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The carbon intensity of electricity is related to the specific method of generation used (e.g. coal, gas, nuclear, wind), and therefore the CO2 emissions associated with national or regional grid electricity depend on the specific combination of generation methods (the fuel mix) which supply the grid. The fuel mix will change due to variations in demand and supply.

The fuel mix will vary according to demand because only certain types of generation methods can be adapted to meet the demand. For example, fossil fuel generation involving gas and coal can be adjusted relatively quickly to meet demand whereas most existing nuclear and renewable generation methods cannot. This means that times of peak demand tend to be more CO2 intensive (i.e. the kgCO2 per kWh is greater) than trough times. For this reason, in the UK, electricity is most carbon intensive during the day and in the winter and least so during the night and in the summer. Another factor that may affect the fuel mix will be the maintenance and (de)commissioning of power stations.

Use of the emission factor

Note: Incorrect use of the real time emission factor will lead to misleading estimates of CO2 savings.

This emission factor must be used with great care. In particular, it will almost always give a misleadingly high "face-value" estimate of potential CO2 savings. For example, let's suppose that the day EF is 0.6 tCO2/MWh and night EF is 0.4 tCO2/MWh and that a factory switches from day to night production so that 1000 MWh of energy consumption now occurs during the night. The CO2 saving will not be 0.6 x 1000 - 0.4 x 1000 = 200 tCO2 (33%). The reason is that the appearance of 1000 MWh demand at night will have to be met by increasing energy generation and this will involve increased gas and coal generation. The size of the real saving will typically be <1% and can in fact be negative.

Fuel mix

The AMEE category home/energy/electricity/realTimeElectricity contains instantaneous CO2 emissions data for electricity consumption based on data describing the current grid fuel mix which are updated every five minutes. At present, AMEE contains such data for the UK national grid only. AMEE receives data on the instantaneous supply of grid electricity for the following types of electricity generation:

  • Closed cycle gas turbine (the CCGT data item value)
  • Open cycle gass turbine (OCGT)
  • Oil (oil)
  • Coal (coal)
  • Nuclear (nuclear)
  • Wind (wind)
  • Pump storage (pumpStorage)
  • Non-pump storage hydro (NPSHYD)
  • Other (other)
  • International import: France (INTFR)
  • International import: Ireland (INTIRL)

AMEE uses this instantaneous fuel mix data, together with specific CO2 emissions factors associated with each type of generation (by reference to the category home/energy/electricity/realTimeElectricity/fuelEmissionFactors), to calculate a weighted average CO2 emissions factor for each 5 minute period.

To use this category, create a profile item and set the quantity of electricity used in the energyUsed profile item value. The returned value represents the associated quantity of CO2 emitted based on the most recent grid fuel mix data.

UK National Grid Data and RealtimeCarbon.org

The Balancing Mechanism Reporting System website is full of information regarding the current state of the national electricity grid in the UK. A license to use part of this data has been granted to Dynamic Demand and the RealtimeCarbon.org project. As part of this project, this data is stored at regular intervals in the AMEE API.

The source for the data is a feed which is updated every 5 minutes: http://www.bmreports.com/bsp/additional/soapfunctions.php?element=generationbyfueltypetable. The feed shows the current mix of the electricity supply, breaking it down by fuel type. Below is an example of the XML data received:

The INST tag states that the current ‘instantaneous’ profile is produced for the time 16:35:00 on 3rd July, 2009. It states that the total generation at this time is 40,350 megawatts.

The FUEL tags show the contribution of each type of fuel to that total. The less obvious ones include CCGT for closed cycle gas turbines, OCGT for open cycle gas turbines, PS for pump storage and NPSHYD for non-pump storage hydro. Also listed are imports over the interconnectors to France (INTFR) and Ireland (INTIRL). The VAL attribute is the amount of generation for that fuel type in megawatts, and PCT gives the percentage of the total.

The Realtimecarbon.org project takes this data and stores it in AMEE every 5 minutes. AMEE then applies emission factors for each of the generation methods, and calculates a total CO2 emission for the current state of the grid. This data is them displayed on the Realtimecarbon.org website, and is available through their API. It is also stored in the massCO2PerEnergy (kg per kWh) data item value in AMEE, for use as described above.

More detail

A technical case study on using this category is available at the AMEE API documentation site.

Detailed information on the assumptions and fuel factors can be found in this page, including how the real time calculated electricity emission factors relate to the grid average value published by DEFRA/DECC and available in the Electricity category in AMEE.


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