Firewood Math

Firewood vs Propane, Oil, Gas and Electric Heat Cost Calculator

By Tam Vu · Updated

On this page
  1. How the comparison works
  2. Worked example
  3. What the calculator leaves out
  4. Sources

Fuel prices are in different units: dollars per cord, per gallon, per therm, per kilowatt-hour. To compare them, convert each to dollars per million BTU of heat that reaches your home. This calculator does that with your prices and your equipment's efficiency, because prices vary by place and season and this site doesn't give a market price.

Cost of heat by fuel

Other fuels (fill in the ones you use)

The 70% wood stove default is a mid-range figure for improved non-catalytic stoves (the U.S. Department of Energy gives 65–75%). Use your stove's own rating if you have it. Not for open fireplaces. Assumes seasoned wood at 20% moisture or less.

How the comparison works

  1. Heat in the fuel. The U.S. Energy Information Administration (EIA) lists the heat in each unit: propane 91,452 BTU per gallon, heating oil 138,500 BTU per gallon, natural gas 100,000 BTU per therm and electricity 3,412 BTU per kWh. Firewood is the USU figure in million BTU per cord for the species you pick (see the BTU chart).
  2. Cost per million BTU in the fuel = price ÷ (million BTU per unit).
  3. Cost per million BTU of useful heat = step 2 ÷ equipment efficiency. A less efficient stove or furnace sends more of the fuel's heat up the flue, so the same fuel costs more per unit of heat delivered.

Worked example

These prices are made up to show the arithmetic. They are not market prices. Red oak is 24.6 million BTU per cord.

Fuel Price Million BTU per unit Cost per million BTU in the fuel Efficiency Cost per million useful BTU
Firewood (red oak) $300 per cord 24.6 $12.20 70% $17.42
Natural gas $1.50 per therm 0.1 $15.00 90% $16.67
Heating oil $4.00 per gallon 0.1385 $28.88 80% $36.10
Propane $3.00 per gallon 0.091452 $32.80 80% $41.01
Electricity $0.16 per kWh 0.003412 $46.89 100% $46.89

Check the propane row: $3.00 ÷ 0.091452 = $32.80 per million BTU in the fuel, and $32.80 ÷ 0.80 = $41.01 per million useful BTU. Check the wood row: $300 ÷ 24.6 = $12.20, and $12.20 ÷ 0.70 = $17.42.

With these made-up prices firewood and natural gas are close, and firewood costs less than half of propane per unit of useful heat. With other prices, other equipment or another species the order can change, which is why the calculator takes your own numbers.

What the calculator leaves out

The comparison covers fuel cost only. It does not include the cost of your time to cut, stack and load wood, equipment or chimney costs, or any difference in how evenly each system heats. It also assumes seasoned wood. Wet wood delivers less heat than the USU figure, so see the seasoning guide. To work out how many cords would replace last winter's fuel, use how much firewood do I need.

Sources

  • U.S. Energy Information Administration, British thermal units (Btu): heat content per gallon of propane and heating oil, per therm of natural gas, and per kWh.
  • Utah State University Extension, Wood Heating, Table 1: million BTU per cord by species.
  • U.S. Department of Energy, Wood and Pellet Heating: stove efficiency 65–75% for improved non-catalytic stoves; 20% moisture standard.