Do You Need an Environmental Permit for Your Diesel Generator in the UK?

If you operate a diesel generator in the UK — whether for emergency standby power, continuous site generation, or grid balancing services, you may be legally required to hold an environmental permit before that generator can run. Many UK operators are unaware of this obligation until they face a compliance enquiry, a planning review, or a site acquisition due diligence check.

This article explains clearly what triggers the permit requirement, who is exempt, which regulator applies in your region, and what the application process involves — with direct links to the official government guidance at every step.

What Law Creates the Permit Requirement?

The environmental permit requirement for diesel generators comes from the Medium Combustion Plant Directive (MCPD), which was incorporated into UK law through the Environmental Permitting (England and Wales) (Amendment) Regulations 2018. The MCPD controls emissions of sulphur dioxide (SO₂), nitrogen oxides (NOx), and particulate matter (dust) from medium-sized combustion plants — which include diesel generators used for any purpose, including emergency standby.

The regulations apply to the site operator, not the equipment supplier or installer, and they are retrospective — meaning generators already in operation before the legislation came into force are still subject to the permit deadlines.

Does Your Generator Need a Permit?

The permit requirement is triggered by the generator's rated thermal input — the energy content of the fuel consumed, not the electrical output. This distinction catches many operators off guard because generators are almost always sold and specified in kVA or kW electrical terms.

The MCPD covers combustion plants with a rated thermal input of between 1 MWth and 50 MWth. As a practical guide, because diesel engines are typically around 35–45% thermally efficient, a generator producing 400 kW of electrical output may have a thermal input of approximately 900 kW to 1.1 MWth — placing it at or very close to the 1 MWth MCPD threshold. Older, less efficient engines reach this threshold at lower electrical outputs.

Critical aggregation rule: The MCPD threshold applies to the combined thermal input of all generators on the same site, under the same operator, that share or could discharge through a common stack. Multiple generators that each fall below 1 MWth individually may still require permits if their combined thermal input reaches or exceeds 1 MWth. 

To check whether your specific generator is in scope, use the GOV.UK guidance on when you need a permit. This is the definitive reference for England and Wales and covers exclusions, aggregation rules, and the full list of combustion plant types.

Who Is Exempt?

Not all diesel generators require a full environmental permit. The MCPD includes several important exclusions:

• Mobile generators placed on the market after 1 January 2017 are covered by the revised Non Road Mobile Machinery (NRMM) Regulations rather than the MCPD — so a towable generator deployed temporarily on multiple sites falls outside MCPD scope
• Mobile generators placed on the market before 1 January 2017 with a drive output above 560 kW remain within MCPD scope
• Offshore platform generators are excluded
• On-farm combustion plant with a thermal input of 5 MWth or less using specified agricultural fuels may be exempt

If you are unsure whether your generator qualifies for an exclusion, consult the Environment Agency or seek advice from an environmental consultant before assuming you are outside the scope.

Emergency Standby Generators - Still in Scope

One of the most widespread misconceptions is that genuine emergency standby generators — units used only when the mains fail, with annual testing — are exempt from the permit requirement. They are not.

A genuine emergency standby generator is still a medium combustion plant and still requires a permit by the applicable deadline. The key difference is that emergency units are exempt from having to meet the emission limit values (ELVs) during genuine emergency operation and testing, as long as they do not exceed 500 hours of operation per year for emergency and testing combined.

Important: Running a standby generator for demand-side response, capacity market services, triad avoidance, or any commercial grid service removes its emergency standby status. Any such operation counts toward annual operating hours for ELV compliance purposes, and the generator must then meet the relevant emission limits.

Who Regulates This in Your Part of the UK?

