Real kWH costs of solid fuel

chrisw2k

Hi

Does anyone have any real world information or case studies on the efficiency of solid fuels and stoves? As it looks like I’ll be paying £700 for a tonne this year Ive been trying to work out approx how much it’s costing per kWh.

Ive attempted to worked this 2 ways and get quite different figures..

Wherever I can find a figure quotes for calorific value of anthracite based solid fuels I get 33,000 kJ/kg which the internet tells me is about 9kWh/KG. If my stove is 70% efficient, 1 tonne would give me 6300kWh which works out to about £0.11/kWh

However, I reckon I load up the stove with approx 8kg each evening in the colder months which burns for around 3 hours at a good temperature. The Nominal output of the stove is 8kWh, so approx 24kWh output in total and therefore £0.24 / kWh.

So I'm thinking either my calculations are off, my stove is nowhere near 70% efficient, or the calorific value of the fuel is far less than I see quotes. Does anyone have any thoughts?

Ive been using Maxibrite for the last few years, however the quality appears to have dropped last year(and they have since ceased trading) so going to give Newburn a go this year.

Reason for this is that Im looking into alternative heating solutions..

Thanks!

matt_drummer

What makes you think that your stove is running at its nominal output of 8kWh?

Surely it depends on what it is burning and how hot it is?

I would think that your 9 kWh/kg is your most accurate measure, it's a known value that I assume can be challenged and suppliers are bound by law to be close to what they quote?

The heat output of your stove is more difficult to measure.

QrizB

The Nottingham Energy Partnership produce a useful table:
https://www.nottenergy.com/advice-and-tools/project-energy-cost-comparison

It's recently been re-formatted; I think I preferred the old one, but the new one looks prettier.

You can substitute your own energy costs and then use the rest of the figures to quote your own price per hWh of usable heat delivered.

chrisw2k said:

However, I reckon I load up the stove with approx 8kg each evening in the colder months which burns for around 3 hours at a good temperature. The Nominal output of the stove is 8kWh, so approx 24kWh output in total and therefore £0.24 / kWh.
So I'm thinking either my calculations are off, my stove is nowhere near 70% efficient, or the calorific value of the fuel is far less than I see quotes. Does anyone have any thoughts?

The effective calorific value of the fuel could be quite a bit lower than your 33MJ/kg. Notts Energy quote 6.7kWh per kg for smokeless fuel, which would mean your 8kg is only 54kWh.
Also, I've never seen a solid furl stove where it ignites immediately, burns at a constant strady heat output then goes immediately cold when the coals are exhausted. Usually there's a period at the start when it's warming upp, and a long tail while the embers burn and the stove cools down. You're probably getting more than 3x8kWh of heat from the stove.

chrisw2k

matt_drummer said:

What makes you think that your stove is running at its nominal output of 8kWh?

Surely it depends on what it is burning and how hot it is?

Yes, ive just assumed if i bring it up to temperature and then close it down slightly its going to run at roughy the nominal output given in the manual. This could be a bad assumption.

I would think that your 9 kWh/kg is your most accurate measure, it's a known value that I assume can be challenged and suppliers are bound by law to be close to what they quote?

You would think so! But it I were getting the quoted values I would be outputting over 50kWh into the house each evening, which, for a small-ish 2.5 bedroom cottage seems rather a lot.

chrisw2k

QrizB said:

The Nottingham Energy Partnership produce a useful table:
https://www.nottenergy.com/advice-and-tools/project-energy-cost-comparison

It's recently been re-formatted; I think I preferred the old one, but the new one looks prettier.

You can substitute your own energy costs and then use the rest of the figures to quote your own price per hWh of usable heat delivered.

chrisw2k said:

However, I reckon I load up the stove with approx 8kg each evening in the colder months which burns for around 3 hours at a good temperature. The Nominal output of the stove is 8kWh, so approx 24kWh output in total and therefore £0.24 / kWh.
So I'm thinking either my calculations are off, my stove is nowhere near 70% efficient, or the calorific value of the fuel is far less than I see quotes. Does anyone have any thoughts?

The effective calorific value of the fuel could be quite a bit lower than your 33MJ/kg. Notts Energy quote 6.7kWh per kg for smokeless fuel, which would mean your 8kg is only 54kWh.
Also, I've never seen a solid furl stove where it ignites immediately, burns at a constant strady heat output then goes immediately cold when the coals are exhausted. Usually there's a period at the start when it's warming upp, and a long tail while the embers burn and the stove cools down. You're probably getting more than 3x8kWh of heat from the stove.

