Calculating carbon emissions for energy data in the UK

I’ve recently been taking a closer look at the Streamlined Energy & Carbon Reporting (SECR) guidelines. While we currently produce figures on CO₂ emissions in Energy Sparks we’re not producing SECR style reports for schools or trusts. This is something we’re planning to add to the product soon, so I’ve been looking at what’s involved and how it’s different to what we’re currently doing.

I’m going to drop my notes in here in case useful for anyone else. I’ll briefly outline how to calculate emissions for SECR and then look at more accurate ways to do it.

I’m not going to summarise SECR itself or look at anything except reporting of emissions for electricity and mains gas. If you’re looking for more details then look at the requirements document. There’s also some specific guidance for multi-academy trusts, as well as plenty of third-party tools and explanations.

A brief look at Scopes 1, 2 and 3

As I’ll be talking about Scope 1, 2 and 3 emissions, its useful to define what those are:

• Scope 1 – your direct emissions, e.g. from burning gas to heat a building
• Scope 2 – your indirect emissions that are caused directly by your activities, e.g. by you consuming electricity from the grid
• Scope 3 – all other indirect emissions that are not directly caused by you, but which relate to your organisations “value chain”, e.g. emissions from the transmission and distribution of electricity or, if you used LPGs, from transport of that to your buildings

Basic SECR Calculations

There are three main greenhouse gases: Carbon Dioxide (CO₂), Methane (CH₄) and Nitric Oxide (N₂O) with CO₂ being the main one.

It’s possible to report these separately but the convention is to report all GHG emissions as a “CO₂ equivalent”, e.g. the warming caused by an equivalent emission of CO₂.

For SECR reporting your reporting needs to include both your annual consumption (in kWh) and your emissions.

Emissions can be calculated from your consumption using a conversion factor that converts a unit of energy into a CO₂ equivalent (kg CO₂e). The conversion factors for the UK are published annually — around June I think — with the entire history available online.

So you need to find and use the right conversion factors, for the right year, to calculate the emissions for different types of energy consumption. For 2024 the electricity and gas figures look like this:

		Unit	kg CO2e	kg CO2e of CO2 per unit	kg CO2e of CH4 per unit	kg CO2e of N2O per unit
Electricity	Consumption	kWh	0.20705	0.20493	0.00090	0.00122
	Transmission & Distribution	kWh	0.0183	0.01811	0.00008	0.00011
Natural Gas	Consumption	m3	2.04542	2.0414	0.00307	0.00095
		kWh	0.20264	0.20223	0.00031	0.0001

The Transmission & Distribution factor for electricity is used when reporting Scope 3 emissions attributable to the supply of electricity to your location. This isn’t a mandatory requirement but I’ve read that its considered best practice to include these.

There are two conversion factors for Natural Gas. This is because gas meters measure volume of gas rather than a kWh value. But bills typically also provide a kWh value instead for consistency.

Actually, some older gas meters are still on Imperial units so they report measurements in cubic-feet or hundreds of cubic-feet. Something to watch for when dealing with energy data.

There is a formula for converting volume measurements of gas into a kWh value, which relies on knowing the calorific value of the gas being used. In the UK the calorific values are regulated to be between around 38 MJ/m³ to 41 MJ/m³. So you can use an average of those, but see below for more details.

Renewables

Energy generated and consumed from solar panels can be included in SECR. Although I find the guidance to be a little unclear.

The 2024 conversion factors includes a note to say that:

Where electricity is onsite generated from a renewable source such as PV then the kWh should be reported under scope 1 with 0 emissions for both the electricity fuel source and 0 emissions for the T&D value.

Greenhouse gas reporting: conversion factors 2024 – condensed set

So my understanding from that is that annual solar generation figures can be directly included in the Scope 1 and Scope 3 emissions.

Annex G of the main SECR guidance also states that solar exports can be used to produce a net kg CO₂e emissions figure:

The emission reduction should be calculated using the grid average factor. Total emissions reductions from generated and exported renewable electricity (and green tariffs if appropriate) must not be greater than gross scope 2 emissions.

SECR, Annex G

So the export emissions for producing that offset are calculated using the electricity consumption intensity factor for that year (e.g. 0.20705 kg CO₂e from the above table).

What about if you’re on a 100% renewable tariff or have purchased offsets of some kind? Well there are no green electrons so you can’t actually consume only 100% renewable energy if you’re consuming any electricity from the grid. At least today.

SECR accommodates green tariffs and offsets by suggesting that you should report both a “location” based emissions (which is what I’ve outlined above) AND a “market” based set of emissions.

The market emissions would be calculated using set of intensity factors provided by your supplier. These figures are also supposed to be backed up, e.g. by a REGO.

Limitations of SECR

SECR is a simplified approach so has some limitations.

The main one is that the intensity factors produced by the government are always out of date. As the 2024 methodology explains, the data used to calculate the electricity consumption intensity factor is based on grid emissions data from 2 years ago.

So the current 2024 electricity consumption factors are based on the state of the National Grid in 2022. The grid has decarbonised significantly over the last few years and continues to do so. So the emissions reported using these factors will be higher than actual.

Because SECR is based on calculating emissions from your total annual consumption, it won’t reflect that the grid carbon intensity varies over the course of the day. Or regionally. The amount of renewable energy in the mix has a significant impact. So the time when you consume energy, and where you’re consuming it, also makes a difference.

For natural gas emissions there’s a similar issue. The Calorific Value of the gas pumped to your home varies from day to day. So the actual emissions will be different from the averages used in calculating the natural gas intensity factors.

More accurate approaches

A more accurate approach to calculating CO₂ emissions is to use half-hourly energy data. You can calculate the emissions attributable to each half-hourly period using:

• Data from the Carbon Intensity API which provides an estimate of the carbon intensity of the grid for each half-hour. It provides both historical estimates and forecasts, as well as regional figures. We harvest these in Energy Sparks on a daily basis.
• By using the daily natural gas calorific values from the National Gas data portal to convert volume of gas consumed to kWh to give a more accurate estimate of the energy consumed, and then calculate the CO₂ emissions

However there’s some caveats here that apply to the data from the Carbon Intensity API. As they explain in their methodology there are some differences in the intensity factors used in SECR:

• The estimated emissions are for CO₂ only. Other greenhouse gases are not estimated. SECR requires reporting CO₂e figures which include all three gases. I suspect the intensity could be adjusted based on the contribution of other gases over time, using the standard GHG factors.
• The estimated emissions cover direct emissions from the electricity generation AND transmission and distribution emissions. SECR requires reporting these as separate figures, with separate emissions factors for each. The electricity generation factor also includes the cost of transporting fuel to power stations

This means the resulting figures aren’t comparable but are likely to be a better overall estimate of CO₂ emissions.

SECR is useful for providing a baseline for comparing businesses but is less helpful for tracking your current emissions or setting targets.

If you think I’ve missed anything or made a mistake, then let me know!