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tl;dr: 1 kWh of heat can be extracted from 0.195/GrossEfficiency(as decimal) kg of dry (18.5% moisture content) firewood.

PPS: I am not trying to suggest that Combustion Efficiency figures are useless. They are useful if what you are primarily concerned about are emissions. And whilst a "complete burn" will release more heat than an "incomplete burn", if your heater needs to draw in twice as much air to do so then you are going to lose a larger fraction of heat through the flue — and your house may actually end up colder. It's called the "Efficiency Paradox". Some modern heaters — with multi-stage burns, convoluted gas paths, and low flue temperatures — defyresolve the paradox and feature very good combustion and gross efficiencies.

tl;dr: 1 kWh of heat can be extracted from 0.195/GrossEfficiency(as decimal) kg of dry (18.5% moisture content) firewood.

PPS: I am not trying to suggest that Combustion Efficiency figures are useless. They are useful if what you are primarily concerned about are emissions. And whilst a "complete burn" will release more heat than an "incomplete burn", if your heater needs to draw in twice as much air to do so then you are going to lose a larger fraction of heat through the flue — and your house may actually end up colder. It's called the "Efficiency Paradox". Some modern heaters — with multi-stage burns, convoluted gas paths, and low flue temperatures — defy the paradox and feature very good combustion and gross efficiencies.

tl;dr: 1 kWh of heat can be extracted from 0.195/GrossEfficiency kg of dry firewood.

PPS: I am not trying to suggest that Combustion Efficiency figures are useless. They are useful if what you are primarily concerned about are emissions. And whilst a "complete burn" will release more heat than an "incomplete burn", if your heater needs to draw in twice as much air to do so then you are going to lose a larger fraction of heat through the flue — and your house may actually end up colder. It's called the "Efficiency Paradox". Some modern heaters — with multi-stage burns, convoluted gas paths, and low flue temperatures — resolve the paradox and feature very good combustion and gross efficiencies.

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PPPS: If, for whatever reason, you don't want to use the EN's 5.14kWh value, you can go to the Phyllis2 database, check the box next to "untreated wood" (or expand the category and select one or more entries as a subset), then click the button labelled "Average of Selected". Results will be shown on the right. Scroll down to the Calorific Values section. Look at the row marked "Net calorific value (LHV)" and the column marked "Mean" (it's the only bold one). That gives you MJ/kg. Divide by 3.6 to get kWh/kg. At the time of writing, the average for all untreated wood was 18.88MJ/kg, so (18.88/3.6=) 5.24kWh/kg. As more and more samples are added to the database, expect the values to change slightlyrefine over time.

PPPS: If, for whatever reason, you don't want to use the EN's 5.14kWh value, you can go to the Phyllis2 database, check the box next to "untreated wood" (or expand the category and select one or more entries as a subset), then click the button labelled "Average of Selected". Results will be shown on the right. Scroll down to the Calorific Values section. Look at the row marked "Net calorific value (LHV)" and the column marked "Mean" (it's the only bold one). That gives you MJ/kg. Divide by 3.6 to get kWh/kg. At the time of writing, the average for all untreated wood was 18.88MJ/kg, so (18.88/3.6=) 5.24kWh/kg. As more and more samples are added to the database, expect the values to change slightly over time.

PPPS: If, for whatever reason, you don't want to use the EN's 5.14kWh value, you can go to the Phyllis2 database, check the box next to "untreated wood" (or expand the category and select one or more entries as a subset), then click the button labelled "Average of Selected". Results will be shown on the right. Scroll down to the Calorific Values section. Look at the row marked "Net calorific value (LHV)" and the column marked "Mean" (it's the only bold one). That gives you MJ/kg. Divide by 3.6 to get kWh/kg. At the time of writing, the average for all untreated wood was 18.88MJ/kg, so (18.88/3.6=) 5.24kWh/kg. As more and more samples are added to the database, expect the values to refine over time.

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PPPS: If, for whatever reason, you don't want to use the EN's 5.14kWh value, you can go to the Phyllis2 database, check the box next to "untreated wood" (or expand the category and select one or more entries as a subset), then click the button labelled "Average of Selected". Results will be shown on the right. Scroll down to the Calorific Values section. Look at the row marked "Net calorific value (LHV)" and the column marked "Mean" (it's the only bold one). That gives you MJ/kg. Divide by 3.6 to get kWh/kg. At the time of writing, the average for all untreated wood was 18.88MJ/kg, so (18.88/3.6=) 5.24kWh/kg. As more and more samples are added to the database, expect the values to change slightly over time.

PPPS: If, for whatever reason, you don't want to use the EN's 5.14kWh value, you can go to the Phyllis2 database, check the box next to "untreated wood" (or expand the category and select one or more entries as a subset), then click the button labelled "Average of Selected". Results will be shown on the right. Scroll down to the Calorific Values section. Look at the row marked "Net calorific value (LHV)" and the column marked "Mean" (it's the only bold one). That gives you MJ/kg. Divide by 3.6 to get kWh/kg. At the time of writing, the average for all untreated wood was 18.88MJ/kg, so (18.88/3.6=) 5.24kWh/kg. As more and more samples are added to the database, expect the values to change slightly over time.

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