Carbon Content of Heat

What is the carbon content of heat?

The debate rages around how to decarbonise our heating needs. Much is talked of what should and should not be done, and what can and cannot be achieved, without putting the relative carbon merits in context. At Erda Energy we measure and record the parameters needed to ensure we know what the carbon content of our heat is. The intention of our chart is simple – to track on a regular basis the carbon content of heat as it is produced or could be produced by various technologies. We aim to put this in a relative context rather than absolute  – which we do elsewhere – to show the carbon intensity (in gCO2) of 1kWh of heat delivered to the treated space.

This article is to provide the calculations and reference points behind the Erda Energy carbon content of heat chart.

This is not as simple a task as it would seem, and far less simple than the current sources of electrical grid carbon available to the public. Electricity – by its very nature – must be balanced precisely between demand and supply, and this balancing is recorded with precision in 5-minute intervals by Elexon.

Heat is complicated, and very rarely measured directly. More often than not we use the measurement of a primary fuel (natural gas, oil, coal, LPG etc,.) as a proxy for heat. We measure these primary fuels closely as they are the basis of revenue, but they do not represent the “end-product” as closely as electricity does. To arrive at the end-product (heat) you need to know what the efficiency of the process is – generally a combustion process. If you have ever driven a car, you will know that the fuel efficiency which the salesman gave you and what you get “on the road” are often times very different. To be (partially) fair to the salesman – much depends on how you maintain your car and your style of driving. It’s exactly the same with your heating system.

We wanted to show the carbon impact of the primary fuel used for heating, and the efficiency of the system you choose to produce that heat – hence the Carbon Content of heat.

Erda Energy operate a number of geo-exchange systems, and as such understanding their performance is critical to our business. We have been collecting detailed data for many years and have built up a substantial set of performance data on our Erda | smart™ platform. We are also able to collect and store data from other sources – putting it alongside our own to create valuable insights. By measuring and collecting data we have been able produce a chart of actual geo-exchange performance vs a range of other heating technologies.

We display an insight of this on our website – 5-minute snap shots of real heating, measured from real buildings. From all of the measurements we take – and those we collect from other sources – we can show both the energy efficiency and the carbon efficiency of our systems.


Wanting to make sure we had our sums right – we asked Oxford University to check our sums for us. Here is the full report.

Having established the “system carbon content” of our heat relative to a gas boiler, we started to widen our comparisons to look at other technologies which are or could be providing heat. Given that we measure and report our own performance under “on the road” conditions, we wanted to put some real-life comparisons up against that – and this is what we found;

This is the carbon content of heat.


To do a comparison against other technologies we need to find some “on the road” data which gives us the efficiency of transferring primary energy into heat. In the absence of a continuous feed of live data for comparison system, we have used the findings of a Carbon Trust report – Micro CHP Accelerator1.

This report was a detailed study of gas fired boilers and Micro CHP systems in a range of domestic and non-domestic situations, analysing many thousands of data points over a period of at least 12 months.

Natural Gas

The starting point for “heat” in the UK – and the counterfactual for all discussions on the way forward is the gas-fired boiler. It’s by far the most prevalent form of heating for domestic and non-domestic properties.

The Carbon Emissions Factor for natural gas is laid out in the building regulations and is currently

184 gCO2/kWh or 0.184 kgCO2/kWh.

The majority of the boilers used in this study were SEDBUK A rated – suggesting their efficiency in producing heat is in the range of 90% to 92%. The systems studied within this initiative found that in real-life, less than 10% of the boilers actually performed as well as the test conditions. They found the aggregate efficiency of all systems to be 85%, and some systems returning only a 74% efficiency (think back to the m.p.g. analogy…).

Therefore – to deliver 1kWh of heat to the treated space – you would need to supply between 1.35 kWh and 1.06 kWh of natural gas (74% and 94% efficient systems respectively).

Consequentially, this amount of gas equates to a system carbon content range of

195.7 – 248.6 gCO2 for every kWh of heat delivered.

UK Grid Carbon

Electricity is an important factor in the debate around decarbonising heat. In its own right power is an important sector to decarbonise. We use data supplied by Elexon to calculate the carbon content of electricity at any given 5-minute period. We feel we have taken a fairly conservative approach to this calculation, and only used data for electricity dispatched into the grid, rather than trying to account for additional wind and solar generation “behind the meter”. There are many great sources of expertise around this data:


Grid Carbon App

Electric Insights

We have stuck to using just the Elexon data, added at factor of 8% for grid transmission losses, and therefore feel we are on the conservative side of reporting UK Grid Carbon.  At some points in time – grid electricity could actually be slightly cleaner than we have allowed for.

In domestic and non-domestic applications electricity can, and often is used directly to provide heat. These are usually resistive heaters for space heating or immersion heaters for hot water. Its estimated2 that around 2.2m homes in the UK use some form of electric heating (storage and/or direct) with an increasing number (100k+) using electrically driven heat pumps.

Whilst there are certainly efficiency and affordability issues around the direct use of electricity for heating, at least for the purposes of the “carbon content of heat” debate – observers can see how clean that heat is in relation to other technologies.


