ESOS · the assessment
The ESOS assessment: what the law asks for, part by part
An ESOS assessment is the assessment of a participant’s energy consumption that the ESOS Regulations 2014 require once in every four-year compliance period.
It is wider than the energy audit: it measures, selects, audits or certifies, reports savings achieved, reviews the last action plan, and ends in a responsible officer’s confirmation and a notification.
For Phase 4 the size test is applied on 31 December 2026 and the completed assessment is notified by 5 December 2027.
Definition
What the law means by an ESOS assessment
The Energy Act 2023 defines the term: section 256(1) lets ESOS regulations require assessments of a participant’s energy consumption and the resulting greenhouse gas emissions, and “such an assessment is referred to as an ‘ESOS assessment’”.
The ESOS Regulations use only the energy limb: regulation 20 places the duty to carry out an ESOS assessment on the responsible undertaking, and Part 4 says what it consists of.
Until 22 July 2026 regulation 20 said the assessment “includes an energy audit”; SI 2026/701 deleted those words, because an ISO 50001 certificate can now stand in for the audit altogether.
The GOV.UK ESOS page describes the same thing in eight steps, from calculating total energy consumption to keeping an evidence pack.
In one line: the assessment is the whole compliance exercise for a phase, and the audit, the report and the notification are parts of it or what follows from it.
Who must carry one out is set by Schedule 1; the ESOS overview runs the size test on your figures.
| Part 4 chapter | What it covers |
|---|---|
| Chapter 1 | The duty to carry out the assessment, the lead assessor and estimates |
| Chapter 2 | Total energy consumption and significant energy consumption |
| Chapter 2A | Organisational purposes, subtotals and intensity ratios |
| Chapter 3 | Energy audits |
| Chapter 3A | The ESOS report |
| Chapter 3B | Savings achieved and the action plan review |
| Chapter 4 | The evidence pack |
Assessment, audit, report, notification
Four terms that are often used as if they were one
| Term | What it is | Who sees it | Provision |
|---|---|---|---|
| ESOS assessment | The whole exercise in Part 4: measurement, significant areas, ratios, audits or ISO 50001, report, savings achieved, review | The participant, the lead assessor and, on request, the regulator | reg 20; Part 4 |
| ESOS energy audit | An analysis of consumption and efficiency that identifies cost-effective savings opportunities, with site visits | In the evidence pack; reviewed by the lead assessor | regs 26–27 |
| ESOS report | The written record of the assessment, including the opportunities, savings achieved and the action plan review | Kept in the evidence pack; shared in part with group undertakings | reg 27A |
| Notification of compliance | The filing to the scheme administrator through MESOS, with the responsible officer’s confirmation | The regulator; most of it is published | regs 29, 31 |
The distinction is practical: an organisation can have every audit done and still be in breach, because the notification is a separate duty with its own penalty.
The audit itself is covered on the ESOS energy audit page, the report fields on ESOS templates, and the filing on ESOS reporting.
On your route
Which parts of the assessment apply to you
Not every participant carries out every part, and the panel beside this text shows which duties apply on each route.
A participant with zero energy consumption is deemed to have done the assessment under regulation 33A, though it still notifies.
One below 40,000 kWh needs no lead assessor under regulation 21(3), but still audits and reports.
Since 22 July 2026 an ISO 50001 certificate covering all total or all significant consumption stands in for the lead assessor, the audit and the report, but not for the measurement or the notification.
A certificate covering only part of the consumption covers only that part, and the rest is audited with a lead assessor.
The trade-offs between the two routes are set out on ISO 50001 and ESOS, and the exemptions that take an organisation out altogether on ESOS exemptions.
Your route, duty by duty · Phase 4
Total energy consumption over the reference period
ISO 50001 certification, valid on 5 December 2027
Is the lead assessor independent of the participant?
Not independent if, in the last 12 months, they were an employee, director, person exercising management control or shareholder, or the spouse or civil partner of one (reg 30(4)).
