The dates that matter
The Energy Savings Opportunity Scheme (ESOS) is now in its fourth phase, and the timeline is the first thing every in-scope organisation should pin down. The Phase 4 qualification date is 31 December 2026, the day your status is assessed, and the notification of compliance deadline is 5 December 2027. Notification is made through the Environment Agency's MESOS online system. After notifying, you publish an action plan and then report delivery through two annual progress updates in the years that follow; DESNZ is expected to confirm the exact Phase 4 progress-update dates in updated guidance.
Who qualifies
You are a large undertaking in scope for ESOS if, on the 31 December 2026 qualification date, you employ 250 or more people, or you have an annual turnover above £44 million and a balance sheet above £38 million. The employee threshold on its own is enough; the financial test requires both figures to be exceeded. If the highest UK parent of a corporate group qualifies, the whole UK group is normally in scope and can comply together. The first piece of evidence to keep is simply the headcount and financial figures you used, and the date they relate to.
What has actually changed: the action plan
The biggest shift from earlier phases is that ESOS no longer ends at notification. Since Phase 3, participants must produce an action plan setting out the energy-saving measures they intend to implement, signed off by a board-level director and submitted via MESOS. Delivery against that plan is then reported in two annual progress updates. In practice this turns ESOS from a four-yearly audit into an ongoing cycle of commitment and accountability. The measures you list are tracked, and any you do not deliver have to be explained.
The 95% rule and your AoSEC
ESOS does not require you to audit every kilowatt-hour. You identify your areas of significant energy consumption (AoSEC), the buildings, transport and processes that together make up at least 95% of total energy use, and audit those. The remaining 5% or less can be excluded as de minimis. The 2023 ESOS amendment raised this threshold from 90% to 95%, so it applies from Phase 3 onwards. Defining it well matters: too narrow and you fall below the 95% threshold; too broad and you spend audit effort on immaterial sites. Your assessment must then be signed off by an approved ESOS Lead Assessor before you notify.
How to prepare now
Start by confirming whether you qualify and gathering 12 months of energy data that includes the qualification date. Decide your compliance route (ESOS energy audits, or ISO 50001 certification covering the same energy) and line up a Lead Assessor early, because assessor availability tightens as the deadline approaches. Crucially, plan for the action plan and progress-update cycle before you notify, not after. Organisations that treat the action plan as a live document, recording delivery as it happens, find the later progress updates straightforward rather than a scramble.