ESOS to carbon reporting: turning an audit into an asset
An ESOS Phase 4 energy audit must cover at least 95% of your total energy consumption.
That same data — kWh of electricity, fuel volumes — is exactly what you multiply by DESNZ conversion factors to produce Scope 1 and 2 emissions for SECR and UK SRS S2.
This is how to treat a compliance audit as a reporting asset, and where its limits lie.
The audit you already have to run is a carbon dataset
ESOS — the Energy Savings Opportunity Scheme — makes large UK organisations measure their energy use and identify cost-effective efficiency opportunities every four years.
From the third compliance period onward, an ESOS assessment must cover assets and activities accounting for at least 95% of total energy consumption, with up to 5% excluded as de minimis[1].
That is a high bar — and it produces precisely the activity data a carbon inventory needs: kilowatt-hours of electricity, and volumes of gas and fuel.
Multiply those figures by the right DESNZ conversion factor and you have Scope 1 and Scope 2 emissions for SECR and UK SRS S2[3].
From kWh to tCO2e
The conversion itself is arithmetic. The DESNZ (formerly DEFRA) UK Government conversion factors for company reporting are published annually, expressed in kilograms of CO2 equivalent per unit of activity[3].
You take each activity figure from the ESOS audit, multiply by the matching factor, and divide by 1,000 to get tonnes of CO2 equivalent.
The GHG Protocol splits the result by scope: fuels combusted on site are Scope 1; purchased grid electricity is Scope 2[4].
| ESOS energy category | Conversion | GHG scope | Feeds |
|---|---|---|---|
| Grid electricity (kWh) | kWh × DESNZ electricity factor | Scope 2 (location-based) | SECR, UK SRS S2 |
| Natural gas, heating oil, LPG (kWh / litres) | Activity × DESNZ fuel factor | Scope 1 | SECR, UK SRS S2 |
| Owned / controlled fleet fuel | Fuel × DESNZ vehicle factor | Scope 1 | SECR, UK SRS S2 |
| Business travel in non-owned vehicles | Distance × DESNZ travel factor | Scope 3 | UK SRS S2 (Scope 3) |
Note the last row.
ESOS focuses on energy the organisation consumes, so it captures owned-fleet fuel cleanly but only touches the edge of value-chain emissions.
For the wider value chain, see our UK SRS Scope 3 reporting analysis, and the underlying figures in our UK emissions factors reference.
Feeding SECR and UK SRS S2
SECR — Streamlined Energy and Carbon Reporting — requires large UK companies and LLPs to disclose UK energy use and associated greenhouse gas emissions in their annual report each year[5].
ESOS data maps onto SECR almost directly: the Scope 1 and 2 figures, the energy totals, and the efficiency actions identified in the audit can support the SECR narrative on measures taken[5].
UK SRS S2 goes further. It requires gross Scope 1, Scope 2 and Scope 3 emissions measured in line with the GHG Protocol Corporate Standard[4].
The FCA’s CP26/5 proposed mandatory UK SRS S2 climate reporting for in-scope listed companies for accounting periods beginning on or after 1 January 2027[6].
For those companies, an ESOS audit run for the 5 December 2027 deadline lands in the same window as the first UK SRS S2 reporting periods — a planning opportunity, not a coincidence to ignore.
What ESOS does not cover for carbon reporting
Treating ESOS as a finished carbon inventory is the mistake to avoid.
Four gaps matter.
Geography. ESOS covers UK operations only. SECR and UK SRS S2 extend to the global operations of the reporting entity, so ESOS data needs supplementing for overseas sites[1].
Frequency. ESOS runs on a four-yearly cycle, with Phase 4 spanning 6 December 2023 to 5 December 2027[1]. SECR and UK SRS S2 are annual, so the years between audits still need their own activity data.
De minimis. Up to 5% of total energy consumption can be excluded from the ESOS audit, but a complete GHG inventory aims for full coverage[1].
Scope 2 method and Scope 3. ESOS does not require a market-based Scope 2 figure, and it does not capture most Scope 3 value-chain emissions that UK SRS S2 expects[4].
Turning a compliance audit into a reporting asset
The difference between a cost and an asset is in the scoping.
A few decisions taken before the Phase 4 audit pay back across every annual report that follows.
