How Long Does a Bag of Logs Last? Burn Time Calculator

Put in the weight of the bag you buy, your stove's rated output and how long you burn, and this works out how many kilograms of logs an hour your stove gets through, how many evenings a bag lasts and how many bags you need a month. The figure that surprises people: a 5kW stove running at its rated output gets through about 1.6kg of dry logs an hour, so a 10kg net is gone in roughly six hours. Wet logs go faster still, because some of the heat in every log goes into boiling off its water.

How the calculator works

A stove's rated output is the heat it puts into the room. To deliver that heat it has to burn more energy than it gives out, because some goes up the flue. So the wood burned each hour is the heat you get, divided by the stove's efficiency, divided by the energy in a kilogram of your logs.

The energy in a kilogram of logs depends mostly on how wet they are, not on the species. Bone-dry wood of almost any kind holds a similar amount of energy per kilogram; what changes is how much of each kilogram is water, and that water has to be boiled off before the wood gives you any heat. We use the standard net calorific value method: about 18.7MJ per kilogram of oven-dry wood, scaled down for the water content, minus the energy needed to evaporate that water. At 20% moisture that comes to roughly 4kWh per kilogram; at 45% it falls to about 2.6kWh.

Two things the sums cannot see. First, stoves burn less efficiently when they are throttled right down, so a "gently" figure may be a little optimistic. Second, bag weights differ between sellers and a net of wet logs weighs more than a net of dry ones while holding less heat. That is why kiln-dried logs often work out cheaper per hour of warmth than a heavier, cheaper bag of damp wood.

Making a bag last longer

• Burn dry wood. Logs under 20% moisture give far more heat per kilogram, and the stove glass and flue stay cleaner.
• Refuel little and often. Two or three logs on a hot bed of embers burn cleaner than a firebox packed full.
• Do not oversize the fire. If the room gets too hot and you open a window, you are paying for logs to heat the garden.
• Store logs off the ground under cover. Dry logs left in the rain can pick moisture back up.