How many bags of pellets or cords of wood you need for the winter

How to work out pellet and firewood use

Start from the heat the stove has to give: rated output × hours burning × average load. A 12,000 W stove (about 41,000 BTU/h) burning 8 hours a day at half output gives 12 × 8 × 0.5 = 48 kWh of heat a day. To make that heat it burns more fuel, because part of the heat goes up the flue: divide by the stove's efficiency, then by the kWh in a kilogram of fuel.

The starting values come from official sources and can be changed in the advanced options. ENplus A1 pellets must hold at least 4.6 kWh/kg, about 7,100 BTU per pound; the calculator uses that minimum so you don't run short, and you can enter the figure printed on your bag. Efficiency starts from the German legal minimums: 85% for a pellet stove, 73% for a wood stove. The EPA notes that ads often quote combustion efficiency and that overall efficiency may be 10% to 20% lower, so use the figure from your stove's spec sheet.

For firewood, water is what matters. The Bavarian forestry institute LWF gives 5.00 kWh/kg for oven-dry hardwood and 5.20 for softwood, minus 0.68 kWh for every kilogram of water that has to boil off. A moisture meter reads on a dry basis: 20% on the meter means 16.7% of the weight is water, and oak then gives 4.05 kWh/kg. From the density of each species (oak 571, beech 558, pine 431, spruce 379 kg of dry wood per solid m³) and 0.7 m³ of solid wood in a stacked cubic metre, the calculator also works out the size of the woodpile.

If you have the spec sheet, you can start from the burn rate in pounds per hour at rated output instead: the calculator multiplies it by the hours and by the average load.

A worked example

In the US. The calculator opens on a 12,000 W pellet stove burning 8 hours a day at half output for 180 days, about six months.

• Heat per day: 12,000 × 8 × 0.5 = 48 kWh
• Energy in the pellets: 48 ÷ 0.85 = 56.5 kWh a day, 12.3 kg or 27 lb of pellets
• For the season: 4,872 lb, 121.8 bags of 40 lb: 122 bags, about 2.4 tons, two pallets of 50 bags and 22 more
• Volume: about 120 ft³ of pellets to keep dry

With seasoned oak (20% on the meter) in a wood stove of the same output and the same hours you need 6,437 lb of wood, 1.7 cords, so 2 cords. A cord of oak at that moisture weighs about 3,833 lb. At an example price of 300 per cord, oak costs 0.078 per pound and a kWh of heat 0.058.

In the UK, in metres. An 8 kW pellet stove with the same hours: 32 kWh of heat a day, 1,473 kg of pellets for the season, 99 bags of 15 kg and 2.3 m³ of pellets. With seasoned beech it takes 1,947 kg, 4.2 stacked cubic metres, so 5 m³ of stacked logs.

How much a 27,000 to 51,000 BTU/h stove burns

From the calculator, 8 hours a day at half output for 180 days, oak at 20% on the meter.

Stove output	Pellets	40 lb bags	Oak
8,000 W (27,300 BTU/h)	3,248 lb	82	1.1 cords
10,000 W (34,100 BTU/h)	4,060 lb	102	1.4 cords
12,000 W (40,900 BTU/h)	4,872 lb	122	1.7 cords
15,000 W (51,200 BTU/h)	6,089 lb	153	2.1 cords

At the same moisture a pound of any wood gives almost the same heat; what changes is how much a cord weighs, and so how much heat it holds.

Wood (20% on the meter)	Weight of a cord	Heat in a cord
Oak	3,833 lb	24.0 million BTU
Beech	3,745 lb	23.5 million BTU
Pine	2,893 lb	18.9 million BTU
Spruce	2,544 lb	16.6 million BTU

Bags, pallets, cords and face cords

In the US pellets come in 40 lb bags, and a ton is sold as a pallet of fifty bags. A pallet holds about 1.4 m³, roughly 49 ft³, of pellets: keep it off the floor and away from damp. In the UK and Europe bags are 15 kg, and a pallet holds for example 70 of them; change the bag size and the count in the advanced options.

Firewood is sold by the cord: wood stacked tight in a space of 128 cubic feet, 4 ft high, 4 ft deep and 8 ft long. A "face cord" is a stack 4 ft high and 8 ft long but only one log deep; with 16 in logs it is a third of a cord. In Washington state, for example, sellers may not use the term, so ask how many cubic feet you are getting. In the UK logs are sold by the cubic metre; ask whether the volume is stacked or loose, because a loose load holds less wood.

Common mistakes

• Leaving out the load. A stove rarely runs at its rated output all day: at full output for the same hours it would burn twice the half-load estimate.
• Reading the moisture meter as water content. The meter reads on a dry basis: 25% on the meter means 20% of the weight is water.
• Buying green wood by weight. At 50% on the meter beech gives 3.11 kWh/kg instead of 4.05: you pay for water and need about 30% more wood.
• Comparing a face cord with a cord. Three face cords of 16 in logs make one cord; a face cord at a third of the price is not a bargain.
• Using the advertised efficiency. Combustion efficiency is the higher number; the heat that reaches the room is less.