What an ESOS Assessment Involves

An ESOS assessment is a structured piece of work with a defined output. It is not an energy survey, and it is not a certification. It is an audit of how your organisation uses energy, reviewed by a registered professional, approved at board level and notified to a regulator.

This is what it involves.

1. Establish the participant and the boundary

Before any measurement, you need to know who is being assessed. That means the participating organisation or corporate group, which entities are inside the boundary, and who the responsible undertaking is for notification purposes.

For a single company this is straightforward. For a group, it rarely is, and getting it wrong at this stage invalidates work done later.

2. Calculate total energy consumption

You measure all energy consumed by assets you hold and activities you carry out, across three areas:

• Buildings, including electricity, gas and other heating fuels
• Industrial processes, where applicable
• Transport, including company vehicles and business travel in vehicles you control

This is done over a twelve-month reference period, and the data, methodology, assumptions and any estimates go into your evidence pack. Estimates are permitted where measurement is not possible, but the basis has to be recorded.

Transport is where organisations most often under-report, usually because fuel cards and mileage claims sit with finance rather than with whoever is compiling the assessment.

3. Identify significant energy consumption

You then identify the assets and activities that together account for at least 95% of total energy consumption.

This significant energy consumption must be covered by a compliant energy audit, or by another applicable route to compliance. The remaining 5% can fall within the permitted de minimis amount and does not need auditing.

The 95% figure is worth stating precisely, because older guidance and some consultancy websites still say 90%. That was the earlier threshold and it is out of date.

4. Calculate energy intensity ratios

Energy intensity ratios relate consumption to a measure of activity: energy per unit of production, per square metre, per employee, per mile travelled.

Absolute consumption on its own tells you little, because a busy year uses more energy than a quiet one without anything having got worse. Ratios let you compare across years and sites, and they are what makes the assessment useful internally rather than only for compliance.

5. Complete the energy audits

The audits examine patterns of consumption and identify practical energy saving opportunities, based on data that is appropriate, proportionate and sufficiently representative.

Recommendations should set out the likely energy and cost savings, the implementation cost and the payback period. ESOS identifies opportunities. It does not generally require you to implement them, though Phase 3 introduced action plans and progress reporting, so what you said you would do is now on the record.

6. Lead Assessor review and board approval

A registered ESOS Lead Assessor reviews the assessment and confirms it meets the requirements. A responsible officer, normally a board-level director or equivalent, must also review and approve the relevant information.

Both are required. An assessment that has not been through both is not complete, and the board-level approval in particular tends to be left until the last fortnight.

7. Notify the scheme administrator

The responsible undertaking submits the notification through the government's Manage your ESOS reporting service by the compliance deadline, which for Phase 4 is 5 December 2027.

The evidence pack

Running alongside all of this is the evidence pack: the underlying data, calculations, audit records, decisions and approvals that demonstrate how you complied.

It is not submitted with the notification. It exists so that you can produce it if the regulator asks, and organisations that assemble it as they go find the exercise considerably less painful than those who reconstruct it afterwards.

Alternative routes

Not every organisation uses the energy audit route.

A certified ISO 50001 energy management system covering your total energy consumption, or the significant energy consumption representing at least 95% of the total, provides deemed compliance with specified duties. See our ISO 50001 guide.

The 2026 amendment regulations also introduced provisions for participants whose calculated total energy consumption is zero kWh.

Display Energy Certificates and Green Deal Assessments are no longer alternative compliance routes for Phase 4.

Where to go next

Check whether you are in scope with our qualification guide, or see the key dates. For help running the assessment, see our ESOS compliance services.