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

Spent last winter on the narrowboat wrestling with exactly this. The temptation is always to just throw more battery at the problem but honestly it rarely solves the root issue.

What actually made the difference for me was layering the approach rather than relying on one heating solution:

• Small Webasto diesel heater as the backbone (runs off the engine tank, barely touches the electrics)
• Decent insulation job on the roof — genuinely transformative
• 12v electric blanket for the sleeping area overnight instead of running the heater at full chat

The electric blanket thing sounds daft but it pulls maybe 40-50w versus a fan heater which would absolutely destroy a modest bank overnight.

On the battery side I've got a Fogstar Drift 200Ah and it's honestly enough for everything except trying to heat the space electrically. That's just thermodynamics working against you — resistive heat is brutal on capacity.

If you're in a static cabin rather than a boat, have you looked at a small log burner? Zero draw, keeps the space properly warm, and you can get decent ones fairly cheap secondhand. Pair that with a Victron setup for your lights and devices and you've essentially decoupled your heating from your battery system entirely.

What's your current setup like? Might be easier to suggest something specific. Also — how insulated is the cabin? That's usually where the answer actually lives before you start adding kit.

JakeShaw96 | 47 posts

@HazelSoul totally agree that more battery capacity is rarely the answer on its own. What made the biggest difference for me in my little Welsh hillside cabin was sorting the insulation properly first - specifically addressing thermal bridging around the window frames which I'd completely overlooked. Lost embarrassing amounts of heat there.

Also switched to a small wood gasification stove for the bulk heating and only lean on the electric system for top-up warmth and overnight. Means my battery bank stays reasonable rather than needing something ridiculous just to run convection heaters all night.

The other thing worth mentioning - a decent thermal mass inside the cabin (stone floor, brick internal wall) holds daytime solar gains surprisingly well into the evening, which genuinely reduces how hard your system needs to work. Simple but effective.

LindaClark90 | 134 posts

@HazelSoul oh this is SO relevant to me right now — I'm trying to heat my garden office/tiny house hybrid and the battery rabbit hole is real 😅

Curious what your actual heat source ended up being though? I've been going back and forth between a small diesel heater (Webasto type) versus just leaning harder on a well-insulated thermal mass setup with minimal electric top-up.

On the narrowboat specifically — did you find the hull itself was losing heat faster than anything else? I'd imagine insulation is trickier to retrofit there than in a cabin with accessible wall cavities?

My current thinking is that decent PIR board insulation sorted first, then right-size the battery second. Running a Victron SmartShunt so at least I can actually see where the power's going rather than guessing!

MarinePhil | 1,247 posts

@HazelSoul this resonates deeply — spent three winters aboard before converting a Sprinter, and the lesson was always the same: thermal mass and insulation first, then size your system around what's left.

On my narrowboat I ran a Webasto diesel heater alongside a modest 200Ah Fogstar lithium bank. The Webasto sips maybe 7-8W once at temperature — virtually invisible to the batteries. Meanwhile my neighbour was burning through a 400Ah setup trying to run a cheap resistive heater. The maths never worked in his favour.

Key thing people miss: a well-insulated space with passive heat retention needs far less active heating, which means your Victron MPPT and modest solar can actually keep pace even in January.

What's your current insulation situation? That'll likely reveal more than any battery upgrade would.

LochFinn | 312 posts

Great thread. One thing I'd add that nobody's mentioned yet — thermal mass is your friend before you even think about electrics. A small woodburner or even a well-insulated rocket mass heater can do the overnight heavy lifting so your battery system only needs to handle the edges of the day, not the whole 14-hour Scottish winter night.

I run a 400Ah lithium setup in my highland cabin and honestly the biggest wins came from sheep's wool insulation in the roof void and sealing draughts around the door frame. Dropped my overnight draw by nearly 40% before I touched the electrical side at all.

@LindaClark90 a garden office/studio is actually a great candidate for this approach — smaller volume means thermal gains stick around longer if the envelope's decent.

VDH_Boats | 89 posts

@HazelSoul narrowboat here too — this thread is basically my diary from two winters ago.

What transformed things for me wasn't battery capacity at all. It was accepting that heat retention overnight is a separate problem from heat generation. I fitted 25mm Kooltherm under every hatch and along the gunwales, and my Webasto's runtime dropped dramatically. The Victron battery monitor finally started telling me things I actually wanted to hear.

One thing worth flagging for anyone on the water specifically — thermal bridging through the steel hull will quietly undo everything if you're not methodical about it. @LochFinn's point about thermal mass is spot on for land-based cabins, but on a boat that mass works against you until the hull temperature comes up.

My Fogstar 200Ah lithium has never once been the limiting factor. The building envelope always was.