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Energy, electricity and mechanics

Firewood energy

Calorific value and useful energy of firewood based on its moisture content and stove efficiency.

kg
%

Moisture on a wet basis (percentage of total weight that is water). Freshly cut wood: 40-60 %. Well seasoned wood (over a year under cover): 15-20 %.

%

Fraction of the combustion heat you actually use. Open fireplace: 10-30 %. Closed stove: 50-80 %.

Result

Net calorific value
— MJ/kg
Useful energy
— MJ
Useful energy
— kWh
Water it would heat from 15 to 60 °C
— L

How it is calculated

Approximate net calorific value based on moisture on a wet basis (w, from 0 to 1): NCV ≈ 19 × (1 − w) − 2.44 × w MJ/kg. Well seasoned wood (w ≈ 0.15-0.20): 14.5-16 MJ/kg; the reference value for completely dry wood (w = 0) is about 18-19 MJ/kg.

Useful energy: E = mass × NCV × stove efficiency. In kWh: E(kWh) = E(MJ) ÷ 3.6.

Water it would heat from 15 to 60 °C (specific heat of water 4.186 kJ/kg·°C, a 45 °C rise, i.e. 0.18837 MJ per litre): litres = E(MJ) ÷ 0.18837.

Keep in mind

  • Season cut firewood split into pieces, stacked and covered but ventilated, for at least 6-12 months (better 1-2 years) to bring moisture down from an initial 40-60 % to the 15-20 % of seasoned wood.
  • Burning wet firewood not only yields less energy: it also produces far more smoke, creosote (chimney fire risk) and polluting particles.
  • Always ventilate any space where combustion happens: carbon monoxide is odourless and can kill without warning. Never use a stove or brazier without a flue in an enclosed space.

See also

More tools in “Energy, electricity and mechanics”