The MCPD has been transposed into UK law separately by each devolved nation, so the regulator and the specific legislative instrument varies depending on where your generator is located:

England — regulated by the Environment Agency (EA) under the Environmental Permitting (England and Wales) Regulations 2016 (as amended 2018)

Wales — regulated by Natural Resources Wales (NRW) under the same regulations; note that NRW no longer issues standard rules permits and requires bespoke applications for all Welsh sites

Scotland — regulated by the Scottish Environment Protection Agency (SEPA) under the Pollution Prevention and Control (Scotland) Amendment Regulations 2017

Northern Ireland — regulated by the Northern Ireland Environment Agency (NIEA) under the Pollution Prevention and Control (Industrial Emissions) (Amendment) Regulations (Northern Ireland) 2018

What Are the Key Permit Deadlines?

The deadline that applies to your generator depends on whether it is classified as new or existing, and on its thermal input size.

• New generators (first commissioned on or after 20 December 2018) — permit required before commissioning; emission limit values apply from day one
• Existing generators 5–50 MWth (commissioned before 20 December 2018) — permit required by 1 January 2024 (this deadline has already passed; if you are in this band without a permit, contact the EA immediately)
• Existing generators 1–5 MWth (commissioned before 20 December 2018) — permit required by 1 January 2029

Two Types of Permit: Standard Rules vs Bespoke

For generators in England, there are two permit routes:

Standard Rules Permit

A standard rules permit is a fixed-format permit issued where the generator meets a pre-defined set of conditions set out in the applicable Standard Rules document. Standard rules permits are simpler and faster to obtain than bespoke permits, and the application fee is lower. They are appropriate for many straightforward standby and prime power generators that are not in or near Air Quality Management Areas (AQMAs) and do not require air dispersion modelling.

Bespoke Permit

A bespoke permit is required where the generator cannot meet standard rules conditions — for example, if it is located in or near an AQMA or a protected habitat site, if detailed air dispersion modelling is needed to demonstrate safe emission levels, or if the site has a complex combustion plant portfolio requiring a tailored assessment.

The Environment Agency now operates an online permit application service for both standard rules and new bespoke applications for standalone MCPs and specified generators in England. The online service calculates the application fee based on the answers you provide.

What Does the Application Require?

Whether you apply for a standard rules or bespoke permit, you will need to provide:

• Your generator's rated thermal input and fuel type (diesel/gas oil is the correct designation — use this even for HVO fuel, as the EA dropdown does not list HVO separately)
• Annual operating hours, distinguishing clearly between emergency and testing use, and any commercial grid service hours
• Details of all other combustion plant on site that may need to be considered for aggregation
• Evidence of your environmental management system (a requirement even for sites with a single emergency generator)
• Confirmation that you can meet the applicable emission limit values (ELVs), including a monitoring plan
• For new or substantially refurbished generators applying from 28 February 2026: a decarbonisation readiness report, demonstrating how the plant could transition toward low-carbon technology (carbon capture or hydrogen conversion readiness)

What Happens If You Operate Without a Permit?

Operating an in-scope generator without the required environmental permit is a criminal offence under Regulation 38 of the Environmental Permitting (England and Wales) Regulations 2016. The Environment Agency has the power to.

• Issue enforcement notices requiring cessation of operation
• Serve stop notices with immediate effect
• Prosecute operators, with unlimited fines and — in the most serious cases — custodial sentences

Beyond criminal enforcement, operating without a permit can affect planning applications for site development, invalidate insurance coverage that is conditional on regulatory compliance, and create significant problems during corporate transactions, site sales, or lease renewals where environmental due diligence is carried out.

What to Do Next

The immediate steps for any UK business operating a diesel generator are:

• Step 1 — Confirm your generator's rated thermal input (not kVA rating) and check against the 1 MWth MCPD threshold
• Step 2 — If multiple generators are on site, check whether their combined thermal input exceeds the threshold
• Step 3 — Confirm which regulator applies (EA, NRW, SEPA, or NIEA) based on where the generator is located
• Step 4 — Check your deadline: if you are in the existing 1–5 MWth band, your deadline is 1 January 2029, and early applications are advisable given expected volume pressure on the EA near the deadline
• Step 5 — Determine whether a standard rule or bespoke permit is appropriate for your site
• Step 6 — If purchasing a new diesel generator, factor the permit application into your project programme — the generator cannot legally operate without the permit in place from commissioning