Standby generators: noise, emissions and the permit line

A standby generator planning noise emissions case has to satisfy three separate regimes, and they are assessed by three different sets of people who do not talk to each other. The planning authority's environmental health officer looks at noise and local air quality. The Environment Agency looks at whether the plant needs an environmental permit. And the fire and building control route looks at fuel storage and enclosure construction. A generator that sails through one can be stopped by another, and the usual casualty is a data centre or hospital scheme where the generator was treated as a mechanical detail until the consultation responses arrived.

Deal with the three in parallel and the generator stops being the critical path item.

Why one plant item raises three issues

The generator is unusual because it is simultaneously a large noise source, a combustion source and a fuel store, and because it almost never runs. That last point is what confuses assessments. For 99 per cent of its life it is silent. For a few hours a year it is tested, and in a power cut it runs for as long as the outage lasts, possibly through the night.

So you are assessing two different scenarios with one piece of plant: a controlled, scheduled test during working hours, and an uncontrolled emergency run at any hour. Councils can condition the first and cannot condition the second. An assessment that only models the test regime will be sent back. An assessment that applies the full emergency scenario as though it were routine will produce mitigation nobody needs.

Noise during testing and during an outage

The assessment method is BS 4142:2014+A1:2019, which compares a rating level against the measured background sound level at the nearest noise-sensitive receptor. The standard's own indication is that a rating level around 10 dB above background indicates an adverse effect of some consequence, 5 dB above indicates an adverse effect, and a level at or below background indicates a low impact, all subject to a context assessment.

Three things the measurement has to get right. The background has to be measured at the correct period, which for emergency running means a night-time measurement during the quietest hours, not a daytime average. The character correction has to be applied, because a diesel generator has a strong low frequency content and a distinct tonal quality, and ignoring the correction understates the rating level by several decibels. And the receptor has to be the actual nearest window, including upper floors, since a generator in a yard can be screened at ground level and fully exposed at second floor.

The practical structure for the report:

1. Test scenario, daytime, with the proposed mitigation in place and the rating level compared against the daytime background.
2. Emergency scenario, night-time, with the same mitigation and the night background, presented as a rare event rather than as a routine condition.
3. Cumulative scenario where more than one generator exists on site, since several units running together add logarithmically and a site with six generators is not 6 dB louder than one, it is around 8 dB louder.

The background measurement work overlaps with site appraisal, and if strategic data has already been used at feasibility stage, say how it compares with the measured values, as discussed in strategic noise mapping for early site appraisal.

Emissions, stack height and dispersion

Diesel generators emit nitrogen oxides, and the question for planning is whether the contribution at nearby receptors is acceptable against the air quality objectives, principally the annual mean nitrogen dioxide objective of 40 micrograms per cubic metre and the 1-hour mean of 200 micrograms per cubic metre not to be exceeded more than 18 times a year.

For a true standby unit the annual mean contribution is trivial, because the plant runs for hours rather than months. The short-term objective is where the assessment has to do real work, since a test run or an outage can produce a measurable hourly concentration.

Stack height is the main design lever and resolving it on a drawing costs far less than resolving it on site. Three rules of thumb that dispersion modelling then confirms:

• Discharge vertically with efflux velocity high enough to carry the plume clear, never through a horizontal louvre at head height.
• Clear the building's recirculating wake, which on a flat-roofed building extends well above roof level.
• Check receptors above you as well as beside you, because a stack at parapet height on a low building next to a taller residential block puts the plume straight at habitable windows.

Dispersion modelling for a standby unit should state the assumed operating hours, the emission concentration taken from the engine manufacturer's data at the relevant load, the stack parameters and the meteorological dataset and years used. An Air Quality Assessment without those stated inputs cannot be checked, and where the site sits in an Air Quality Management Area the authority will look at them closely. The monitoring arguments that sit behind the baseline figures are covered in diffusion tubes against continuous analysers.

Where an environmental permit is also needed

This is the regime applicants miss. The Environmental Permitting (England and Wales) Regulations 2016 as amended brought combustion plant into permitting, with the medium combustion plant band running from 1 megawatt to 50 megawatts of rated thermal input, and a separate set of controls for specified generators.

Three points to establish early:

• Rated thermal input, not electrical output. A generator rated at around 1 MW electrical will have a thermal input well above that, which can push it over a threshold the project team assumed it was under.
• Aggregation. Several units discharging through a common stack, or capable of being treated as a single plant, may be aggregated, so a bank of generators can cross a threshold that no individual unit reaches.
• Operating hours. The controls on standby plant turn substantially on how many hours a year it runs, and the regime has transitional dates for existing and new plant.

The thresholds, the hours tests and the transitional dates have all been amended since the regulations were made, so check the Environment Agency's current guidance rather than relying on a figure from an older report. Include a short permitting position statement with the planning application saying which category the plant falls into and whether a permit will be sought. Officers appreciate it and statutory consultees stop asking.

Fuel storage is a fourth consent thread. An above-ground oil container over 200 litres at commercial premises falls within the oil storage regulations, which require secondary containment sized to the tank and specified bund construction.

Siting, enclosures and attenuation

The options, in descending order of effectiveness and ascending order of cost:

Two engineering constraints to respect. Silencer grade affects exhaust back pressure, and over-silencing can derate the engine, so the acoustician and the mechanical engineer have to agree the specification rather than the acoustician specifying it alone. And an enclosure needs enough air through it to keep the engine cool, which means the attenuated openings have to be sized for airflow before they are sized for noise.

Testing hours as a planning condition

Expect, and preferably offer, a condition restricting testing to daytime hours on weekdays, excluding weekends and bank holidays, with a stated maximum duration per test and a stated frequency.

Offer it yourself for a practical reason. The insurance and resilience regime behind the generator will have its own testing requirement, and if you draft the condition you can align the two. A condition imposed without reference to the maintenance schedule can leave the operator in breach of either the permission or the resilience standard.

Also offer a record-keeping obligation: a log of test dates, times and durations, made available to the authority on request. It costs nothing, it answers complaints, and it is far better evidence than a retrospective reconstruction.

What to submit with the application

A Noise Impact Assessment covering both scenarios with the character corrections applied and the mitigation specified. An air quality assessment with dispersion modelling for the short-term objective and its inputs stated. A plant schedule giving sound power levels by octave band for each unit. An elevation drawing showing the stack height relative to the building and to the nearest receptor windows. Fuel storage details with the bund specification. The proposed testing regime. And the permitting position statement.

The one that gets left out is the octave band data, and without it the acoustician's mitigation cannot be verified by anyone else. Ask the supplier for it at tender stage, because it is much harder to extract once the order is placed.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect noise and air quality work to be prepared by a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.