Anyone else struggling to keep their garden office cool in summer without hammering the battery?

It's a real balancing act, isn't it. I've been dealing with this exact problem in my static caravan setup and a lot of the same principles apply to a garden office.

The biggest win for me was passive measures first before throwing cooling capacity at the problem:

• Reflective insulation on the roof made a massive difference — I used a foil-backed multi-layer product and it genuinely cut peak internal temps by several degrees
• Ventilation positioning matters enormously; cross-ventilation with a small 12V fan pulling air through rather than a big inverter-driven unit is far kinder on the battery
• Shade — obvious but often overlooked; even a sail shade over the south-facing roof helps

On the electrical side, if you're running an air conditioning unit off a battery bank, you really need to be honest about the draw. A small portable AC pulling 700W-900W will drain even a decent 200Ah lithium setup embarrassingly quickly on a hot day when solar input is competing with the load simultaneously.

My Victron kit lets me monitor exactly what's happening in real time, which at least helps you make informed decisions rather than just watching the state of charge mysteriously plummet.

Has anyone tried the 12V/24V compressor-based units rather than standard inverter AC? I've seen a few crop up — Fogstar and others are starting to stock compatible kit — and the efficiency figures look promising on paper. Would be curious whether anyone has real-world runtime data from a garden office context specifically.

What's your current setup — lithium or AGM, and what capacity are you working with?

Curly63 | 847 posts

@VoltAlison great point about passive measures, they're so underrated. One thing I'd add that made a massive difference for me was thermal mass. I put down a thick slate floor in my office and it absorbs the morning cool, then releases it slowly through the afternoon. Completely free once it's installed.

Also worth mentioning window film - I used a reflective tint on the south-facing glass and it knocked the temperature down noticeably without blocking much light. Cost me about £40 and took an afternoon to fit.

If you do end up running a fan or small cooler, consider running it from a timer during peak solar hours only so you're essentially using generation rather than eating into storage. Works a treat on sunny days when you actually need it most.

Passive measures are ace but don't sleep on night purging either — open everything up once temps drop and flush that stored heat out before you sleep. Cheap as chips and works a treat.

Also worth checking your roof insulation situation. My shepherd's hut was losing ridiculous amounts of heat through the roof in summer (and winter tbh). Chucked in some proper Kingspan and the difference was night and day.

On the electrical side, if you do need active cooling, a 12v compressor unit running directly off your battery bank is way more efficient than inverting to run a standard portable AC. Keeps your Victron MPPT happy too — less demand spike nonsense. 🔋

Has anyone actually measured how much difference window film makes vs just getting proper external blinds? I went down a rabbit hole on this last summer when my Fogstar 48V setup was getting hammered by a little 12V fan and a portable AC unit I'd panic-bought.

Wondering if there's a point where the passive measures genuinely cap out and you're just better off right-sizing the battery to handle the cooling load rather than fighting it? Like, what's the realistic ceiling on passive gains for a standard 3x3m garden office?

Also — does anyone run a thermostat-controlled relay to cut the AC if SOC drops below a threshold? Feels like that could save a lot of grief on cloudy stretches without having to manually babysit it.

@WonkySparky ran this exact comparison on my van a couple of summers ago using a cheap IR thermometer. External blinds won by a country mile — roughly 8–10°C difference on the interior glass surface versus window film alone. The physics makes sense: you want to intercept the radiation before it hits the glass and converts to heat inside. Film just slightly reduces transmission after the damage is already being done.

For the garden office crowd, external roller blinds (the proper European-style ones, not the flimsy interior Ikea nonsense) combined with decent roof insulation will slash your cooling load significantly before you even think about running an aircon unit. Less cooling demand = smaller inverter draw = Victron MPPT not screaming at you all afternoon.

Thermal mass inside the structure helps too — stone/tile flooring absorbs heat during the day and releases it slowly rather than letting air temps spike.