Scope the audit to the GHG Protocol organisational boundary where practical, so the energy data reconciles with your Scope 1 and 2 inventory rather than fighting it[4].
Record activity data in the units the DESNZ factors use — kWh and litres — so conversion is a multiplication, not a re-measurement[3].
Keep the audit trail and assumptions documented, because the same evidence supports assurance of both SECR figures and, in time, UK SRS S2.
For the broader regime overlap, see our ESOS and SECR analysis and the ESOS Phase 4 guide.
ESOS to carbon reporting: frequently asked questions
Can ESOS energy-audit data be used directly for carbon reporting?
Partly. An ESOS Phase 4 assessment must cover assets and activities accounting for at least 95% of an organisation’s total energy consumption, with up to 5% excluded as de minimis. That energy data — fuel volumes and electricity in kWh — is exactly the activity data you multiply by DESNZ conversion factors to produce Scope 1 and Scope 2 emissions for SECR and UK SRS S2. But ESOS is a four-yearly UK-only energy audit, so it does not by itself provide the annual, global, market-based figures those reporting regimes require. The audit is a strong foundation, not a finished carbon inventory.
When is the ESOS Phase 4 deadline?
The qualification date for ESOS Phase 4 is 31 December 2026, and the notification-of-compliance deadline is 5 December 2027. Organisations qualify if, on the qualification date, they have 250 or more employees, or a turnover above the financial threshold and a balance sheet above it. The compliance period runs from 6 December 2023 to 5 December 2027.
How do you convert kWh to tCO2e?
You multiply each activity figure — kWh of grid electricity, kWh or litres of gas and fuel — by the matching emission factor from the DESNZ (formerly DEFRA) UK Government conversion factors for company reporting, which are published annually in kgCO2e per unit. Gas and other fuels combusted on site produce Scope 1 emissions; purchased grid electricity produces Scope 2 emissions on a location-based method. Dividing the resulting kgCO2e by 1,000 gives tonnes of CO2 equivalent (tCO2e).
What does ESOS not cover for carbon reporting?
ESOS covers UK operations only, on a four-yearly cycle, and allows up to 5% of total energy consumption to be excluded as de minimis. It does not require market-based Scope 2 figures, does not extend to most Scope 3 value-chain emissions, and does not align its boundary with the GHG Protocol organisational boundary that SECR and UK SRS S2 use. It also does not produce the annual time series those regimes need. So ESOS gives you robust Scope 1 and 2 activity data, but you still build the full inventory separately.
How does ESOS data feed UK SRS S2?
UK SRS S2 requires gross Scope 1, Scope 2 and Scope 3 greenhouse gas emissions measured in line with the GHG Protocol Corporate Standard. The ESOS energy audit gives you well-evidenced UK energy activity data that converts cleanly into Scope 1 and 2 under that methodology. You then have to extend coverage to global operations, add market-based Scope 2, and build out Scope 3 — none of which ESOS supplies. The FCA’s CP26/5 proposed mandatory UK SRS S2 climate reporting for in-scope listed companies for accounting periods beginning on or after 1 January 2027.
- Comply with the Energy Savings Opportunity Scheme (ESOS) — GOV.UK / Environment Agency · Phase 4 qualification 31 Dec 2026, compliance deadline 5 Dec 2027; 95% coverage / 5% de minimis
- Energy Savings Opportunity Scheme (ESOS): guidance — GOV.UK / DESNZ · Phase 4 changes; compliance deadline 5 December 2027
- UK government conversion factors for company reporting of greenhouse gas emissions — GOV.UK / DESNZ · Activity data × factor (kgCO2e/kWh) for Scope 1, 2 and 3; published annually
- GHG Protocol Corporate Accounting and Reporting Standard — Greenhouse Gas Protocol · Scope 1, 2 and 3 definitions and organisational boundary
- The Companies (Directors’ Report) and Limited Liability Partnerships Regulations 2018 (SECR) — legislation.gov.uk · Streamlined Energy and Carbon Reporting — annual UK energy and GHG disclosure
- CP26/5: Aligning listed issuers’ sustainability disclosures with international standards — Financial Conduct Authority · Proposed mandatory UK SRS S2 from accounting periods beginning on/after 1 Jan 2027