Firewood is not bought by the winter: it is bought by the kilowatt-hour

«How much wood for one winter?» is the wrong question, and whoever sells wood knows it: the honest answer depends on four numbers the winter does not contain. The stove does not burn steres: it burns kilowatt-hours — the ones the house loses, multiplied by the share you ask of the wood, divided by the firebox efficiency. The stere, the quintal and the pallet are only the number's clothing. This calculator does the count in the right order: from the home's energy class to the useful kilowatt-hours, from the kilowatt-hours to the true quintals — with the rule that decides everything and that no price list prints: the calorific value belongs to the water that is not there.

The formula: area, class, share and efficiency

The model is a multiplication with an honest divisor. The useful energy is E_u = A·f·w/100: the area times the annual demand of the energy class, times the share the wood covers. The efficiency η turns useful energy into energy demanded at the wood: E_l = E_u/(η/100) — and this is where the open fireplace loses the match. The mass follows from the calorific value: 4.0 kWh/kg for hardwood seasoned two summers at 15-20% moisture. The outputs check each other: quintals per year, kilograms per season day (a conventional 180 days), and the two energy balances — useful and at the wood.

The scale of the energy class: the house commands, not the stove

The table runs the heat demand over the base case (80 m², wood share 70%, efficiency 75%):

The same stove in the same 80 m² goes from 9 to 47 quintals without anything changing in the flue: the envelope changed. The high-class home burns a pallet where the sieve burns a woodshed — which is why the demand f is an input and not a hidden constant: the right number sits in the energy certificate, or is estimated from historic consumption.

A complete numerical example

Typical home: 80 heated m², mid-class demand (120 kWh/m²·year), a stove covering 70% of the heating at 75% efficiency. The useful energy asked of the wood is 80 × 120 × 0.70 = 6,720 kWh; the efficiency turns it into 8,960 kWh demanded at the wood; at 4 kWh/kg that is 2,240 kg: 22.4 quintals a year, that is 12.4 kg per season day — two-three full loads. It is the «stove works» regime: the wood cuts a real slice of the bill, and the stock is bought in summer.

How to read the demand

The four bands tell four relationships with wood. Below 10 quintals it is the light supplement: the evenings, the mid-seasons — quality counts more than quantity. Between 10 and 25 the stove works: summer stock, ventilated space, loading rhythm. Between 25 and 45 it is true wood heating: the woodshed is a small warehouse with its rotations. Above 45 the volumes are logistics — and the number should be interrogated before organising it: such a high demand usually speaks of the envelope, and insulating yields more than stacking.

The moisture: the calorific value belongs to the water that is not there

The 4.0 kWh/kg number does not belong to the wood: it belongs to dry wood. Freshly cut wood is water for half its weight, and every kilo of water in the firebox drinks energy to evaporate instead of giving it: green wood yields half, dirties the glass and fattens the flue with creosote — the fuel of chimney fires. Proper seasoning is physics, not patience: two summers under a ventilated roof, stacks lifted off the ground, split pieces (bark holds water in). The few-euro moisture meter is the only instrument that certifies the number: below 20% wood is wood; above, it is water wearing wood's clothes.

The stere, the quintal and the cord: the units that confuse

Wood is sold in three languages. The quintal (100 kg) is the honest unit: weight, which with known moisture becomes energy. The stere is a cubic metre of stack, air included: it weighs 350-450 kg by species and stacking — the beech stere and the poplar stere are two different fuels with the same name. The commercial pallet is typically a generous stere of pressed logs. The practical conversion: a demand in quintals divided by 4 gives roughly the steres of hardwood. And the purchase rule: by weight with declared moisture beats any volume — because air and water do not heat.

International perspective: cords, steres and the same multiplication

Wood is universal, the units are not. North America measures in cords (3.62 stacked m³, ~2 tonnes of dry hardwood ≈ 20 quintals); Europe in steres and quintals; everywhere the heat demand is written in kWh/m² and the efficiency in percent. The multiplication underneath is identical — area times class times share, divided by efficiency and calorific value — which is why the calculator works in kilowatt-hours and converts at the end: the energy number is the lingua franca, quintals and cords are the dialects.

Limits of the model and correct use

The model photographs the average seasonal demand: it does not model the harsh or mild winter (±15-20% year to year — the stock is sized on the full winter, not the average), nor the domestic hot water if the stove produces it, nor thermo-stoves with buffer tanks that shift the efficiencies. The demand f is taken from the certificate or estimated with judgement: to compute it from the real losses the heat load calculator does the upstream count; and if the number comes out in the high band, the insulation cost calculator says what it costs to stop buying quintals — because the cheapest wood is the wood you do not need.

The scale of the share: from the evening fire to the main heating

The share w says what role the stove plays in the house. The table runs it over the base case (80 m² at 120 kWh/m², efficiency 75%): the category jump arrives at 90%, where the wood stops helping and starts commanding.

From the evening fire to the wood-only home run 26 quintals — but the true difference is not the quantity: it is the responsibility. Above 90% the stove has no plan B covering it: the breakdown, the flu, the week away become heating problems — which is why the verdict, from there up, asks for the plan B and the full stock by September.

The scale of the efficiency: the open fireplace eats the forest

The efficiency η is the count's silent divisor. The table runs it over the base case (useful energy fixed: 6,720 kWh):

The useful-energy column does not move — the house always asks the same kilowatt-hours — but the wood does: the 40% open fireplace burns double the modern stove for the same warmth. It is the single most profitable number of wood heating: going from 40 to 75 efficiency is worth twenty quintals a year in the base case — the insert or the stove pays for itself in a few seasons, at current wood prices.