Thanks QrizB, this sounds a bit more sensbile, my stove is rated at 70% efficiency so based on this somewhere around 14-15p/kWh is likely where im at.

You are of couse correct with the burn period, there will still be some output as it burns out..

Thanks

FreeBear

This study might be of interest - http://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/933099/burning-wood-consult-bsria-report1.pdf

It primarily looks at the effects of moisture content and how that affects heat output. But it also provides some data for coal, so may give the OP some real world numbers to compare. Should point out the cost per KW the report gives is based on 2020 prices, so doubling the numbers should put you in the ballpark for costs today.

Effician

Can you give some info on the stove,

Multifuel or coal only

Defra spec.

Also what's the flue draw like. how often do you refuel/ riddle etc

I'd be disappointed if i only got 3hrs of very good heat from 6kg of logs, never mind the extra 3hrs + of residual heat.

Log burner is our only form of heating by the way so have to wring out as many watts as we can.

Qyburn

chrisw2k said:

You would think so! But it I were getting the quoted values I would be outputting over 50kWh into the house each evening, which, for a small-ish 2.5 bedroom cottage seems rather a lot.

Is that necessarily unrealistic, considering the stove will probably be putting out heat all through the night, admittedly at a reducing rate. For comparison that figure would equate to around 5l of oil which would be pretty good for Winter.

We had a Rayburn running cooking central heating and HW in a former house, a 1950s bungalow. Roughly speaking it took one coal scuttle full per day or anthracite or Furnacite. That would be around 15kg I would think.

chrisw2k

FreeBear said:

This study might be of interest - http://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/933099/burning-wood-consult-bsria-report1.pdf

It primarily looks at the effects of moisture content and how that affects heat output. But it also provides some data for coal, so may give the OP some real world numbers to compare. Should point out the cost per KW the report gives is based on 2020 prices, so doubling the numbers should put you in the ballpark for costs today.

Thanks FreeBear, this is exactly the sort of study I was looking for.  I'll take a good look tomorrow but if im reading this correctly, efficiency of smokless fuel is around 5-6kWh which is a bit less than what Nottingham Energy suggest, and probably around the figure i think im getting (working out to  15p/ kWh based on this years prices).

chrisw2k

Effician said:

Can you give some info on the stove,

Multifuel or coal only

Defra spec.

Also what's the flue draw like. how often do you refuel/ riddle etc

I'd be disappointed if i only got 3hrs of very good heat from 6kg of logs, never mind the extra 3hrs + of residual heat.

Log burner is our only form of heating by the way so have to wring out as many watts as we can.

Hi Effician

Im not so much worried that the stove isnt doing what its supposed to, Im just trying to get a rough figure on how much its costing me per kWh.

Its a Hunter Herald 8 with back boiler, the draw is good and it burns lovely with anthracite solid fuels (ive stuggled to keep pure anthracite nuggests in however).

As with everything, its not clear what the manufacturer means by nominal output, ive read this can mean the output with having to refuel every 1-2 hours which i do not do - 1 scuttle, approx 8kg will last the evening unless its really cold. Im guessing i get a solid 8kWh for the first hour or so and then it tailing off over the evening.

chrisw2k

Qyburn said:

chrisw2k said:

You would think so! But it I were getting the quoted values I would be outputting over 50kWh into the house each evening, which, for a small-ish 2.5 bedroom cottage seems rather a lot.

Is that necessarily unrealistic, considering the stove will probably be putting out heat all through the night, admittedly at a reducing rate. For comparison that figure would equate to around 5l of oil which would be pretty good for Winter.

We had a Rayburn running cooking central heating and HW in a former house, a 1950s bungalow. Roughly speaking it took one coal scuttle full per day or anthracite or Furnacite. That would be around 15kg I would think.

Yeah, Im thinking 50kWH would be the equivelent to me having about 8 of those 2kwh electic radiatiors running full power for 3 hours. 1 is enough to keep the downstairs warn in the mornings when I havnt lit the fire, and thats not constantly on..

For me, i get about 3 skuttles from a 25kg bag, hence by 8Kg.