ESOS Compliance UK: Phase 3 Requirements and Submission Guide

The Energy Savings Opportunity Scheme places a legal obligation on large UK enterprises to audit their energy use and identify cost-effective savings opportunities. Unlike voluntary reporting frameworks, ESOS compliance is mandatory for organisations that meet the qualification thresholds, and failure to comply attracts civil financial penalties administered by the Environment Agency.

Phase 3 of the scheme runs from 6 December 2023 to 5 December 2027, with a compliance notification deadline of 5 June 2027. Organisations that completed Phase 2 audits but have since changed in size, structure, or energy profile need to reassess their obligations from the start of Phase 3.

Which organisations must comply with ESOS

ESOS applies to large UK undertakings and their corporate groups. An organisation qualifies if, on the qualification date (the last day of the relevant compliance period), it meets either of the following size thresholds:

• More than 250 employees, or
• An annual turnover exceeding EUR 50 million (approximately £43 million at 2024 exchange rates) AND an annual balance sheet total exceeding EUR 43 million

The qualification date for Phase 3 is 31 December 2026. Organisations that were large on that date must comply. The assessment covers the entire corporate group, not just a single entity, so a parent company is responsible for ensuring that the group audit covers all UK undertakings within scope.

Subsidiary companies, joint ventures, and overseas-owned entities with UK operations may all fall within scope depending on their relationship to the qualifying undertaking. ESOS regulations set out specific rules on group structures that organisations with complex corporate arrangements should review with their lead assessor.

The 4-year audit cycle and what it must cover

ESOS requires a comprehensive energy audit covering all significant energy consumption across the organisation's UK activities. The audit must address:

• Buildings: energy used in all UK premises, including rented, leased, and owned properties
• Industrial processes: any energy used in manufacturing, processing, or other operational activity
• Transport: energy consumed by vehicles used for business purposes, including company fleet and grey fleet

The audit must account for at least 90% of total energy consumption across these three areas. If a route to compliance covers less than the full estate, the organisation must demonstrate that the uncovered portion falls within the 10% de minimis allowance.

Energy consumption data for the 12-month reference period must be collected and verified. The reference period is typically the most recent 12 months of available data before the compliance notification deadline, though organisations may use an earlier period if they can justify it.

Routes to compliance

ESOS allows organisations to demonstrate compliance through several routes. Not all organisations need to commission a standalone ESOS audit from scratch:

• ESOS energy audit: a full audit conducted or supervised by a qualified lead assessor, covering all areas of significant energy consumption
• ISO 50001 certification: an organisation holding a current, valid ISO 50001 energy management system certification can use this as a route to compliance, provided it covers at least 90% of energy consumption
• Display Energy Certificates: for buildings only, a valid DEC can satisfy the buildings element of the audit
• Green Deal assessments: valid assessments may be used for the buildings element where applicable

Most organisations without existing ISO 50001 certification will use the ESOS energy audit route. The lead assessor signs off the audit and takes professional responsibility for its adequacy.

Lead assessor requirements and responsibilities

Every ESOS audit must be led or signed off by a qualified lead assessor. The lead assessor must hold one of the qualifications or memberships approved by the Environment Agency, which include:

• Membership of an approved professional body at the relevant grade (including the Energy Institute, CIBSE, IMechE, and others listed in the Environment Agency's approved list)
• Holding an approved ESOS-specific qualification

The lead assessor does not need to be an employee of the organisation. Many businesses engage external energy consultancies whose staff hold the required credentials. However, the organisation remains legally responsible for ensuring the audit is compliant, regardless of who carries it out.

Lead assessors must not sign off an audit where they have a conflict of interest. The Environment Agency has issued guidance on independence requirements. Organisations should verify that their chosen lead assessor appears on the published list of recognised professional bodies before commissioning work.

The notification process and ESOS portal

Compliance is notified to the Environment Agency through the ESOS online notification system. The responsible officer (a director or equivalent senior individual) must submit the notification confirming that the organisation has completed its audit and identified energy-saving opportunities.

The notification must include:

• The organisation's details and Companies House number
• Details of the lead assessor, including their qualification and professional body membership number
• Confirmation of the routes to compliance used
• The reference period for the energy data
• A statement that the audit covers at least 90% of energy consumption

Notifications for Phase 3 must be submitted by 5 June 2027. The Environment Agency has historically allowed a short grace period past formal deadlines in cases of genuine administrative difficulty, but this is not guaranteed and should not be relied upon.

