About this calculator
How much pellets, firewood, gas or oil will the house burn in a year? The calculator starts from the building's annual heat demand, divides it by the appliance's efficiency (or a heat pump's SCOP) and then by the fuel's net calorific value.
The heat demand comes from an energy performance certificate, a heat loss calculation or past bills. If you only know the floor area, enter it with your own estimate of the specific demand in kWh per m² a year.
No prices are built in — fuel prices change too often. For the yearly cost, multiply the quantity by your own price: tonnes of pellets × price per tonne, stacked m³ × price per stacked m³, m³ of gas or kWh of electricity × your tariff.
Formula
Q = annual heat demand, kWh
(or area × specific demand)
E = Q / η energy to buy, kWh
Q / SCOP heat pump
Q direct electric
fuel = E / NCV
Default fuel properties (all editable):
| Fuel | Net calorific value | Source |
|---|---|---|
| Wood pellets | 4.6 kWh/kg | EN ISO 17225-2 class A1 minimum (16.5 MJ/kg) |
| Natural gas | 10.19 kWh/m³ | Amber Grid, monthly average, March 2025 (0 °C) |
| LPG | 12.87 kWh/kg, 0.53 kg/l | Wood Fuels Handbook, sec. 2.10 |
| Heating oil (extra light) | 10.0 kWh/l | Wood Fuels Handbook, sec. 2.10 |
Firewood depends on species and moisture (Wood Fuels Handbook, AIEL, 2008):
NCV_M = (NCV₀ × (100 − M) − 2.44 × M) / 100 MJ/kg
ρ_M = ρ₀ / 1.135 / (1 − M/100) kg per solid m³
solid m³ = mass / ρ_M, stacked m³ = solid m³ / k
M— moisture, % of the wood's weight as burned.NCV₀— 18.5 MJ/kg for dry broadleaved wood, 2 % more (18.87) for spruce and pine.ρ₀— oven-dry density (ÖNORM B 3012): birch 640, alder 490, aspen 450, ash 670, oak 670, beech 680, hornbeam 750, spruce 430, pine 510 kg/m³.1.135— the handbook's reference 13.5 % volume swelling of wood from dry to fully wet; the wood is taken at its swollen volume.k— solid wood in a stacked m³ (0.7 by default, as in the firewood calculator).
Checked against the handbook's own table of energy per stacked m³ of 33 cm logs — using the handbook's own stacking factors for such logs, k = 1/1.61 = 0.62 for beech and 1/1.55 = 0.65 for spruce — this model gives beech at 15 % moisture 1868 kWh (table: 1888) and spruce 1252 kWh (table: 1320). With the calculator's default k = 0.7 the same model gives 2105 kWh (beech) and 1358 kWh (spruce) per stacked m³.
Worked example
A house needs 15 000 kWh of heat a year; the boiler's seasonal efficiency is 85 %:
E = 15 000 / 0.85 = 17 647 kWh
pellets: 17 647 / 4.6 = 3836 kg → 256 bags of 15 kg
gas: 17 647 / 10.19 = 1732 m³
oil: 17 647 / 10.0 = 1765 l
birch, 20 % moisture:
NCV = (18.5 × 80 − 2.44 × 20) / 100 = 14.31 MJ/kg = 3.98 kWh/kg
mass = 17 647 / 3.98 = 4439 kg
ρ = 640 / 1.135 / 0.8 = 705 kg per solid m³ → 6.30 solid m³
stacked = 6.30 / 0.7 = 9.0 stacked m³
heat pump, SCOP 3.2: 15 000 / 3.2 = 4688 kWh of electricity
FAQ
What efficiency should I enter? The seasonal efficiency from the appliance's data sheet or energy label. The default 85 % is only an example figure. A modern pellet or condensing gas boiler does better; an old solid-fuel boiler or a stove can be well below. Only a condensing gas boiler can exceed 100 %, because the efficiency is counted against the net calorific value — at most by the ratio of gross to net calorific value, about 1.11 for Lithuanian natural gas (Amber Grid: 11.29 / 10.19 kWh/m³), so the gas input stops at 110 %. For the other fuels the input stops at 100 %.
Why does moisture matter so much? Water in the wood has to be evaporated and that heat is lost: birch at 20 % moisture gives 3.98 kWh/kg, at 40 % only 2.81 kWh/kg. Wet wood also burns dirtier. Buy dry firewood or let it season, and measure with a moisture meter.
What is a stacked m³? A stacked cubic metre (stere) is a 1 × 1 × 1 m stack of logs with the air gaps between them. How much solid wood it holds depends on log length and how neatly they are stacked — the Wood Fuels Handbook's figures run from about 0.6 to 0.8.
Why is the default SCOP only 3.2? 3.2 is the EU ecodesign minimum for a low-temperature heat pump — a deliberately cautious value. Use your unit's SCOP for your climate and flow temperature: it can be well above 4 with underfloor heating and lower with hot radiators.
Assumptions and limits
- Efficiency is one seasonal figure, entered by you; the calculator does not model part-load, standby or distribution losses separately.
- Fuel properties are typical values: the pellet value is the A1 minimum, the gas value one month's published average.
- Firewood mass per m³ uses a single 13.5 % swelling for all species; against the handbook's table this is within −5 % to +8 %. The stacked-m³ result depends most on the stacking factor you enter.
- No prices and no costs are calculated; multiply the quantity by your own price.
- Domestic hot water is included only if it is part of the heat demand you enter.