From heat demand to bags
Wood pellets are compressed sawdust, sold in bags of about 40 lb in North America, or delivered in bulk to a storage bin. To budget for a season you need three numbers: how much heat your home needs, how efficiently your stove or furnace turns pellets into heat, and how much energy each pound contains.
The calculation
- Pellets needed = heat demand ÷ (efficiency × energy content)
- Bags = pellets ÷ bag weight, rounded up
- Cost = bags × price per bag
- Cost per million BTU = cost ÷ heat demand in MMBtu
- Bulk storage volume = pellet weight ÷ about 650 kg/m³ (about 40 lb per cubic foot)
Example
A home needs 40 million BTU of heat over a winter. The pellet stove is 80% efficient and the pellets contain 8,000 BTU per pound:
- Pellets: 40,000,000 ÷ (0.80 × 8,000) = 6,250 lb, about 3.1 tons
- Bags: 6,250 ÷ 40 = 156.25, so 157 bags
- Cost at $7 a bag: $1,099
- Cost per million BTU of useful heat: $27.48
- Storage in bulk: about 154 cubic feet
Estimating the heat demand
The easiest way is from last winter’s bills. Multiply the fuel you used by its energy and the efficiency of the old system:
- Natural gas: 1 therm = 0.1 MMBtu
- Heating oil: 1 gallon ≈ 0.139 MMBtu
- Propane: 1 gallon ≈ 0.0915 MMBtu
- Electricity: 1 kWh = 0.003412 MMBtu (resistance heating is 100% efficient)
For example, 600 gallons of heating oil burned at 80% efficiency is about 66.7 MMBtu of useful heat.
Comparing fuels
The cost per million BTU of useful heat lets you compare pellets with other fuels. Pellet prices vary by region and season, and buying a full pallet in spring is usually cheaper than buying bags in January.
Choosing pellets
Look for pellets certified under the PFI or ENplus standards, with low ash and moisture. Premium pellets burn hotter and leave less ash, so the stove needs less cleaning. Keep bags dry and off concrete floors: wet pellets swell and can jam the auger.