Combined Heat and Power.

Combined heat and power, or CHP, is a technology which uses a primary fuel input (typically natural gas) to generate both heat and electrical power simultaneously. However this introduces further debate and complexity when assessing the relative carbon benefits of this technology compared to others.

Micro CHP (defined as being up to 50kW of electrical output) was tested within the Carbon Trust Accelerator report in a number of domestic and non-domestic settings. The findings and results are published in the Carbon Trust report in the same way as for the Gas boilers. This technology is not as mature as gas boilers so there isn’t an industry wide benchmark such as SEDBUK from which to compare various manufacturers – the headline efficiency though is “up to” 92%3.

Within the Carbon Trust report the findings of efficiency are reported and documented in figure 3.16

Figure 3.16 from the Carbon Trust Micro CHP accelerator report1.

Findings from the study reveal the “on the road” efficiency of the Micro CHP technology tested is 52% for heat and 22% for electricity. That means – in the context of our comparison – to deliver 1kWh of heat into a treated space you would need to use 1.92 kWh of gas.


The combustion of 1.92 kWh of gas therefore produces 353.8g of CO2. Burning this gas and committing this amount of carbon to the atmosphere however would also produce 0.422 kWh of electrical energy, and we need to assign this a carbon value.

Within the interim report of the same study (November 20074) there is a discussion as to “how” to value the carbon of the electrical energy produced. Methods such as the “marginal plant method” and “grid mix” where put forward within the report, and at the time it was determined that the marginal plant method portrayed CHP in a more carbon positive light.

Given there is no conscious decision by the operators of CHP to produce electricity in times of high grid carbon, nor by the national grid to displace “dirty” grid electric when a Micro CHP operates – the grid just sees demand and balances it with the supply which is available. We therefore use the “grid mix” method and have netted-off the carbon value of 0.442 kWh of electricity as if it were produced from the grid at the time is was supplied and assigened it that value.  The rest of the carbon output is atributed to heat.

As an example – at a time when the grid carbon factor is 300 gCO2/kWh we would therefore arrive at a carbon content of the heat as follows:

353.8 – (0.442 x 300) = 221.2 gCO2 for each 1 kWh of heat supplied.

As we use a record of the grid carbon every 5 minutes we can use the live grid data and give insight into how the dynamic carbon content of heat from CHP technology compares to other technologies, and roll this up to averages across days, weeks and months.



The future of decarbonised heat..? There is much discussion around the future use of the natural gas transmission network, and how-to re-purpose it such that it’s not left stranded by the decarbonation needs of heat. Hydrogen is being considered for that task – with headlines of “significant savings”, but what are those savings which would justify investigating the practicalities which need to be explored. We have therefore tried to consider the carbon implications of this technology against what is actually happening today.

Hydrogen generated by SMR and using CCS is considered the most scalable technology given current maturity. Significant research and has been undertaken, and large scale trials are proposed. We have taken the findings of one such report – H21 Leeds City Gate5 – as a basis for our assessment of the potential carbon impacts.

Chart 7.1 within the report sets out their findings on the carbon content of hydrogen. There are various “scope” steps to capture the emissions associated with Hydrogen, and this report considers Scope 1+2+3 emissions and as such the carbon content for this type of hydrogen to be 85.83 gCO2/kWh.

NOTE: this creates a slight comparison discrepancy between natural gas and Hydrogen. The Hydrogen factor includes a scope of emissions not currently recognised for natural gas in the building regulations. Its anticipated that this correction (from 184 to 209.28 gCO2/kWh) for natural gas will be made soon and that the scope 3 emissions will be the UK’s most accurate assessment of the carbon associated with Hydrogen.

The next projection necessary is how this primary fuel will be actually translated to heat in the treated space. As this fuel supply is still in development, there is obviously no “on the road” data for the efficiency of operation. However, as this is a combustion technology, and that if this primary fuel is adopted there would need to be a boiler upgrade program, we have taken a view that boiler efficiency could be improved based on the lessons learned from the Carbon Trust Micro CHP Accelerator report, and it would be reasonable to assume all efforts are made to do that by following the various recommendations set out in the report. Accordingly we should be more optimistic in relation to boiler efficiency and therefore take a range of 85% to 94%.

Therefore – to deliver 1kWh of heat to a treated space – you would need to supply between 1.17 kWh to 1.06 kWh of Hydrogen. Accordingly, this amount of gas equates to a range of

91.3 – 100.9 gCO2 for every kWh of heat delivered.

Hydrogen can also be created via electrolysis. At the moment this process is not at a maturity where grid-scale application could be considered. If it does – we will be sure to include it in our comparisons.


Using this data, and the dynamic carbon content of grid electricity, we can keep a regular eye on which technologies actually produce the cleanest heat.


1.  Carbon Trust Micro CHP Accelerator. Final report – March 2011

2.  Electric for heating in homes

3.  Dachs mini-CHP datasheet

4.  Carbon Trust Micro CHP Accelerator.  Interim report – November 2007

5.  Leeds H21 Report

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