Anyone else struggling to keep a small cabin warm without going overboard on battery capacity?

Living in a tiny house on a marina berth means I've been wrestling with exactly this problem for the past two winters.

The temptation is always to just throw more battery capacity at it — I nearly went down the rabbit hole of a massive Fogstar Drift lithium bank — but honestly the heating strategy matters far more than raw storage.

A few things that have genuinely helped me:

• Insulation first, always. Sounds obvious but I added 50mm PIR board to my roof void last autumn and the difference was night and day
• Propane as the primary heat source rather than electric. Keeps the draw off the batteries almost entirely
• A small 12V electric blanket on a timer instead of heating the whole space overnight

The bit I'm still not sure about is whether a Webasto or Espar diesel heater would make more sense long-term vs. the propane setup. Diesel is obviously more convenient at a marina, but the upfront cost is eye-watering.

Anyone running a diesel heater in a small space — are you finding the low-power mode actually useful, or does it cycle on and off annoyingly? My space is roughly 20m² so I suspect even the smallest unit would overpower it on anything above the minimum setting.

Also curious what battery sizes people are actually running for a similar footprint. I've got 200Ah of lithium at the moment with a 400W solar array, which just about gets me through a cloudy December day if I'm careful.

@MV_Marine the marina context is interesting — condensation and salt air will wreck your battery terminals faster than anything if you're not on top of it.

Rather than oversizing capacity, I'd look hard at your loads first. A decent 12v diesel air heater (Webasto or even a cheap Chinese clone) will shift your heating load almost entirely off the battery bank. Mine draws maybe 10W once it's running — negligible compared to any resistive heating setup.

My setup runs 200Ah LiFePO4 (Fogstar Drift cells) and it handles the cabin perfectly once I stopped trying to run a fan heater off it. That was the turning point honestly.

Worth checking your Victron BMV data too — you might find heating isn't actually your biggest draw once you dig into it.

@MV_Marine mate, the secret isn't more battery — it's stopping the heat escaping in the first place, because you can't Victron your way out of a draughty cabin any more than you can bail out a boat with a teaspoon.

@KentCruiser is bang on — insulation first, battery second, otherwise you're just heating the marina with a Victron invoice.

@Jock90 and @KentCruiser have covered the insulation angle well, so I'll add a different angle from my shepherd's hut experience.

The real efficiency gains come from separating your heating loads by time-of-use. Rather than running a single system 24/7, I run a diesel Webasto on a timer for the coldest overnight hours, which draws almost nothing from the battery bank, then a small 12V fan to redistribute retained heat during the day. The battery capacity (200Ah Fogstar Drift LiFePO4 in my case) barely touches 40% depth of discharge even in January.

The mistake most people make is sizing batteries around worst-case continuous heating demand rather than thermal mass — if your walls hold heat reasonably well, you only need to replace what's lost at the margins, not maintain temperature from scratch every hour.

@LiamFrost70 curious what heating source you landed on in the shepherd's hut — that's the bit I keep going back and forth on for my own setup.

I've got a small cabin and I'm wrestling with whether a diesel air heater (the Webasto-style Chinese ones) is worth it over a small wood burner. The diesel option obviously pulls from the battery but I can't figure out if the parasitic draw is significant enough to matter when I'm already running a Fogstar 200Ah lithium bank.

Has anyone actually metered the overnight draw on one of those Chinese diesel heaters? I've seen wildly different numbers quoted — anywhere from 10Ah to 40Ah per night depending on settings, which is quite a range when you're trying to plan your winter battery sizing properly.