What ESOS is and who must comply
GOV.UK describes ESOS as "a mandatory energy assessment scheme for organisations in the UK that meet the qualification criteria". It applies to large UK undertakings and their corporate groups, mainly businesses, but also not-for-profit bodies large enough to qualify (GOV.UK).
ESOS phase 4 qualification, from GOV.UK (checked 2 November 2026)
| Test | Rule |
|---|---|
| Qualification date | 31 December 2026 |
| Large undertaking | A UK company employing 250 or more people, or with turnover above £44 million and balance sheet total above £38 million |
| Groups | A corporate group qualifies if at least one UK member is a large undertaking |
| Overseas companies | UK registered establishments take part if their global group's UK activities qualify |
| Public sector | Usually does not need to comply |
| Notification deadline | 5 December 2027 |
Phase 4 dates and duties
- Total energy consumption: Calculate the energy used by buildings, industrial processes and transport
- Significant energy consumption: Identify the areas covering at least 95% of the total, to be audited
- Energy intensity ratios: For buildings, transport, industrial processes and other uses
- Route to compliance: An energy audit or ISO 50001 certification; DECs and Green Deal assessments no longer count
- Lead assessor: Sign-off by an ESOS lead assessor, unless an exemption applies
- Audit data: Verifiable consumption over a 12-month period beginning no earlier than 6 December 2022
- Board sign-off: By one board level director, then notify through the online system
- Action plans: Progress against action plan commitments must be included, with reasons where they weren't met
Source: GOV.UK. The regulator can issue civil sanctions, including financial penalties, where an organisation does not meet its obligations. Phase 3 participants also owe an annual progress update on their action plan by 5 December 2026.
What an ESOS audit recommends
An ESOS audit looks at where the organisation uses energy and identifies energy saving opportunities. The notification records the opportunities identified, and the scheme administrator publishes most of the information in it (GOV.UK). For a building with a large daytime electricity load and a suitable roof, on-site solar is one of the measures that can come up, alongside lighting, controls, heating and process efficiency. Phase 4 also lets participants include net zero considerations voluntarily, using the PAS 51215-1 and -2:2025 standards.
Acting on solar recommendations
If your audit points to solar, the next step is a site survey for each building that matters: the roof, the electricity supply, the network operator's limits and how much of the output the site will use. A survey turns a recommendation into a costed proposal your board can approve, and into progress you can report.
- Start with the biggest daytime users: Sites with the largest daytime electricity load usually give the best return
- Use your audit data: The same 12 months of consumption data sizes the system: see half-hourly data
- Check the paperwork: Leases, planning and the network operator, before committing
- Consider how to pay: Buying outright, finance, or a PPA with no upfront cost
See half-hourly data, solar PPAs, SECR and scope 2 emissions and choosing an installer.
Note: General information from GOV.UK, not compliance advice. Your ESOS lead assessor confirms what applies to you.
Sources
GOV.UK's ESOS page (updated 2 September 2026 for phase 4) was read on 2 November 2026. The full rules are in the ESOS Regulations 2014 as amended in 2026 and the phase 4 compliance guidance linked from it.
Frequently asked questions
What is ESOS?
The Energy Savings Opportunity Scheme: a mandatory energy assessment for large UK undertakings and their corporate groups. GOV.UK says qualifying organisations must carry out an assessment every 4 years, including an audit of the energy used by their buildings, industrial processes and transport.
Who has to comply with ESOS phase 4?
An organisation qualifies if, on 31 December 2026, it meets the ESOS definition of a large undertaking: a UK company that employs 250 or more people, or has an annual turnover above £44 million and an annual balance sheet total above £38 million. Corporate groups qualify if at least one UK group member is a large undertaking. Public sector organisations usually don't need to comply.
What is the ESOS phase 4 deadline?
GOV.UK says the deadline for submitting a notification of compliance for phase 4 is 5 December 2027. Phase 3 participants also have an annual progress update on their action plan due by 5 December 2026.
What changed for ESOS phase 4?
Display Energy Certificates and Green Deal assessments can no longer be used as compliance routes; progress against action plan commitments must be included in the assessment, with an explanation where commitments have not been met. Participants can also voluntarily include net zero considerations using the PAS 51215-1 and -2:2025 standards.
Does installing solar count towards ESOS?
Solar is not a route to compliance: you still need the assessment. But an ESOS audit identifies energy saving opportunities, and on-site generation is a measure many organisations consider alongside efficiency. Acting on the opportunities, and reporting progress against an action plan, is what phase 4 asks about.
Related pages
- Solar and carbon reportingHow on-site solar affects Scope 2 emissions, location-based and market-based reporting, SECR, carbon reduction plans, supplier questionnaires and REGOs, in plain terms.
- Carbon reduction plansBidders for larger central government contracts must publish a carbon reduction plan; on-site solar cuts the scope 2 emissions it reports.
- Energy data analysisHow half-hourly consumption data shapes solar sizing and payback: getting the data, profile classes, baseload, weekend and seasonal use, self-consumption and batteries.
- Climate Change LevyElectricity you generate and use on site carries no Climate Change Levy, so the levy is part of what each solar kWh saves.