ESOS
The Energy Savings Opportunity Scheme
Reviewed 10 August 2026by Julian Russell, Managing Director
Overview
What is ESOS?
The Energy Savings Opportunity Scheme is a mandatory energy assessment scheme for large UK organisations and corporate groups. Qualifying organisations must measure energy used by buildings, industrial processes and transport, identify significant consumption, assess energy saving opportunities and complete the required reporting.
The Energy Savings Opportunity Scheme (ESOS) is a mandatory energy assessment scheme for large UK organisations and corporate groups. Organisations that qualify must measure energy used by their buildings, industrial processes and transport, identify significant areas of consumption, assess energy saving opportunities and complete the required reporting.
ESOS operates in four year compliance periods. The immediate milestones are the second Phase 3 annual progress update on 5 December 2026 and the Phase 4 compliance deadline on 5 December 2027.
This guide explains how ESOS works, who is likely to qualify and what organisations should prepare. It is general information rather than legal advice, and our ESOS compliance service covers the assessment itself. Organisations close to a threshold, or with a complex group structure, should check the latest government guidance.
What does ESOS mean?
ESOS stands for the Energy Savings Opportunity Scheme. It was introduced through the Energy Savings Opportunity Scheme Regulations 2014 and is administered across the UK by environmental regulators.
The scheme is intended to help large organisations understand how they use energy and identify cost effective opportunities to reduce consumption, costs and carbon emissions. An ESOS assessment considers energy used by buildings, industrial processes, transport and other organisational energy uses.
Qualifying organisations usually need to complete an assessment every four years and notify the scheme administrator by the relevant deadline.
Who qualifies for ESOS?
ESOS mainly applies to large UK undertakings and to corporate groups containing at least one large UK undertaking. It can apply to businesses, not for profit bodies and other non public sector undertakings.
The Phase 4 thresholds are unchanged from Phase 3, and the guidance published on 22 July 2026 confirms them. A UK undertaking qualifies if, on the qualification date, it either:
- employed 250 or more people; or
- had both an annual turnover above £44 million and an annual balance sheet total above £38 million.
A corporate group qualified if at least one UK group member met the definition of a large undertaking. UK establishments of overseas companies could also be brought into scope through their wider UK corporate group.
The government decided not to proceed with a proposed change aligning Phase 4 qualification thresholds with Streamlined Energy and Carbon Reporting, so the two schemes continue to use different tests. Our qualification criteria guide works through the detail, including the group structures that catch people out.
ESOS Phase 4 qualification date
The Phase 4 qualification date is 31 December 2026. Whether an organisation qualifies is determined by its position on the qualification date, not simply by its size when the assessment is carried out.
Qualification can be more complicated where:
- the organisation is close to an employee or financial threshold;
- companies have joined or left a corporate group;
- acquisitions, disposals or insolvency proceedings have occurred;
- an overseas parent has several UK establishments;
- accounts cover an unusual period; or
- the organisation qualified in an earlier phase but has since changed size.
What changed for ESOS Phase 4?
The Energy Savings Opportunity Scheme (Amendment) Regulations 2026 came into force on 22 July 2026 and introduced several changes for the compliance period ending in 2027. Key changes include:
- reporting estimated energy savings achieved since the previous compliance deadline, in kWh;
- identifying the measures that produced those savings;
- reviewing measures recorded in the previous ESOS action plan;
- explaining why relevant action plan measures were not implemented;
- an additional, final annual progress update;
- removal of Display Energy Certificates and Green Deal Assessments as alternative compliance routes;
- updated treatment of ISO 50001 certified energy management systems;
- additional Lead Assessor notification responsibilities; and
- clearer provisions for organisations with zero energy consumption and certain insolvency situations.
Detailed Phase 4 guidance and changes to the online reporting system are expected before the 5 December 2027 compliance deadline. Monitor official updates rather than relying on older Phase 3 summaries.
ESOS deadlines
| Requirement | Relevant date |
|---|---|
| Phase 3 notification of compliance | 5 June 2024 |
| Phase 3 action plan | 5 December 2024 |
| First Phase 3 annual progress update | 5 December 2025 |
| Second Phase 3 annual progress update | 5 December 2026 |
| Phase 4 qualification date | 31 December 2026 |
| Phase 4 compliance deadline | 5 December 2027 |
ESOS action plans and annual progress updates
Phase 3 introduced an ongoing reporting cycle after the main assessment. An ESOS action plan sets out the measures the organisation intends to implement, the expected completion dates, the estimated energy savings and how those savings were estimated.
Annual progress updates report what happened during the following reporting periods. They record implemented measures and estimated savings, and explain changes where planned measures were not completed as expected. For the second Phase 3 annual progress update, the important date is 5 December 2026.