- Measure total energy consumption (the ESOS assessment, reg 20)Requiredreg 20; Part 4 Ch 2
- Appoint a lead assessorRequiredreg 21(1)
- Carry out an ESOS energy auditRequiredreg 26; Part 4 Ch 3
- Produce the ESOS reportRequiredreg 27A
- Keep an evidence packRequiredreg 28
- Notify compliance through MESOS by 5 December 2027Requiredreg 29(1)
- Responsible officers who confirm1reg 30(3)–(4)
- Action plan and three progress updatesRequiredregs 34A, 34B
SI 2014/1643 regs 20, 21, 26–30, 33, 33A and Part 6A, as amended by SI 2026/701.
An orientation aid, not advice.
Step by step
The Phase 4 assessment in eleven steps
The order follows the Regulations, because each step feeds the next.
| Step | What happens | Provision |
|---|---|---|
| 1. Qualify | Test each UK undertaking on 31 December 2026, using the two-consecutive-period rule | Sch 1 ¶¶1, 1A, 11; reg 15 |
| 2. Map the group | Identify the participant, normally the highest UK parent, and the responsible undertaking | reg 17 |
| 3. Fix the reference period | Twelve consecutive months including 31 December 2026, ending on or before 5 December 2027 | reg 22(5) |
| 4. Measure total consumption | All energy supplied to and consumed by the participant, in energy units or by spend | Part 4 Ch 2 |
| 5. Identify significant areas (optional) | Areas making up not less than 95% of the total; if not identified, cover the total | reg 25 |
| 6. Calculate intensity ratios | Subtotals and at least one ratio for each organisational purpose with consumption, in kWh | regs 25A–25C |
| 7. Choose a route and appoint a lead assessor | Audits, ISO 50001, or both; an assessor unless exempt | regs 21, 26, 33 |
| 8. Audit | Analyse, visit representative sites, identify and categorise opportunities, estimate costs and savings | regs 26–27 |
| 9. Report | The ESOS report, with savings achieved since 6 December 2023 and the review of the Phase 3 plan | regs 27A, 27D, 27E |
| 10. Confirm and notify | Lead assessor review; one or two responsible officers confirm; notify through MESOS | regs 21, 29–31 |
| 11. Keep the evidence | The evidence pack, kept until at least 5 December 2035 | reg 28 |
Steps 1 to 6 apply to every participant with any energy consumption, whatever its route.
After the notification come the action plan, due between 6 December 2027 and 5 December 2028, and three annual progress updates; the ESOS action plan page covers both.
The same steps are set against the Phase 4 dates on the Phase 4 compliance guide, and the requirements they implement are listed on ESOS requirements.
Measuring energy
What counts in total energy consumption, and what does not
Total energy consumption is the energy supplied to and consumed by the participant’s assets and activities in the UK, whichever group member holds them.
The Environment Agency’s guidance puts four things out of scope: energy the organisation does not use and supplies to a third party, energy consumed outside the UK or offshore area, international travel or shipping that neither starts nor ends in the UK, and energy used in flares at petrochemical works.
For transport the test is who is supplied with the fuel: company cars, fleet vehicles you operate and personal or hire cars on business use are in, while trains, flights and taxis you do not operate are out.
Data should be verifiable where reasonably practicable: invoices, delivery notes, meter readings, stock records and half-hourly or smart meter outputs.
Where there are gaps, the guidance allows estimates by direct comparison, pro-rata extrapolation or benchmarking, recorded in the evidence pack with the reasons, and notified to the scheme administrator where they affect total consumption.
Consumption measured in other units or by spend is converted to kWh for the intensity ratios; the guidance points to the fuel properties tab of the government conversion factors, and to the average biofuel blend for forecourt petrol and diesel.
From Phase 4 the Environment Agency publishes the total, and where applicable the significant, consumption attributable to each organisational purpose.
Climate Change Agreements, the UK ETS and SECR do not automatically count as ESOS compliance.
Their data can feed the calculation, but the guidance warns that ESOS covers more energy than those schemes, so their figures alone are unlikely to be enough.
Who confirms it
The lead assessor’s review and the responsible officer
A lead assessor from one of the seven approved registers reviews the assessment, considers whether it meets the Regulations, and tells the responsible undertaking; the ESOS lead assessor page lists the registers.
