Most operators discover the UK generator permitting regime the hard way: during a site audit, a tender submission, or an insurance review. It is genuinely complicated, the guidance is spread across several documents, and almost nobody selling generators explains it.
This article explains what applies, to whom, and by when. It is not legal advice, and if you are close to a threshold you should confirm your position with your regulator or an air quality consultant. But it should give you a clear enough picture to know whether you have a problem.
The short answer
Two separate regimes can apply to a generator in England and Wales, both sitting under the Environmental Permitting Regulations:
- Medium Combustion Plant (MCP) rules, which apply to combustion units with a rated thermal input of 1 MWth or more and less than 50 MWth, regardless of fuel or operating hours
- Specified Generator (SG) rules, which apply to electricity-generating plant between 1 MWth and 50 MWth, and to smaller units in certain circumstances
Both can apply to the same machine. If they do, you apply for the permit with the stricter controls and the earlier deadline.
The critical point most operators miss: a genuine emergency backup generator is excluded from Specified Generator controls, but that exclusion does not remove the MCP obligation. If the set is 1 MWth or more, it still needs a permit by the relevant MCP deadline. The exclusion narrows your obligations. It does not delete them.
Step 1: Work out your thermal input, not your kVA
Everything in this regime is measured in megawatts thermal (MWth), which is the fuel energy going into the engine. Your generator is sold in kVA, which is electrical output coming out. They are not the same number and the ratio depends on engine efficiency.
As a very rough orientation, 1 MWth arrives somewhere around 400 to 500 kVA of electrical output for a modern diesel set. That is a guide for working out whether you need to look at this seriously, not a number to rely on.
Do not estimate the real figure. Either ask the manufacturer for the rated thermal input, or calculate it using the AMPS method for determining thermal input, which is the approach the Environment Agency directs operators to. Note that it uses net calorific value, not gross.
If your set is a 100 kVA standby unit sitting behind a small office, you can stop reading here. You are nowhere near the threshold.
Step 2: Check whether you are actually excluded
Several categories of generator fall outside Specified Generator controls. The ones that matter most in practice:
Emergency backup generators. Since 1 January 2019, a backup generator used only to provide power at a site during an emergency is excluded from SG controls. But read the next section carefully, because the definition of "emergency" is narrower than most people assume, and the MCP obligation still stands.
Mobile generators. SG controls generally do not apply to a generator designed to be moved. But they do apply if it is performing a function a static generator could do. Regulators will look at how long it has been in one location, and six months or more in a single position is treated as functioning like a static generator. Long-term hire on a fixed site is a common trap here.
Construction sites. A mobile generator on a construction site is outside SG controls. If only part of a site is under construction, the regulator will assess whether the set is really doing a static job.
Planned maintenance and emergencies. Mobile sets used for planned maintenance or emergency cover are outside SG controls. Water utilities replacing pumps, data centre and telecom maintenance, and switchgear upgrades are the examples the guidance gives.
Offshore and nuclear. Offshore generators, and generators with a defined nuclear safety role under an ONR site licence, are excluded.
Step 3: The 50-hour testing rule, and what breaks the exclusion
This is where operators most often lose their exclusion without realising.
If you are relying on the emergency backup exclusion, you must not carry out more than 50 hours of testing per year for each backup generator, unless you have written agreement from your regulator to increase it. You must record the hours for each set to demonstrate you meet the criteria. Exceeding the limit without agreement will attract enforcement action.
Actual emergency use is different. There is no hour restriction on genuine on-site emergency running, though operators are expected to minimise it.
What is not emergency use:
- Providing a balancing service, whether procured or not
- Demand side response, including triad avoidance and fast frequency response
If your standby asset earns revenue from the grid in any form, it is not an emergency backup generator for these purposes. This catches a lot of sites that added a grid services contract to an existing standby set without revisiting their permitting position.
Data centres get a narrow carve-out: a data centre using its on-site emergency backup generator when transmission frequency is unstable remains excluded, provided the generator is not part of a formal agreement or contract.
Step 4: Aggregation, and why one small generator can become a big problem
If you have more than one generator on a site, operated by the same operator for the same purpose, you aggregate them into a single Specified Generator and apply for one permit for the site.
Generators still count as being for the same purpose even if they use different fuels or technologies.
This is how sites with several sub-threshold sets end up in scope. Four 300 kVA generators are individually well under 1 MWth. Aggregated, they are comfortably over it.
Note that operating hours are aggregated differently from what most people expect. You do not add each generator's hours together. Each site hour counts once, however many machines are running in it. Three generators running for the same 400-hour period is 400 hours, not 1,200.
For MCP purposes the aggregation rule is different again: you only aggregate new MCPs, and only where they discharge through a common windshield. Existing plant is not aggregated even where it shares a stack.
Step 5: Find your deadline
Specified Generators are split into two tranches based on when they came into operation or contracted with the grid.
