ESOS energy audit requirements

What an ESOS energy audit is

An ESOS energy audit is a systematic assessment of an organisation's total UK energy consumption, identifying cost-effective opportunities to improve energy efficiency. The requirement is established in the Energy Savings Opportunity Scheme Regulations 2014 (SI 2014/1643) and updated by the ESOS Amendment Regulations 2023 (SI 2023/1182).

Unlike voluntary energy assessments, an ESOS audit has specific mandatory content requirements, must be conducted or approved by an accredited lead assessor, must achieve a minimum coverage threshold, and must receive board-level approval before the compliance notification is submitted to the Environment Agency.

Failure to complete the audit exposes the organisation to civil penalties of up to £90,000 under the Regulatory Enforcement and Sanctions Act 2008.

ESOS audits must be completed every four years, aligned to the ESOS compliance cycle.

Phase 4 covers the period from 6 December 2023 to 5 December 2027, with the compliance deadline of 5 December 2027 for notification to the Environment Agency.

The 95% coverage requirement

ESOS Phase 4 requires energy audits to cover at least 95% of the organisation's total UK energy consumption.

This replaces the earlier 10% de minimis threshold (which allowed 10% to be excluded) with a stricter 5% de minimis, meaning no more than 5% of total energy consumption may be excluded from audit coverage.

The 95% threshold applies to the organisation's total UK energy consumption, not just its "significant energy consumption."

Organisations must first calculate their total energy consumption, then identify which assets and activities make up at least 95% of that total — these are the "areas of significant energy consumption" that must be audited or covered by an alternative compliance route such as ISO 50001.

Buildings, transport, and processes

ESOS audits must cover all three energy consumption categories across UK operations:

Buildings

All properties occupied or managed by the organisation that contribute to UK energy consumption.

This includes head offices, regional offices, warehouses, manufacturing facilities, retail premises, and any other buildings where the organisation controls energy use.

Leased buildings where the organisation pays energy bills are included; landlord-controlled energy costs may be excluded where the tenant has no control.

Transport

All transport energy used for organisational purposes — road vehicles (cars, vans, HGVs), rail, air, and maritime where the organisation controls or funds the energy use. DESNZ guidance

provides specific instructions on how to treat grey fleet (employee-owned vehicles used for business travel), hired vehicles, and company cars versus pool vehicles.

Transport energy can be expressed using separate freight and passenger intensity ratios, or combined if the organisation has both.

Industrial processes

Manufacturing equipment, production lines, data centre infrastructure, and any other energy-consuming processes beyond buildings and transport.

For organisations with significant process energy (manufacturing, food production, data centres), process energy typically represents the largest component of the 95% coverage requirement.

Lead assessor requirements

All ESOS energy audits in Phase 4 must be conducted or approved by a certified lead assessor on an approved register. DESNZ guidance

specifies that approved registers include the Chartered Institution of Building Services Engineers (CIBSE) and the Energy Institute (EI).

• Lead assessors must hold current accreditation on the CIBSE or Energy Institute approved registers
• MICIBSE (Member of CIBSE) or TIET (Technician in Energy and the Environment) grades are typically required
• Lead assessors must have completed applicable continuing professional development (CPD) — Phase 4 strengthened CPD requirements versus Phase 3
• The lead assessor must sign off the ESOS assessment and can be an employee of the qualifying organisation or an external consultant
• Lead assessors employed internally must be demonstrably independent from the energy activities being assessed

Phase 4 introduced enhanced lead assessor competency requirements through the ESOS Amendment Regulations 2023.

Organisations should verify that their lead assessor's accreditation is current and that they meet the updated Phase 4 competency framework before commissioning the audit.

Data requirements

ESOS audits must be based on 12 consecutive months of verifiable energy consumption data.

The reference period must include the qualification date of 31 December 2026 and must end before the compliance deadline of 5 December 2027.

In practice, this means the reference period can start as early as 1 January 2026.

Verifiable data sources for ESOS include:

• Utility invoices and smart meter readings for electricity and gas
• Fuel purchase records and expense data for transport fuels
• Automatic Meter Reading (AMR) data from building energy management systems
• Travel expense data and fleet management systems for transport
• Sub-metering data for process energy where available
• Estimates supported by documented calculations where primary data is unavailable

Where data gaps exist, estimates are permitted but must be supported by documented methodology and calculations.

The DESNZ Phase 4 guidance sets out specific requirements for data quality and estimation approaches.

Conversion factors for energy content (kWh equivalents) must be stated in the assessment.

