Sizing a battery bank for a garden office — am I overcomplicating this?

I'm converting a 3x4m timber cabin into a full-time home office and trying to work out the correct battery capacity. I've done the load calc: two monitors, a laptop, a small fan heater (used sparingly), LED lighting, and a router — roughly 1.8–2kWh per day in summer, potentially 3–3.5kWh in a grim January. I've got 600W of panels on the roof (four 150W Renogy mono panels), and a Victron SmartSolar 100/30 MPPT already sourced.

The bit I'm stuck on is whether to go 24V or 48V for the battery bank. I've been looking at Fogstar Drift 100Ah 24V lithium units — two in series would give me 48V/100Ah (4.8kWh usable at ~95% DoD), which seems adequate for summer but marginal in winter. Alternatively, two in parallel at 24V gives me 24V/200Ah (4.8kWh same maths), but then I'd need to upsize the MPPT.

Is there a genuine practical reason to favour 48V over 24V at this scale, or is it mainly relevant when you're pulling higher currents? In my van build I ran 12V throughout and it was fine, but that was a much smaller system. I keep reading that lower voltage means fatter cable runs and higher losses, but at these modest loads in a static install, does it actually matter enough to influence the decision?

Moorey13 | 847 posts

@FormerMariner1 the fan heater is your wild card there — even "used sparingly" it'll absolutely dwarf everything else on that list combined. A 2kW heater running just 2 hours daily is 4kWh on its own, which changes your battery sizing conversation completely.

My honest advice: don't size the battery around the heater at all. Insulate the cabin properly and use a small 400-500W panel-fed system for the office kit, then run a separate mains spur or a small propane/infrared heater for the thermal load. Trying to store enough capacity for electric heating off-grid gets expensive very quickly.

What's your solar array looking like, and are you totally off-grid or just wanting grid independence during daylight? That'll change the recommendation significantly.

LindaClark90 | 203 posts

@FormerMariner1 I went through almost exactly this for my garden office last year! One thing that caught me out — have you factored in your inverter's idle draw? Mine was pulling nearly 20W just sitting there doing nothing, which added up over a long working day.

Also, what's your solar input looking like? In winter here a 3x4m roof gives you maybe 2-3 usable hours on a good day, so your battery has to carry far more of the load than summer calculations suggest.

What battery chemistry are you going with? I ended up with Fogstar Drift LiFePO4 and honestly couldn't be happier — the usable capacity vs lithium cost made sense for my setup. Are you planning Victron for the management side?

FinnTaylor78 | 134 posts

@FormerMariner1 worth flagging that on my narrowboat I made the same mistake of undersizing because I calculated average loads rather than peak loads. The heater will likely be your dominant draw by a huge margin — everything else is almost negligible beside it.

Have you considered whether a small inline electric radiator (oil-filled, thermostatically controlled) might actually be more predictable to plan around than a fan heater? The wattage stays consistent rather than cycling unpredictably.

Also — what's your charging source? Solar only, or do you have a grid fallback? That changes the sizing calculation quite significantly. On my emergency backup setup I always plan for 2-3 days autonomy minimum, but a garden office with grid access nearby might change that equation completely.

What depth of discharge are you planning to work to on whatever battery chemistry you go with?

DodgyMechanic | 312 posts

@FormerMariner1 what's your solar input situation? Because the battery size question is almost meaningless without knowing how quickly you're replenishing. I ran a similar calc for my garden office and kept going round in circles until I stopped fixating on battery capacity and worked backwards from worst-case solar harvest in December/January.

Also — are you planning lithium or AGM? Because your usable capacity changes dramatically. 100Ah AGM is effectively 50Ah. With Fogstar Drift or similar LiFePO4 you're getting 80-90% usable.

What days of autonomy are you designing for? Two cloudy days? Five? That single assumption will drive your bank size more than anything else in your load calc.

ST_Builds | 287 posts

@FormerMariner1 the fan heater is the killer here — even "used sparingly" it'll dwarf everything else combined. I'd almost treat it as a separate problem and just use a small 230V immersion or oil rad on a timer directly from grid/inverter rather than baking it into your battery calc.

For the actual office loads (monitors, laptop, lighting etc.) you're probably looking at 300-500Wh real daily usage. I'd size for 3 days autonomy minimum on that figure — so roughly 150-200Ah at 24V in LiFePO4.

Done a similar thing for my shepherd's hut. Went with Fogstar Drift cells and no regrets. Don't overspec chasing the heater load, you'll end up with a massive bank that's overkill 350 days a year.