What are the routes to ESOS compliance?
ESOS compliant energy audits. The standard route is to measure total energy consumption, identify significant energy consumption and complete compliant audits covering the relevant assets and activities. A registered Lead Assessor normally reviews the assessment.
ISO 50001. A certified energy management system can provide deemed compliance with specified ESOS duties. For Phase 4 this can apply where the certified system covers either all of the participant's total energy consumption, or its significant energy consumption, meaning the areas representing at least 95% of the total. Where ISO 50001 covers only part of the organisation's energy use and does not meet one of those conditions, the remaining consumption is addressed separately.
Zero energy consumption. The 2026 amendment regulations introduced specific deemed compliance provisions for a participant whose calculated total energy consumption is zero kWh.
Display Energy Certificates and Green Deal Assessments are no longer alternative compliance routes for Phase 4.
What is an ESOS Lead Assessor?
An ESOS Lead Assessor is a competent energy professional who belongs to an approved professional register. For most assessments using the energy audit route, the Lead Assessor reviews the work and confirms that it meets the relevant ESOS requirements.
Our Lead Assessor guide explains what the role covers and when to appoint one. The assessor may be an appropriately registered employee or an external specialist. Involve them early enough to review the proposed scope, data and audit approach before the compliance deadline. The 2026 amendment regulations also require Lead Assessors to notify their approval body after completing an assessment.
What happens if an organisation does not comply?
Environmental regulators can investigate non compliance and impose civil sanctions, including financial penalties and publication penalties. The outcome depends on the breach and the regulator's enforcement approach.
An organisation that has missed a deadline should not assume the obligation has disappeared. Check the latest official guidance, complete outstanding work and contact the relevant regulator where appropriate.
Preparing for ESOS Phase 4
Organisations do not need to wait until 2027 to begin. Useful preparation includes checking the likely qualification position and corporate group boundary, reviewing the Phase 3 assessment and evidence pack, identifying gaps in energy data across buildings, transport and industrial processes, confirming a suitable 12 month reference period, deciding whether to use energy audits or ISO 50001, and scheduling Lead Assessor and board level review before the deadline.
Early preparation reduces deadline pressure and creates more time to act on worthwhile energy saving opportunities.
Why get certified
Key Benefits of ESOS
Legal Compliance
Meet mandatory ESOS requirements and avoid penalties of up to £50,000.
Cost Savings
Identify practical energy saving measures that reduce operational costs.
Carbon Reduction
Support your organisation's net-zero and sustainability objectives.
Strategic Insight
Gain a comprehensive understanding of your energy consumption profile.
How we work
Our Certification Process
Calculate total energy consumption
Energy consumed by assets held and activities carried out, normally covering buildings, industrial processes and transport over a suitable 12 month reference period. Data, methodology, assumptions and estimates are retained in the evidence pack.
Identify significant energy consumption
The assets and activities that collectively account for at least 95% of total energy consumption. This must be covered by a compliant audit or another applicable route. The remainder can fall within the permitted de minimis amount.
Calculate energy intensity ratios
Ratios relating consumption to a suitable measure of activity, such as energy per unit of production, floor area, employee or distance travelled, so performance can be compared over time.
Complete the required energy audits
Audits examine consumption patterns and identify practical savings, based on proportionate and representative data. Recommendations consider likely savings, implementation cost and payback. ESOS identifies opportunities, it does not require every measure to be implemented.
Prepare the ESOS report and evidence pack
The report records the principal findings and required information. The evidence pack holds the underlying data, calculations, audit records, decisions and approvals needed to demonstrate compliance if the regulator asks.
Obtain the required review and approval
A registered ESOS Lead Assessor reviews the assessment, and a responsible officer at board level reviews and approves the relevant information.
Notify the scheme administrator
The responsible undertaking submits the required notification through the government's Manage your ESOS reporting service by the applicable deadline.
Common questions
Frequently Asked Questions
Yes. ESOS is mandatory for UK organisations and corporate groups that meet the qualification criteria. It is not a voluntary certification scheme. Whether an organisation qualifies depends on the legal definitions, its size on the relevant qualification date and, where applicable, the structure of its corporate group.
Guides & insights
Related Resources
ESOS Phase 3 vs Phase 4: What Actually Changed
The compliance routes that were removed, the ISO 50001 exemption that widened, and the new reporting requirements introduced by the 2026 amendment regulations.
ESOS Lead Assessors: What They Do and When to Appoint One
What a registered ESOS Lead Assessor is, what they can and cannot sign off, and why appointing one early costs less than appointing one late.
What an ESOS Assessment Involves
The seven stages of an ESOS assessment, from calculating total energy consumption through to notifying the Environment Agency, and what each one requires.
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