Tranche A covers generators already operating before 1 December 2016, or holding a qualifying grid contract before 31 October 2017. Tranche B is everything else.
| Category | Permit needed by | Comply with conditions by |
|---|---|---|
| Tranche B (any size) | 1 Jan 2019, or the commissioning date | Same as permitting date |
| Tranche A, 5 to 50 MWth, over 500 mg/Nm³ and over 50 hrs/yr | 1 Oct 2019 | 1 Jan 2025 |
| Tranche A, 5 to 50 MWth, otherwise | 1 Jan 2025 | 1 Jan 2025 |
| Tranche A, 1 to 5 MWth | 1 Jan 2030 | 1 Jan 2030 |
| Tranche A, under 1 MWth | 1 Jan 2030 | 1 Jan 2030 |
For MCP obligations, the deadlines run on a different clock:
| MCP category | Permit needed by | Comply by |
|---|---|---|
| New MCP (in operation on or after 20 Dec 2018) | 20 Dec 2018 or before commissioning | On issue |
| Existing MCP, over 5 to under 50 MWth | 1 Jan 2024 | 1 Jan 2025 |
| Existing MCP, 1 to 5 MWth | 1 Jan 2029 | 1 Jan 2030 |
If you are buying a new generator in 2026, you are Tranche B and a new MCP. Your permit must be in place before the set is commissioned. Not after. This is the single most practically important line in this article, and it belongs in your project programme alongside the lead time for the machine itself.
One trap on second-hand plant: an existing MCP becomes a new MCP if it is substantially refurbished at more than 50% of the cost of a comparable new unit, or altered in a way that worsens its emissions. That resets you to the new-plant deadline. You also need traceable proof of the date it was first put into operation, tied to a serial number, and if you bought it second-hand that date may belong to a previous operator at a different site.
What kind of permit will you need?
There are three types, in increasing order of cost and difficulty:
- Standard rules permit (low risk) — only available in England, only where your site has Tranche B generators exclusively, not a mix of A and B
- Simple bespoke permit (low risk) — no detailed air dispersion modelling required
- Complex bespoke permit (high risk) — requires site-specific air quality modelling
Which one you need is decided by screening. You can use the Environment Agency's specified generator screening tool if you meet its criteria, or the SCAIL Combustion tool. If your emissions "screen out" as low risk you can apply for a simple bespoke permit. If they "screen in", you are into a complex bespoke permit and site-specific dispersion modelling, which needs a consultant.
Natural Resources Wales no longer issues standard rules permits at all. In Wales, everything is bespoke.
Full detail on the application process, information requirements and charges is in the Environment Agency's guidance on applying for a specified generator permit.
What changed in 2026
Two amendments matter if you are permitting in 2026.
Standard rules permits withdrawn for new applicants. From 28 February 2026 the Environment Agency withdrew several standard rules permit sets (SR2018 No 2, 3, 5, 6, 8 and 9) for new applicants. Existing holders are being moved onto simple bespoke permits free of charge. The practical effect is that fewer new applicants will get the simplest, cheapest route.
Decarbonisation readiness. From 28 February 2026, operators applying for a permit for new or substantially refurbished in-scope generation must declare how they meet carbon capture readiness or hydrogen conversion readiness requirements under Schedule 25C. This is a new section on the application form and it applies to new build, not to existing plant, unless you opt in voluntarily.
Alongside these, the Environment Agency confirmed that the NOx limit for specified generators operating under 500 hours remains 190 mg/Nm³, added hydrogen and gas oil substitutes such as HVO to the relevant standard rules permits, and relaxed stack configuration restrictions for low-use generators.
Devolved nations
Everything above is England and Wales. The regime exists across the UK but the regulator and the detail differ: SEPA in Scotland, NIEA in Northern Ireland, and Natural Resources Wales in Wales. If you operate across borders, check each separately rather than assuming.
A practical checklist
- Get the rated thermal input in MWth for every generator on site, from the manufacturer or by the AMPS method
- Aggregate them if they are same site, same operator, same purpose
- Check whether any exclusion genuinely applies, and be honest about grid contracts
- If you rely on the emergency exclusion, confirm you are logging test hours and staying under 50 per set per year
- Identify Tranche A or B, and new or existing MCP, for each unit
- Find your permitting date and compliance date, and check whether they have passed
- If you are buying or commissioning new plant, build the permit into the project programme before commissioning
- Where you are near a threshold or in an AQMA, get an air quality consultant involved early
Where this affects your purchase decision
If you are specifying a new set anywhere near 400 kVA and upward, ask your supplier for the rated thermal input in MWth and the emissions data at the quotation stage. Not at delivery. The permit needs to be in place before commissioning, and permit determination takes time.
We publish full specifications and pricing for our diesel generator range openly, including the larger sets from 500 kVA upward where these rules start to bite. As an AMPS member and an authorised dealer for the manufacturers we represent, we can supply the emissions documentation and thermal input figures your permit application will need.
If you are working out where a project sits against these thresholds, talk to our team. We will give you the machine data. For the permit application itself, use your regulator or a specialist air quality consultant.
This article summarises Environment Agency guidance current at the time of writing and is intended as general information, not legal or regulatory advice. Check the current position with your regulator before making decisions.