At a Glance
- Work out the room’s volume in cubic metres, divide by 14 for average insulation, 25 for very good and 10 for poor, and that’s roughly your kW.
- Most cabins, shepherd’s huts and narrowboats come out at 3kW to 5kW. Very few small spaces need more.
- Oversizing is the common mistake. A stove run low to avoid overheating burns dirty and tars the flue.
- In England, a stove can need a permanent air vent even below 5kW in a very airtight dwelling; new UK stoves must meet Ecodesign standards.
If you’re heating a cabin, a shepherd’s hut or a boat with wood, the first question is always the same: how many kW log burner do you need? The answer is a method rather than a number, and that method only takes five minutes.
This article is for small fixed or floating homes. Vans are different enough to get their own piece, a wood burner in a van, and heating water from your stove is covered separately too.
The right size depends on the space, the insulation and the climate. Here’s how to work through each.
Start With Room Volume
Measure the length, width and height of the space in metres and multiply them together. That’s the volume in cubic metres.
Now divide by 14. That gives a rough kW figure for a space with average insulation. If the insulation is very good, divide by 25 instead. If it’s poor or there’s none, divide by 10.
A 6m by 3m hut with a 2.4m ceiling is about 43 cubic metres. Divided by 14 that’s roughly 3kW. Divided by 25 for a well-insulated hut it’s under 2kW. Divided by 10 for a draughty one it’s just over 4kW.
The different results show why insulation matters when you choose a stove.
Why Insulation Changes Everything
A stove doesn’t heat a room so much as replace the heat the room loses. A well-insulated cabin in daily use loses heat slowly, so a small stove keeps up.
An uninsulated shed or a steel narrowboat hull in cold water loses it fast, so the same stove struggles.
Think of the room as a bucket with holes in it. The stove is the tap. Insulation plugs the holes, and the better you plug them the smaller the tap you need.
If you haven’t insulated yet, it’s nearly always cheaper to do that than to buy a bigger stove and feed it. There’s a guide to insulation options for your off-grid home in the Reading Room.
Improve the insulation before deciding whether you need a larger stove.
Why Bigger Is Worse
The mistake almost everyone makes in a small space is buying a stove that’s too big, on the grounds that you can always turn it down.
You can’t, not properly. A wood burner wants to run hot. Choke the air to keep a 6kW stove from roasting a hut and the fire smoulders, the glass blackens, and tar and creosote build up in the flue until it becomes a chimney fire waiting to happen.
You’ll also burn more wood, not less, because a smouldering fire wastes most of it.
A small stove run hot and bright is cleaner, cheaper and safer than a big one run low.
If the sums say 3kW, buy a 3kW to 4kW stove, not the 5kW that looks reassuring.
How Many kW You Need
The calculation above gives you an estimate for your own space. If you only need a rough starting point, the typical ranges below can help, but they still need to be interpreted in light of insulation and layout.
- A small insulated cabin with one or two rooms usually lands between 3kW and 5kW. Larger open-plan cabins with a mezzanine can climb higher, and a cabin with several separate rooms may need a second heat source or a safely designed way to duct warmth to other rooms, rather than one oversized stove.
- A shepherd’s hut is small and often well built, so 2kW to 4kW is the common range. A wood burning stove for a shepherd’s hut is nearly always at the tiny end of what’s made.
- A narrowboat is long and thin with a cold steel hull, so a narrowboat stove of 4kW to 5kW is typical for a full-length boat, with the heat drifting down the cabin from one end.
These ranges are only a starting point, so do the calculation for your own space before choosing a stove.
Rules That Affect Size
Two rules bear on the kW figure in the UK, and both are worth knowing before you buy.
In England, Approved Document J’s permanent-vent guidance depends on both the stove’s output and the dwelling’s airtightness. A stove over 5kW generally needs a vent, but a very airtight cabin can need one even below 5kW. Have a competent installer or Building Control check the requirements for your installation.
The guidance is on GOV.UK. Wales, Scotland and Northern Ireland have their own versions, so check yours. A stove below 5kW still needs a ventilation check.
Across the UK, any new stove sold since 1 January 2022 has to meet Ecodesign limits on emissions and efficiency.
An Ecodesign stove burns cleaner and squeezes more heat from the same log, which is one more reason a small modern stove outperforms a big old one.
On a boat, the installation will be looked at under the Boat Safety Scheme, and the Scheme’s solid fuel stove advice covers flue, clearances and alarms.
Outside the UK, the sizing method still works, but check the installation rules in your own area, because they vary.
The regulations affect how a stove must be installed, so check them before settling on a size.
Final Thoughts
Measure the space, be honest about the insulation, divide, and then resist the urge to round up.
For most cabins, huts and boats, a stove between 3kW and 5kW will be plenty. Run at the right temperature, a smaller stove will generally keep you warmer than an oversized one that is constantly throttled back.
If you would rather have someone fit it, choose an installer with experience of the kind of home you are heating.
To find out more about off-grid living return to our Reading Room.