate how much a cheap panel heater draws versus what their array is actually pushing in at low winter sun angles.

If heating is the main drain, consider pre-heating the space aggressively between 11am-2pm when your panels are actually earning. Thermal inertia does the rest. Pair that with a decent PIR-controlled setback thermostat and you'll reduce afternoon draw significantly without touching the battery bank spec.

What's your current heating solution — resistive, infrared, or a heat pump unit?