Pimithi
Norsk

Firewood Energy Calculator

How many kWh of heat you get from firewood — by sacks or weight, moisture content and stove efficiency — and what the same heat costs as electricity.

A 40 L sack of dry birch weighs about 17 kg, pine 13 kg and spruce 11 kg.

Share of water in the wood as sold (wet basis). Dry firewood is 20% or less.

Modern clean-burning stove 75–80%, older stove 50–60%, open fireplace about 15%.

To compare: what the same heat would cost from electric heaters.

Uses typical calorific values for wood; real results vary with species, how dry the wood really is and how you fire the stove. Burn only dry wood (at most 20 % moisture) — wet wood gives less heat and more soot and smoke.

About this tool

Enter how much firewood you have (number of sacks and the weight of one sack, or the total weight), its moisture content and the efficiency of your stove. The calculator finds the net calorific value of the wood from its moisture (19 MJ per kg of oven-dry wood, minus the water and the energy needed to evaporate it), the total energy content, the useful heat that reaches the room, and what the same heat would cost from direct electric heating.

Frequently asked questions

About 4.1 kWh per kg for air-dry logs at 20% moisture, and about 5.3 kWh per kg for oven-dry wood (Forest Research typical net calorific values). Per kg, all species give about the same; dense hardwoods like birch, oak and beech simply pack more kg into each sack.

Water adds weight but no energy, and boiling it off uses energy. Net value = 19 × (1 − w) − 2.44 × w MJ/kg: at 20% moisture that is 4.09 kWh/kg, at 40% only 2.90 kWh/kg — about 29% less heat, plus more soot and smoke.

Around 75–80% for a modern clean-burning stove (SINTEF gives a typical total efficiency of 80%), 50–60% for an older stove and only about 15% for an open fireplace, where most of the heat goes up the chimney.

It depends on sack size and species. Norsk Ved lists a 40-litre sack of dry birch (≤ 20% moisture) at about 17 kg, pine at 13 kg and spruce at 11 kg; a 60-litre birch sack is about 22 kg. Weigh one sack on a bathroom scale if you are unsure.

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