Since 22 July 2026 the assessor must then notify their own approval body within seven days, naming at least two contacts, one of them the responsible officer.
The responsible officer is a director, or a person exercising management control, of an undertaking in the participant, and regulation 30 asks for one where the lead assessor is independent of the participant and two in any other case.
Under regulation 31 the officer confirms, to the best of their knowledge, that the participant is within the scheme, that the undertaking has complied with Parts 4 to 6, that the information notified is correct, and that they have seen and considered the audit recommendations and any route relied on.
The guidance says the directors will need to see the ESOS report and evidence pack to give those confirmations.
The lead assessor is not responsible for compliance: the guidance says the compliance bodies will not hold them responsible, and the undertaking remains solely responsible for the notification.
Where you are
ESOS assessments in England, Wales, Scotland and Northern Ireland
ESOS is one UK-wide scheme, and the assessment is the same wherever the sites are.
What changes with geography is the regulator, and it follows the registered office of the responsible undertaking, not the location of any site.
An organisation registered in Wales, in Swansea or anywhere else, is regulated by Natural Resources Wales; one registered in England, in London or Cornwall alike, by the Environment Agency.
Scotland’s regulator is the Scottish Environment Protection Agency and Northern Ireland’s the Northern Ireland Environment Agency, while the Secretary of State regulates organisations whose activities are wholly or mainly offshore.
Whoever the regulator, the Environment Agency as scheme administrator runs the guidance, the helpdesk and MESOS for all of them.
Lead assessors are not regional appointments either: the registers list assessors with their contact details and specialisms, and the guidance asks undertakings to choose one with experience of their sector.
Budgeting
What drives the effort, without a price list
No regulator publishes a price for an ESOS assessment, and this site gives none.
What the GOV.UK ESOS page does say is that the costs of an assessment are estimated to be significantly outweighed by the savings from implementing its recommendations.
The guidance shows what drives the work: how many sites must be visited to be representative, how much verifiable data exists, how many organisational purposes apply, and how much of the consumption an ISO 50001 certificate already covers.
Organisations with many identical sites can expect to need fewer visits, the guidance says, and a regulator will look for a documented justification of the sample.
There is no charge for submitting the notification through MESOS.
Quotes come from lead assessors on the approved registers; the ESOS consultancy page sets out what to ask before appointing one, and the size test is worked through case by case in a sister reference on ESOS thresholds.
The rest of the scheme, phase by phase, is on the Energy Savings Opportunity Scheme and ESOS Phase 3 pages, and the regulator’s view of a compliant assessment on ESOS compliance guidance.
Check your understanding
Six statements about the assessment, true or false
The mistakes these statements test are the ones that change the work: treating the audit as the whole job, dropping the fourth organisational purpose, and assuming a certificate ends the duty to notify.
Each answer names the provision it rests on.
A missed or incomplete assessment is a breach of regulation 45, the most serious of the five; the ESOS penalties page sets out what it can cost.
True or false?
01An ESOS assessment and an ESOS energy audit are the same thing.
02Total energy consumption is measured over twelve months that include 31 December 2026.
03An intensity ratio is needed for buildings, transport and industrial processes only.
04Energy consumed abroad counts towards total energy consumption.
05The responsible officer confirms they have seen the audit recommendations.
06ISO 50001 over all significant consumption removes the duty to notify.
6 statements.
Pick an answer to see the provision behind it.
Answers rest on the provisions named beside each one.
Nothing you pick is saved.
Frequently asked
ESOS assessments, answered
What is an ESOS assessment?
An ESOS assessment is the four-yearly assessment of a participant’s energy consumption that the ESOS Regulations 2014 require of every large undertaking and its UK group.
It measures total energy consumption, may identify the areas making up at least 95% of it, calculates intensity ratios, covers the significant areas with energy audits or ISO 50001, and records the result.
A lead assessor reviews it unless an exemption applies, and a responsible officer confirms it in the notification.
What is the difference between an ESOS assessment and an ESOS energy audit?
The audit is one part of the assessment.
The assessment is the whole of Part 4 of the Regulations: measurement, significant consumption, intensity ratios, audits or ISO 50001, the ESOS report, savings achieved and the action plan review.
