ISO 50001 vs ESOS · regulation 33
ISO 50001 and ESOS: what a certificate deems, and what it never does
An ISO 50001 energy management system is the one alternative to the ESOS energy audit in Phase 4, under regulation 33 of the ESOS Regulations 2014.
From 22 July 2026 a certificate covering all of a participant’s total or significant energy deems three duties: the lead assessor, the audit and the report.
It never deems the notification, and the action plan still follows.
What it deems
Three duties deemed, two never touched
Regulation 33 is headed “deemed compliance”, and the word matters: the certificate does not exempt the participant from ESOS, it stands in for particular duties.
Since regulation 24 of SI 2026/701, those duties are the lead assessor, the audit, and the ESOS report, which is what makes the Phase 4 route stronger than Phase 3’s.
The same amendment removed the duty to carry out an ESOS assessment from what the route deems; only the zero-energy route under regulation 33A deems that.
The Explanatory Note puts it in one sentence: a participant whose total or significant consumption is covered is deemed to have complied with the duty to appoint a lead assessor, carry out an ESOS audit and produce an ESOS report.
The audit it replaces is described on ESOS energy audits.
| Duty | Deemed by the ISO route? |
|---|---|
| Appoint a lead assessor (reg 21) | Yes |
| Carry out the energy audit (Part 4 Ch 3) | Yes |
| Produce the ESOS report (Part 4 Ch 3A) | Yes — new in Phase 4 |
| Carry out an ESOS assessment (reg 20) | No |
| Notify compliance (reg 29) | No |
| Action plan and progress updates (Part 6A) | No |
The test
Total or significant, not 100%
The trigger is all of the total, or all of the significant, consumption falling under the certified system.
Significant consumption is defined by regulation 25(2): assets and activities that together account for not less than 95% of the total, measured in energy units or by energy spend.
Identifying significant consumption is the participant’s choice; one that does not make it needs the certificate to cover everything.
So the arithmetic, not the category, decides it: a certificate that covers buildings only is enough if buildings are at least 95% of the energy, and not enough otherwise.
The “100% of energy supplies” test that circulated for years belonged to Phase 3; the GOV.UK overview has stated the Phase 4 test correctly since its rewrite on 2 September 2026.
“Paragraph 3 applies where all of the participant’s— (a) total energy consumption, or (b) significant energy consumption falls under the certified energy management system.”
“Paragraph 3” is the instrument’s own drafting, for paragraph (3).
Mixing routes
Partial certification: the rest is audited
Regulation 33(2) now applies “in relation to any part of” the participant’s energy that falls under the system, so a partial certificate still counts for its part.
The part it covers is deemed compliant; the part it does not is audited, and a lead assessor is appointed for that audit.
Together the two must cover all of the total, or all of the significant, consumption.
The notification then carries both sets of information: the ISO route’s tables and the audit’s.
The step order for a mixed route is on ESOS compliance.
Still owed
What the ISO route leaves you to do
A certificate removes the audit work for the energy it covers. It removes almost nothing else.
| Still required | Provision |
|---|---|
| Calculate total energy consumption, and significant consumption if elected | regs 22, 25 |
| An energy intensity ratio for each organisational purpose | reg 25C |
| Notify by 5 December 2027, including Tables A, C, F, G, J and K of Schedule 3 | reg 29(1)(ad) |
| Responsible officer confirmation, now covering the certified system relied on | reg 31(b), as amended |
| An evidence pack | reg 28 |
| An action plan by 5 December 2028, then three progress updates | Part 6A |
Table J, which the ISO route notifies too, includes the energy savings achieved in the compliance period, and its combined figure is published.
The action plan duty follows any notification, so an ISO participant owes one exactly as an audited one does; see the ESOS action plan.
The certificate
Which certificates count, and from whom
Regulation 33(4)(c) defines ISO 50001 as the 2011 or the 2018 edition; a later edition would not be within it until the Regulations are amended.
ISO publishes the standard; certification is done by certification bodies, and ISO itself does not certify anyone.
The certifier must be accredited by the United Kingdom Accreditation Service, by a national accreditation body of an EU member state, or by a member of the International Accreditation Forum.
A self-declared or unaccredited certificate is outside the regulation.
The Environment Agency’s Phase 4 guidance says the certificate must have been issued on or after the start of the compliance period, 6 December 2023, and must remain valid on 5 December 2027.
The GOV.UK overview puts the start as “after 5 December 2023”, which is the same set of days.
| Requirement | Rule |
|---|---|
| Edition | ISO 50001:2011 or ISO 50001:2018 — reg 33(4)(c) |
| Certifier | Accredited by UKAS, an EU national accreditation body, or an IAF member — reg 33(4)(a) |
| Issued | On or after 6 December 2023 — EA guidance |
| Valid | On the compliance date, 5 December 2027 — EA guidance |
Side by side
ISO 50001 vs the ESOS audit, for Phase 4
| ESOS energy audit | ISO 50001 route | |
|---|---|---|
| What it is | A one-off audit of the significant (or total) consumption | A certified, continuing energy management system |
| Lead assessor | Required, from an approved register | Not required for the covered energy |
| Audit content (reg 27) | Required: costs, benefits, payback, programme | Deemed |
| ESOS report | Required | Deemed (new in Phase 4) |
| Site visits and audit record | Required (reg 26(3A), (9)) | Not required for the covered energy |
| Notification by 5 Dec 2027 | Required | Required |
| Responsible officer | One or two, by the lead assessor’s independence | Confirmation includes the certified system |
| Action plan and three updates | Required | Required |
The route suits an organisation that already runs a certified system across most of its energy, since the certificate must be valid on 5 December 2027.