What the ESOS audit must contain

The ESOS energy audit report must contain specified content elements established in SI 2014/1643 as amended by SI 2023/1182:

• Total energy consumption for the reference period, broken down by energy type (electricity, gas, transport fuels, other)
• Identification of areas of significant energy consumption (the 95%+ subset)
• For each area of significant energy consumption: analysis of energy use patterns, identification of efficiency improvement opportunities, and cost-benefit assessment of recommended measures
• Energy intensity ratios — at least one metric expressing energy consumption relative to business output (e.g. kWh per £m revenue, kWh per tonne product, kWh per square metre)
• Prioritised recommendations for energy efficiency measures, including estimated payback periods
• Progress against Phase 3 action plan recommendations — a new Phase 4 requirement under SI 2023/1182
• Information on how identified measures will be implemented (the action plan)

The standardised template for ESOS compliance notifications, introduced by the 2023 Amendment Regulations, requires organisations to include more detailed compliance information than in earlier phases.

This includes the energy intensity ratio, data quality information, and action plan commitments.

Action plan requirement — new in Phase 4

From Phase 4, all qualifying organisations must develop an ESOS action plan identifying energy efficiency measures and implementation timelines.

This was introduced by SI 2023/1182 Part 6A and represents the most significant change from earlier phases.

The action plan must include specific energy efficiency measures identified in the audit, estimated energy savings and implementation costs for each measure, timelines for implementation, and evidence of board-level approval. Annual progress updates against the action plan must be submitted — organisations cannot simply complete the audit and forget about it until Phase 5.

The submission deadlines, MESOS process, and public disclosure rules are covered in our ESOS action plan guide.

The Phase 4 action plan deadline is 5 December 2028 — one year after the compliance notification deadline.

This gives organisations time after notification to formalise their action plans, though the board-approved action plan must exist before the compliance notification is submitted.

Alternative compliance routes in Phase 4

Phase 4 offers two compliance routes for the energy audit requirement.

Organisations must choose one route that covers at least 95% of total UK energy consumption:

Route 1: ESOS energy audit

The standard route — a full energy audit by a certified lead assessor covering at least 95% of UK energy consumption.

This is the route most organisations will use.

The audit must meet all content requirements and be signed off by an accredited lead assessor before board approval and compliance notification.

Route 2: ISO 50001 certification

Organisations holding a UKAS-accredited ISO 50001 energy management certification covering 100% of their UK energy consumption can use ISO 50001 as their ESOS compliance route.

The ISO 50001 certificate must be valid at the Phase 4 compliance date of 5 December 2027.

Under this route, no separate ESOS energy audit is required, but the organisation must still submit an ESOS compliance notification and a summary report showing energy usage — board sign-off and notification remain mandatory.

For partial ISO 50001 coverage (less than 100% of UK energy), a combined approach is possible: ISO 50001 evidence covers the certified portion, and a standard ESOS audit covers the remaining energy consumption up to the 95% threshold.

Removed routes — DECs and Green Deal Assessments

Display Energy Certificates (DECs) and Green Deal Assessments (GDAs) were removed as standalone ESOS compliance routes by SI 2023/1182.

These routes were available in Phases 1, 2, and 3 but are no longer valid for Phase 4.

Organisations that relied on DECs or GDAs for all or part of their Phase 3 compliance must now adopt either the standard ESOS energy audit route or ISO 50001 certification.

DEC and GDA evidence from Phase 3 may still be used as supporting data to inform the Phase 4 audit, but cannot satisfy the audit requirement as standalone compliance evidence.

Board sign-off requirement

Board-level sign-off is a mandatory element of ESOS Phase 4 compliance.

A director or equivalent senior officer must approve the ESOS assessment and action plan, and be named in the compliance notification submitted to the Environment Agency.

This requirement exists under SI 2014/1643 as strengthened by Phase 4 guidance.

Board sign-off serves two functions: it ensures senior-level accountability for the energy audit findings and action plan, and it creates a formal governance record that the organisation's leadership has reviewed the energy compliance position.

The Environment Agency may request evidence of board approval during enforcement activity.

For organisations with complex group structures, the responsible undertaking (typically the ultimate UK parent) provides board sign-off for the group ESOS compliance.

Subsidiary companies within the group are not required to provide separate board sign-off — though they must receive a copy of the ESOS report under the Phase 4 reporting sharing requirements introduced by SI 2023/1182.

5 December 2027 deadline

All ESOS Phase 4 obligations — energy audit completion, action plan development, board approval, and compliance notification to the Environment Agency — must be fulfilled by 5 December 2027.

Notification is made through the MESOS (Manage your ESOS) digital service.

Site energy audits can begin in 2026 — organisations do not need to wait until the qualification date of 31 December 2026 before starting audit work.

Starting early reduces the risk of capacity constraints in the lead assessor market as the deadline approaches, spreads costs across the compliance period, and enables energy savings to be implemented and measured before the deadline.

For context on penalties for missing the deadline, see the ESOS penalties and enforcement guide.

The Environment Agency has actively enforced Phase 3 non-compliance and is expected to maintain this approach for Phase 4.

Frequently asked questions