The woodshed: the warehouse that decides the quality

The calculated quintals go somewhere, and the where decides the how they will burn. The right woodshed is covered on top and open at the sides: the roof stops the rain, the wind does the drying work. The stacks sit lifted off the ground — pallets or bearers — because rising damp undoes two summers of seasoning; split pieces expose the live wood, and the bark goes up, playing roof tile. The rotation is a warehouse's: burn the old stack, season the new — and the calculator's quintals, divided by 4, say how many metres of stack the roof must hold.

The economic balance: wood is compared in kilowatt-hours

Whether wood pays is decided in the calculator's own unit. The useful wood kilowatt-hour costs the quintal price divided by 400 and by the efficiency: as an example, a quintal at 20 with a 75% stove gives useful kilowatt-hours at about 6.7 hundredths of that unit — to be compared with the useful kWh of gas, pellets or a heat pump at current tariffs. The structural rule does not change with the market: wood wins where it costs little and the labour of managing it is not counted; it loses where time is expensive and storage is missing. The calculator gives the kilowatt-hours: the prices, the day's market.

The flue: the count's silent partner

No demand burns well without the right draught. The sized flue — section and height for the stove's power — decides the combustion as much as the wood; the air intake is the forgotten half: the stove that cannot breathe smothers and makes creosote even with perfect wood; the yearly sweep is the cheapest insurance of the house — the chimney fire is born in the dirty duct, not in the firebox. Whoever moves to main-heating volumes should have the flue checked before ordering the quintals: the bottleneck cannot be stacked.

From the count to the season: the firewood calendar

The calculated demand is spent well with the right calendar. Summer: buy — low prices, wide choice, seasoning that finishes at home. September: the stock is in the shed, all of it; winter does not wait for deliveries. Season: burn the oldest stack, with the moisture meter as referee on doubtful loads. Spring: flue sweep and end-of-season count — the quintals truly burnt, compared with the calculation, tune next year's number better than any estimate. The demand is a model; your home's true consumption, measured over one season, is the definitive number.

Frequently asked questions

How much firewood does it take to heat a home?

It is computed in kilowatt-hours, not in winters: E = area × energy-class demand × share covered by wood, divided by the stove efficiency and the calorific value (4 kWh/kg dry). For 80 m² in a mid class with a 75% stove covering 70%: about 22 quintals a year.

How many kilowatt-hours does a kilo of wood give?

About 4.0 kWh if seasoned two summers at 15-20% moisture (hardwood). Green wood gives about half: the water evaporates drinking energy. That is why the number to ask the supplier is not the price but the moisture — and a few-euro meter certifies it.

How much does a stere of wood weigh?

350-450 kg by species and stacking: the stere is a cubic metre of stack, air included. Beech and poplar in the same stere are different fuels. Practical rule: quintals divided by 4 ≈ steres of hardwood; and buying by weight with declared moisture beats any volume.

Where do I find my home's heat demand?

In the energy performance certificate (the kWh/m²·year number) or from historic consumption: the annual heating gas cubic metres × 9.6 divided by the m² give an honest estimate. New homes sit at 40-60, average ones at 100-150, uninsulated buildings above 200.

What efficiency does a wood stove have?

The open fireplace stops at 30-50% (it mostly heats the flue), modern stoves sit at 75-85%, inserts and thermo-stoves in between. For the same house, moving from an open fireplace to a modern stove nearly halves the quintals: the single most profitable upgrade of wood heating.

How much wood do I burn per day?

The calculator divides the annual demand by a conventional 180 season days: in the base case 12.4 kg a day — two-three full loads. Cold snaps go above the average, mid-seasons below: the number serves logistics, not the thermostat.

When is the best time to buy firewood?

In summer: it costs less, the choice is wide and it arrives in time to finish seasoning under your roof. Buying in October means paying more and burning someone else's wood of unknown seasoning — the classic double mistake of the first winter.

How do you season firewood?

Two summers under a ventilated roof: stacks lifted off the ground, exposed to wind, covered on top but open at the sides; split pieces, because bark holds water in. The goal is below 20% moisture: from there the wood gives its 4 kWh/kg and the glass stays clean.

Hardwood or softwood?

At equal dry weight the energy is similar; at equal volume the hard species (beech, oak, hornbeam) contain much more and burn longer. Resinous woods light quickly but dirty the flue more: good for starting, less for the regime. The calculator's demand is in weight: it holds for both.

Does the calculator work for pellets?

The logic yes, the numbers no: pellets sit at ~4.8 kWh/kg with 85-90% efficiencies. As a quick rule, the demand in wood quintals × 0.75 gives an estimate of the equivalent pellet quintals. The fine count deserves its own parameters.

What is creosote and why does it matter?

The condensate of damp or smothered combustion: tar that lines the flue and is the fuel of chimney fires. It is prevented with dry wood, proper draught and yearly flue cleaning — the maintenance that is never skipped, whatever the demand.

What does the calculated demand NOT tell you?

It does not model the harsh winter (±15-20% year to year: the stock is sized on the full winter), it does not cover hot water if the stove produces it, it does not size the stove in kW (that is the heat load) and it remains an average: it serves buying and storing, not lighting.

Updated July 2026