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Firewood energy and cost calculator

Firewood energy depends almost entirely on moisture content. Wet wood spends its own energy boiling off water.

kg/m3

Stacked hardwood is 350 to 500 kg/m3 dry; softwood nearer 250.

%
%
per kWh
Usable heat
3678kWh
= Exact Firewood energy
Cost per usable kWh
0.0680
Energy in the wood
4904 kWh
Mass of the load
1200
Energy lost boiling off water
586 MJ
Cost of the same heat from gas
257.46
Saving against gas
7.46
Show the working
  1. Volume of wood = 3
  2. Stacked density = 400
  3. Moisture content = 20
  4. Stove efficiency = 75
  5. Price for the load = 250
  6. Gas price to compare = 0.07
  7. Mass of the load = 3 * 400 = 1200
  8. dry_mass = 1200 * (1 - 20/100) = 960
  9. gross_mj = 960 * 19 = 18 240
  10. Energy lost boiling off water = 1200 * 20/100 * 2.44 = 585.6
  11. net_mj = 18 240 - 585.6 = 17 654.4
  12. Energy in the wood = 17 654.4 / 3.6 = 4904
  13. Usable heat = 4904 * 75/100 = 3678
  14. Cost per usable kWh = 3678 > 0 ? 250 / 3678 : 0 = 0.067 971 7
  15. Cost of the same heat from gas = 3678 * 0.07 = 257.46
  16. Saving against gas = 257.46 - 250 = 7.46

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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    The formula

    • dry wood holds about 19 MJ per kg
    • evaporating the water costs 2.44 MJ per kg of water
    • usable heat = net energy x stove efficiency

    Wood at 50% moisture delivers barely half the usable heat of the same wood at 20%, and tars up the flue doing it.

    Season firewood for at least a year, split and under cover, and check it with a moisture meter.

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