Since SI 2026/701, regulation 20 no longer says the assessment includes an audit, because a participant whose ISO 50001 certificate covers all total or significant consumption needs no audit at all.
Who has to carry out an ESOS assessment?
A UK undertaking that on the qualification date employs at least 250 persons, or has both an annual turnover in excess of £44 million and an annual balance sheet total in excess of £38 million, and every UK undertaking in its group.
For Phase 4 the qualification date is 31 December 2026.
When must the Phase 4 ESOS assessment be completed?
The assessment must be complete and notified through MESOS by 5 December 2027.
The notification must be made after the qualification date of 31 December 2026, and the ESOS report must be produced before the compliance date.
Is a lead assessor needed for an ESOS assessment?
Yes, unless total energy consumption is under 40,000 kWh, an ISO 50001 certificate covers all total or all significant consumption, or consumption is zero.
Where a lead assessor is appointed and is independent of the participant, one responsible officer confirms the notification; otherwise two.
How much does an ESOS assessment cost?
No regulator publishes a price, and this site does not give one.
The GOV.UK ESOS page says the costs of an assessment are estimated to be significantly outweighed by the savings from implementing its recommendations.
Submitting the notification through MESOS is free.
Quotes come from lead assessors on the seven approved registers.
Does a Climate Change Agreement or UK ETS count as an ESOS assessment?
No. The Environment Agency says participation in those schemes does not automatically count as ESOS compliance.
Their data can feed the calculation of total energy consumption, and an audit done for a CCA can count as an ESOS audit if a lead assessor conducted, verified or reviewed it and it meets the ESOS minimum standards.
Who is the ESOS regulator in Wales or Scotland?
The regulator follows the registered office of the responsible undertaking: Natural Resources Wales for Wales, the Scottish Environment Protection Agency for Scotland, the Northern Ireland Environment Agency for Northern Ireland and the Environment Agency for England.
The Environment Agency runs the guidance, the helpdesk and MESOS for all of them.
Are universities exempt from ESOS?
Not automatically.
Public bodies are generally excluded, but the Environment Agency says ESOS has been implemented on the basis that it applies to higher education institutions that self-declare as private sector and meet the other qualification requirements.
Sources
Primary sources
Every figure, date and status on this page traces to the instrument’s owner.
Secondary commentary is never the source for a number.
- legislation.gov.ukEnergy Act 2023, Part 11, section 256 — requirement for assessment of energy consumption
Defines an “ESOS assessment” and the power to require it.
- legislation.gov.ukThe Energy Savings Opportunity Scheme Regulations 2014 (SI 2014/1643)
Part 4 sets out the assessment chapter by chapter.
- legislation.gov.ukSI 2014/1643, Schedule 1 — the large undertaking test
Who must carry out an assessment.
- legislation.gov.ukSI 2014/1643, regulation 25C — energy intensity ratios
A ratio for each organisational purpose.
- legislation.gov.ukSI 2014/1643, regulation 31 — the responsible officer’s confirmation
What the director or equivalent confirms in the notification.
- legislation.gov.ukThe Energy Savings Opportunity Scheme (Amendment) Regulations 2023 (SI 2023/1182)
The 95% floor, intensity ratios, the ESOS report and action plans.
- legislation.gov.ukThe Energy Savings Opportunity Scheme (Amendment) Regulations 2026 (SI 2026/701)
The Phase 4 changes: routes, savings achieved, the action plan review.
- Environment AgencyHow to comply with ESOS phase 4
Sections 4 to 12: measurement, routes, audits, the report, sign-off and notification.
- GOV.UK (Environment Agency)Energy savings opportunity scheme (ESOS): find out if you qualify and how to comply
The eight steps in summary, the registers and the regulators; rewritten 2 September 2026.
- GOV.UKEnvironment Agency
The UK scheme administrator and the regulator for England.
- ISOISO 50001 — energy management
The standard behind the deemed compliance route.
- Department for Energy Security and Net ZeroGreenhouse gas reporting: conversion factors
The guidance points to the fuel properties tab for converting energy units into kWh.