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Print at Dec 15, 2025, 9:14:03 PM

Posted by okh at Jan 15, 2016, 11:21:20 AM

Re: Off grid solutions
A crisp -18°C outside and time to reassess some energy investements.

Extra house insulation a while back, brought heating costs down by some 20%.
New electric floor heating in basement (up to 120W/m³) keeps house relatively temperature stable. (An old house in a cold climate could require as much as 200W/m³, modern houses should be able to manage with 1/4th or less).

Firewood is blazing of in a relatively new stove (installation cost around €4000) delivering 2500-11000 W. No reliance on the grid, except for frost protection. Cost is interesting, and it is well worth comparing firewood quality and prices. Dry firewood will deliver approximately the same kWh pr kg regardless of whether it is heavy or light wood. The variables are the cost, moisture content and not least, stove efficiency.

The long and short of it is that a good stove with good firewood will deliver up to 4 kWh pr kg of firewood. But a quick test revealed that the actual cost pr kWh here ranged from €0,1 to €0,2 depending on the firewood quality and price. Seemingly similar sacks of firewood vary greatly in sack weight, dampness and price. So much so that the price/kWh more than doubles from one retailer to another. That was surprising.

ok


                                Stove efficiency
                                Open fireplace    Modern stove
Wood moisture        kWh/kg      15%    50%    80%    85%    90%
0% (does not exist)     5,3     (0,8)  (2,7)  (4,3)  (4,5)  (4,8)
Very dry wood 10%       4,7      0,7    2,4    3,8    4,0    4,2
Dry wood 15%            4,4      0,7    2,2    3,5    3,8    4,0
Relatively dry 20%      4,1      0,6    2,1    3,3    3,5    3,7
Damp firewood 25%       3,8      0,6    1,9    3,1    3,2    3,4

Normal firewood will have a moisture content between 10% and 20%. Moisture over 20% is not acceptable. Getting good efficiency from a stove (85%), requires a modern construction and knowing how to use it.

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Dec 15, 2025, 9:14:03 PM