Explainer
ESOS, the Energy Savings Opportunity Scheme, requires large UK organisations to audit their energy use every four years. Here is who qualifies, what Phase 4 requires, the key dates and the penalties.
By Christopher Fagan, Managing Partner, PEF Consulting Partners
The Energy Savings Opportunity Scheme is a mandatory energy assessment scheme for large undertakings in the UK. The Environment Agency administers it and regulates it in England, with Natural Resources Wales, SEPA and NIEA regulating in the devolved nations. Every four years, qualifying organisations must measure their total energy use, have it audited and identify cost effective ways to save energy. Since Phase 3 they must also publish an action plan and report progress against it.
You qualify if, on the qualification date of 31 December 2026, your organisation or any UK group member is a large undertaking: 250 or more employees, or annual turnover over £44 million and a balance sheet total over £38 million. If one UK group company qualifies, the whole UK group is in scope. The 2026 reforms kept these thresholds and did not add an energy use test.
| Date | What happens |
|---|---|
| 5 December 2026 | Second progress update on the Phase 3 action plan due |
| 31 December 2026 | Phase 4 qualification date |
| 5 December 2027 | Phase 4 compliance deadline: notify compliance through MESOS |
| 5 December 2028 | Phase 4 action plan due |
| 5 December 2029 and 2030 | Annual progress updates on the Phase 4 action plan, with a third update by 5 December 2031 under the reformed regulations |
Display Energy Certificates and Green Deal assessments no longer count towards compliance.
Failing to carry out an assessment can bring a penalty of up to £50,000 plus up to £500 for each working day, capped at 80 days. Failing to notify can bring up to £5,000 plus the same daily penalty, and the regulator can publish details of the breach.
ESOS is often treated as a compliance exercise, but the audit identifies savings that pay back. Use the same data for SECR and your carbon reduction plan, put the opportunities into an action plan with owners and budgets, and track the savings. That turns a four yearly audit into a cost reduction programme. The assessment itself must be reviewed by a lead assessor registered with an approved body; we work alongside your assessor on the data, the action plan and delivery. The government has also said it will consult on reforming ESOS and SECR later in 2026.
Related reading: carbon accounting and reporting in the UK, SECR reporting, cost optimisation and benefits realisation, ESG consultancy and compliance.
UK organisations that are large undertakings on the qualification date, or part of a UK group containing one: 250 or more employees, or turnover over £44 million and a balance sheet over £38 million.
Qualification is assessed on 31 December 2026 and compliance must be notified through MESOS by 5 December 2027. The Phase 4 action plan follows by 5 December 2028.
A competent energy professional registered with an approved body who reviews the assessment and confirms it meets the scheme requirements. Under the reformed rules the lead assessor must notify their approval body within seven days.
The regulator can impose a penalty of up to £50,000 plus daily penalties for failing to assess, up to £5,000 plus daily penalties for failing to notify, and can publish the breach.
If ESOS Phase 4 is coming up, start the data work now. The organisations that leave it to the last quarter pay more and save less.