The Energy Savings Opportunity Scheme is now in its fourth phase, with a compliance deadline of 5 December 2027. For large UK companies that qualify, that date may feel distant — but the experience of Phase 3 shows that organisations that leave assessment to the final months face significant difficulties finding accredited Lead Assessors and risk non-compliance. Here is what you need to know now.
What is ESOS and who qualifies?
ESOS is a mandatory energy assessment scheme for large UK undertakings. You qualify if your organisation meets at least one of the following thresholds: 250 or more employees, or annual turnover exceeding £44 million and an annual balance sheet total exceeding £38 million. Qualification is assessed at group level — if your parent company qualifies, all UK subsidiaries are brought into scope.
Once in scope, your organisation must carry out energy audits covering at least 90% of total energy consumption across buildings, transport, and industrial processes, and submit a compliance notification to the Environment Agency by the deadline.
What has changed in Phase 4?
Phase 4 introduces several important updates from Phase 3. The qualification thresholds have been updated to align with the Companies Act large company definition. The compliance route via ISO 50001 certification remains available — organisations with a certified energy management system covering their full energy footprint can use this as an alternative to energy audits.
The biggest change in Phase 4 is the stronger expectation of alignment with net zero commitments. The Environment Agency has signalled that ESOS action plans should demonstrate how energy efficiency measures connect to the organisation's broader decarbonisation pathway. This is not yet a hard requirement but organisations that treat ESOS as a pure compliance exercise — rather than as a foundation for their net zero strategy — are likely to find the scheme's requirements tightening further in Phase 5.
The Lead Assessor requirement
Every ESOS assessment must be led and signed off by an accredited ESOS Lead Assessor. Lead Assessors are accredited by approved bodies including Sterling Accreditation, CIBSE, and the Energy Institute. They are personally responsible for the quality of the assessment and the compliance notification.
Finding a qualified Lead Assessor with available capacity is consistently the bottleneck in ESOS compliance. In the final months before the Phase 3 deadline, many organisations found that Lead Assessors were fully booked. Phase 4 is likely to follow the same pattern — organisations that engage a Lead Assessor early will have more time, more flexibility, and better quality outcomes.
What a good ESOS Phase 4 assessment covers
A compliant ESOS assessment is not simply a list of energy-saving measures. It should cover:
Buildings: a detailed audit of energy consumption across your property portfolio, identifying the measures available to reduce consumption with associated costs, savings, and payback periods.
Transport: an assessment of energy used in business travel — grey fleet, company vehicles, HGVs, rail, and flights — with recommendations for modal shift, electrification, and efficiency improvements.
Industrial processes: where applicable, an assessment of energy used in manufacturing, data centres, or other operational processes.
The assessment must be supported by 12 months of actual energy consumption data. Estimated or modelled data is not acceptable without clear justification.
What to do now
The most important action is to establish whether your organisation qualifies for Phase 4 and, if so, to appoint a Lead Assessor early. A good Lead Assessor will help you structure the data collection process, identify the most cost-effective audit scope, and ensure the compliance notification is accurate and complete.
Organisations that also want to use ESOS as the foundation for their net zero strategy — rather than treating it as a compliance burden — should ask their Lead Assessor to align the audit findings with their GHG Protocol carbon footprint and CRREM pathway analysis. This produces a richer, more actionable output at minimal additional cost.
The Phase 4 deadline is 5 December 2027. Starting now gives you over two years — enough time to do this properly. Get in touch to discuss our ESOS Phase 4 Lead Assessor service.