How much firewood does it take to produce the equivalent of 1kWh of electrical heating?

Question

Let's assume 1kWh of electricity is used to run a fan heater. Heat pumps are more efficient, but have a range of different efficiencies, so perhaps aren't a good base comparison.

Assuming a good quality log burning fireplace, how much firewood (by weight) does it take to produce the equivalent of that 1kWh of heating? This stove claims combustion efficiency of 89.1%, but 70% is perhaps a reasonable standard to work with.

Obviously water content of the wood would make a big difference too, so let's assume either 10% or 20%, whichever is easier. I understand 20% is considered to be dry, when dealing with firewood.

Answer 1

Per Google, 1 kWh = 3412 BTU. (And a fraction, which I'll neglect.) Here's a table (one of many found with Google) of BTU/cord for various kinds of wood: https://chimneysweeponline.com/howood.htm It varies a lot by species, but let's say 20 million BTU per cord, and 3500 lbs/cord as average for hardwoods. So 1 lb of average hardwood gives you 5714 BTU. Multiply by the efficiency of your stove (70 - 89.1%) gives 4000 - 5091 BTU. So it takes roughly 0.67 - 0.85 lbs (304 to 386 grams) of firewood to equate to 1 kWh of electric heat.

Answer 2

I would present a different analysis to jamesqf, because his does not take into consideration the latent heat of evaporation, or at least does not mention it.

If we're assuming a regular hardwood used typically for firewood (here in the UK at least), which has a net calorific value of 4.06 kwh/kg at a moisture level of 20%, if this is burnt in a stove with an efficiency level of 70% then it is safe to calculate that it would take a 0.352kg piece of firewood to produce 1kWh of electricity.

If the moisture content of the firewood was 10% then it would take a 0.307kg piece of firewood to produce 1kWh of electricity.

In calculating these figures I have used a net calorific value for dry firewood of 5,251. However, and this is a big HOWEVER, this calculation assumes that the 1kWh produced can be perfectly turned into electricity by a thermoelectric generator. This is a naive assumption, hence I would like to present a probable calculation about how much firewood we would need to produce 1kWh of electricity, because that's really what the question is asking. The problem however is that currently I know too little about this process, let me do some research and I may be able to provide a better answer in the not too distant future.

Answer 3

Efficiency numbers are, unfortunately, misused/abused by companies that manufacture and sell wood heaters.

"Combustion Efficiency" is not actually a direct measure of how much heat is produced by burning wood. It's a direct measure of "how completely (or cleanly) the wood has burned". A stove that has an 89.1% combustion efficiency burns 89.1% of all the available fuel (under test conditions). 10.9% of the fuel either remains behind in the firebox (usually in the form to charcoal) or escapes up the chimney (usually in the form of carbon monoxide). In any case, it's not a measure of heat but, because the number tends to be higher than the other types of efficiency ratings, marketing departments love to use it.

If you want to know how much heat is produced, then you need to look at either "Net Efficiency" or "Gross Efficiency" ratings.

"Net Efficiency" ratings describe how much heat you can theoretically extract from firewood using the appliance in question. Contrary to what most people understand by 'net' it does not factor in how much of the heat is wasted vapourising the moisture in the wood.

"Gross Efficiency" ratings describe how much heat you actually extract from firewood using the appliance. The energy taken to vapourise moisture has been taken into consideration.

Wood heaters in Europe are tested according to EN standards, and those tests use firewood that has an 18.5% moisture content. If your heater is described as having a Net Efficiency, and burns regular (dry) firewood, you can convert that to a Gross Efficiency by multiplying by 0.91 (e.g. Net 81% * 0.91 = Gross 73.7%).

The 0.91 conversion number comes from 'Table E4: Efficiency conversion factors' in the 'The [UK] Government’s Standard Assessment Procedure for Energy Rating of Dwellings'. The latest version is SAP2012. Direct link to PDF. Table E4 is on page 64.

Gross Efficiency is what you are after to do the math. Gross Efficiency is useful. Once you have a Gross Efficiency figure, the rest is easy.

Each kilogram of (EN Standard) dry wood contains 5.14 kWh of "potential heat" so 1 kWh of energy is contained in (1/5.14=) 0.195 kg of wood.

If your wood heater's Gross Efficiency was (say) 70% then the actual amount of wood that you would need to produce 1 kWh of heat is (0.195/0.70=) 0.279 kg.

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

PS: In many Western counties, wood heaters have to undergo independent testing before they can be sold to ensure that they comply with local regulations. The test results may be commercial-in-confidence, but reputable wood heater manufacturers/vendors — with nothing to hide — should be willing to provide a copy of either the entire report or an extract. This document is almost always more useful than the glossy brochure the Sales department provides as it contains all of the efficiency ratings that were measured/calculated.

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.

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.

Answer 4

The link below gives you net calorific values and energy densities for different woods with different moisture content.

http://www.woodenergy.ie/woodasafuel/listandvaluesofwoodfuelparameters-part1/

Answer 5

Let’s look at this anther way.

• Your house will have some insulation
• There will be heat input from the people and electrical equipment in the house.
• There will be heat lost from the fireplace including when the stove is not in use.

Therefore having a fireplace will increase the number of hours a year you need to heat compared to electrical heating.

So the questions as asked does not lead to a useful result, in some homes the wood burned will have a negative effect on the heat needed, assuming that the fireplace would not otherwise be there.