Civil penalties for non-compliance

The Environment Agency enforces ESOS compliance and can impose civil penalties on organisations that fail to comply. The penalty structure under the Energy Savings Opportunity Scheme (Amendment) Order 2017 includes:

• Up to £50,000 for failing to carry out a compliant audit
• Up to £5,000 per day for continuing non-compliance after a compliance notice is issued
• Up to £5,000 for providing false or misleading information
• Up to £500 per day for failing to keep adequate records

The Environment Agency publishes enforcement notices and penalty decisions. Phase 1 and Phase 2 enforcement actions have been taken against named organisations, and the register of enforcement actions is publicly accessible. Reputational exposure is therefore a consideration alongside the financial penalty.

Record-keeping obligations

Organisations must retain all evidence used to support their ESOS compliance for a minimum of six years from the compliance notification date. Records that must be kept include:

• Energy consumption data and the source documents (utility bills, meter readings, fleet fuel records)
• The audit reports and any supporting calculations
• Evidence of the lead assessor's qualifications
• Documentation of the routes to compliance used
• Evidence that the responsible officer reviewed and approved the notification

The Environment Agency may request evidence at any point during the six-year period. Organisations that cannot produce adequate records may face penalties even if their underlying audit was compliant.

Changes introduced in Phase 3

ESOS Phase 3 introduced several changes relative to Phase 2. The most significant are:

• The qualification date shifted from 31 December 2022 (Phase 2) to 31 December 2026 (Phase 3)
• Updated guidance on covering transport energy, including grey fleet, was clarified
• The Environment Agency increased its audit and investigation activity following Phase 2 compliance experience
• ISO 50001 as a compliance route remained available but organisations using it must ensure their certification scope covers the required 90% threshold

The Department for Energy Security and Net Zero (DESNZ) and the Environment Agency publish updated technical guidance on the gov.uk ESOS pages. Organisations should not rely on guidance documents produced for Phase 1 or Phase 2 without checking for Phase 3 updates.

Frequently asked questions

Editorial disclaimer: This article explains the ESOS framework as set out in legislation and Environment Agency guidance and does not constitute legal or compliance advice for any specific organisation.

Does ESOS apply to charities and public sector bodies?

ESOS applies to large UK undertakings as defined by the regulations, which primarily targets private sector businesses. Public bodies are generally subject to separate energy reporting obligations under the Greening Government Commitments framework. However, charities that operate commercial subsidiaries meeting the size thresholds may need to assess whether those entities fall within scope. Legal advice is appropriate for complex group structures.

What counts as "significant energy consumption" for audit purposes?

The Environment Agency's guidance defines significant energy consumption by reference to the 90% threshold: the audit must cover enough areas of use to account for at least 90% of total UK energy consumption. Organisations typically prioritise the largest consuming activities first - large buildings, heavy process plant, and high-mileage fleet - and may exclude minor energy uses that in aggregate fall within the 10% de minimis allowance.

Can the same lead assessor cover multiple group companies?

Yes. A qualified lead assessor can sign off an ESOS audit covering an entire corporate group, provided they have access to sufficient data from all group entities and the audit genuinely covers each entity's energy consumption. Group audits are common in large organisations and can reduce overall compliance costs compared to commissioning separate assessments for each subsidiary.

What happens if an organisation's size falls below the threshold mid-phase?

The qualification test is assessed on the qualification date (31 December 2026 for Phase 3). If an organisation was large on that date, it must comply regardless of subsequent size changes. If it was not large on that date - for example, because it had disposed of a business unit - it does not qualify for Phase 3, even if it was large during earlier parts of the phase period.

Is there any obligation to implement the energy savings identified?

ESOS does not legally require organisations to implement any of the energy-saving measures identified in the audit. The obligation is to conduct the audit and notify the Environment Agency. However, DESNZ has signalled interest in strengthening implementation requirements in future phases, and the audit findings are increasingly relevant to voluntary sustainability reporting and net-zero commitments.

How we verified this

This article draws on the ESOS regulations (The Energy Savings Opportunity Scheme Regulations 2014 and subsequent amendments) as published on legislation.gov.uk, the Environment Agency's ESOS guidance pages at gov.uk, and DESNZ guidance on Phase 3 requirements. Penalty figures reflect the Energy Savings Opportunity Scheme (Amendment) Order 2017. No aggregator or consultancy-produced content was used as a primary source.

Sources

• Environment Agency: ESOS guidance - gov.uk
• Energy Savings Opportunity Scheme Regulations 2014 - legislation.gov.uk
• ESOS Amendment Order 2017 - legislation.gov.uk
• ESOS document collection - gov.uk

For more on energy auditing requirements, see secr-reporting-uk-guide and business-energy-audit-uk.