ISO’s own page is clear that certification to ISO 50001 is possible but not obligatory; for ESOS, only a certified system counts.
A participant with a building-only certificate and a large fleet usually ends up mixing the two routes.
What changed
From Phase 3 to Phase 4, for the ISO route
In Phase 3 the certificate had to cover total energy consumption, and it did not deem the report.
SI 2026/701 widened the trigger to total or significant, added partial cover, and brought the report within the deeming.
It also closed the other alternatives, Display Energy Certificates and Green Deal assessments, which is why ISO 50001 is now the only alternative to the audit; see ESOS Phase 3 vs Phase 4.
Whether you are in Phase 4 at all is the test on ESOS Phase 4 thresholds, and the scheme as a whole is on the ESOS hub.
The Phase 4 guidance treats the route in its section 9, and the approved registers for any audit that is still needed are on the GOV.UK ESOS overview.
The 2014 base text is the revised ESOS Regulations, and the Phase 3 amendments are SI 2023/1182.
Frequently asked
ISO 50001 and ESOS: questions people ask
Can ISO 50001 replace the ESOS energy audit?
Yes, for the energy it covers.
Under regulation 33 as amended from 22 July 2026, where all of a participant's total or significant energy consumption falls under a certified ISO 50001 system, the participant is deemed to have complied with the duties to appoint a lead assessor, carry out the energy audit and produce the ESOS report.
Where the certificate covers only part, it deems compliance for that part and the rest is audited.
Does ISO 50001 have to cover 100% of energy for ESOS?
No. From Phase 4 it must cover all of either the total or the significant energy consumption.
Significant consumption is the assets and activities making up not less than 95% of the total, identified by the participant if it chooses.
What does the ISO 50001 route not remove?
The notification of compliance by 5 December 2027, with its Schedule 3 tables and the responsible officer's confirmation; the energy intensity ratios; the evidence pack; and the action plan and progress updates that follow any notification.
It also never deems the duty to carry out an ESOS assessment as such, which only the zero-energy route does.
Which ISO 50001 certificates count for ESOS?
A certificate to ISO 50001:2011 or ISO 50001:2018, the two editions named in regulation 33(4)(c), issued by a certification body accredited by UKAS, by a national accreditation body of an EU member state, or by a member of the International Accreditation Forum.
The Environment Agency says it must have been issued on or after 6 December 2023 and be valid on 5 December 2027.
Is ISO 50001 vs the ESOS audit a choice for the whole group?
No. The route works energy by energy: a certificate over one part of the group's consumption deems compliance for that part, and the rest is covered by an audit.
A group can use both, provided together they cover all of its total or significant consumption.
Do ISO 50001 participants still need an ESOS action plan?
Yes.
Regulation 34A requires an action plan after any notification of compliance, whichever route was used.
Only a participant deemed compliant because it used no energy is carved out.
Sources
Primary sources
Every figure, date and status on this page traces to the instrument’s owner.
Secondary commentary is never the source for a number.
- legislation.gov.ukESOS Regulations 2014, regulation 33 — deemed compliance through a certified energy management system
Reg 33(4)(a) accreditation; (4)(c) the 2011 and 2018 editions. Read with SI 2026/701 reg 24.
- legislation.gov.ukSI 2026/701, regulation 24 — new reg 33(2A), “total or significant”, and “any part of”
- legislation.gov.ukSI 2026/701 — Explanatory Note (with the correction slip applied)
The deemed duties: appoint a lead assessor, carry out an audit, produce the report.
- legislation.gov.ukThe Energy Savings Opportunity Scheme (Amendment) Regulations 2026 (SI 2026/701)
Made 23 June 2026, in force 22 July 2026: the Phase 4 changes. Made under Energy Act 2023 ss.254–260 and 263.
- legislation.gov.ukESOS Regulations 2014, regulation 25 — significant energy consumption
Not less than 95%, by energy units or spend; identification is elective.
- legislation.gov.ukESOS Regulations 2014, regulation 29 — notification of compliance
Reg 29(1)(ad): what an ISO-route participant must still notify.
- legislation.gov.ukESOS Regulations 2014, Part 6A (regulations 34A–34B) — action plans and progress updates
Consolidated to 22 July 2026: contents, windows, kWh basis, the third progress update, the zero-energy carve-out.
- legislation.gov.ukThe Energy Savings Opportunity Scheme Regulations 2014 (SI 2014/1643), revised text
In force 17 July 2014. Consolidation of the 2026 amendments is partial — read with SI 2026/701.
- legislation.gov.ukThe Energy Savings Opportunity Scheme (Amendment) Regulations 2023 (SI 2023/1182)
- Environment AgencyHow to comply with the Energy Savings Opportunity Scheme (ESOS) phase 4
Published 30 July 2026. A restatement of the Regulations, never the source over them; where it and the SI differ, the SI governs.
- Environment AgencyComply with the Energy Savings Opportunity Scheme (ESOS) phase 4 — publication page
- Environment Agency / GOV.UKEnergy savings opportunity scheme (ESOS): find out if you qualify and how to comply
Rewritten for Phase 4 on 2 September 2026; seven approved lead assessor registers; PAS 51215-1 and -2:2025 voluntary.
- ISOISO 50001 — Energy management
The standard’s owner; ISO publishes the standard and does not certify.
- UKASUnited Kingdom Accreditation Service
The UK’s national accreditation body, appointed by government.
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