Standby generators and emissions in an energy statement

A standby generator is assessed on the hours it runs for testing, not on the hours it would run in an outage. That single point explains most of what councils ask for. The emergency case is accepted as a rare event. The weekly or monthly test, at a predictable time, next to someone's bedroom window or a school playground, is the one the environmental health officer has to be satisfied about.

So a submission that leads with resilience and buries the test regime in an appendix gets queries. One that states the test schedule, the flue height and the emissions on the first page usually does not.

When a generator needs permission

A generator in an external enclosure, on a roof, or in a container in a yard is normally development, because it is either a building operation or something that materially affects the external appearance of the building. A unit installed inside an existing plant room, with no external flue or louvre changes, often is not.

Some external plant on industrial land benefits from permitted development rights in Part 7 of the General Permitted Development Order 2015, subject to height limits, distance from boundaries and the exclusion of land near dwellings. Those limits are tight and the article is amended regularly, so read the current text rather than relying on a summary. A lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm the position.

Separate consents may run alongside. Depending on the thermal input and the date of installation, the generator may be a specified generator under the Environmental Permitting (England and Wales) Regulations 2016 and require an environmental permit from the Environment Agency. Where the plant burns fuel in a furnace, chimney height approval under the Clean Air Act 1993 may also be needed from the local authority. Ask the environmental health team early which regimes they consider apply, because the answer changes the information you need to produce.

Testing regimes and their emissions

Set out the test regime in numbers. How often, on which day, at what time, for how long, and at what load. A monthly off-load run of fifteen minutes is a different proposition from a quarterly full load test of two hours, and councils condition what you tell them.

The permitting regime also turns on operating hours. Generators that run only for testing, within a limited number of hours a year, are treated differently from those that operate commercially in a balancing or capacity market, and the Environment Agency publishes the current thresholds and emission limit values. If the asset might ever be used for grid balancing, say so now, because retrofitting abatement to a unit permitted on a testing-only basis is expensive and sometimes impossible within the enclosure.

Flue height and dispersion

Flue height is the variable that resolves most air quality objections, and it costs far less to get right on the drawings than after an objection. The principles are simple: discharge vertically, above the roofline and above the zone of turbulence created by the building, clear of openable windows and air intakes on your own building and on neighbours, and with sufficient efflux velocity to carry the plume upward.

The conventional approach is to determine a height using the chimney heights memorandum method as a starting point, then test it with dispersion modelling where the setting is sensitive. An under-height flue beside a taller neighbouring building produces downwash, which puts the plume at window level, and no amount of filtration fixes geometry.

Two practical cautions. Flue height interacts with planning on visual grounds, so show it on the elevations and the roof plan at the correct height, with the enclosure and any acoustic louvres. And check the flue against the building's own ventilation intakes, because a generator whose plume is drawn into the fresh air plant is a problem for the occupier rather than the neighbour.

What an air quality assessment should show

An Air Quality Assessment for standby plant is a report type where councils expect a suitably qualified author, and professional review is strongly advised. What makes one acceptable:

1. The baseline, using local monitoring data and background maps, with a statement of whether the site lies in an Air Quality Management Area.
2. The plant schedule: rated thermal input, electrical output, fuel, engine emission standard, abatement if any, and manufacturer's emission data at the tested load.
3. The operating scenarios modelled, matching the test regime stated in the planning documents.
4. Dispersion modelling where the setting requires it, with the model named, the meteorological data years stated, and the receptor heights and locations shown on a plan.
5. Results expressed against the relevant air quality objectives at identified receptors, including any school, care home or residential facade within the modelled area.
6. A conclusion on whether mitigation is required, and what form it takes.

The commonest weakness is a report that models the emergency scenario and omits the test scenario, or that models a generic engine rather than the one specified. Both invite a request for a revised assessment and a delay of weeks.

Where the scheme includes combined heat and power as well, the air quality argument gets harder, and our note on combined heat and power and the air quality objection covers that ground.

Noise during testing and in an outage

Testing noise is assessed against the measured background sound level at the nearest noise sensitive facade, following the method in BS 4142:2014+A1:2019. Because testing is deliberate and schedulable, councils usually accept a condition restricting it to weekday daytime hours, which removes the hardest part of the assessment.

Outage operation is treated as an emergency and is not normally held to the same limit, but say so explicitly rather than leaving it to inference, and set out how long a run is realistic.

The design measures that work are an acoustic enclosure with the attenuator sized for the duty, exhaust silencing, anti-vibration mounts on the base frame, and louvre positions facing away from the nearest receptor. A Noise Impact Assessment should state the enclosure's insertion loss and the sound power level of the whole package rather than of the bare engine, since the two differ substantially.

An illustrative example. A small food manufacturer in a mixed use area installs a 300 kVA diesel standby set in an acoustic container in the yard, 18 metres from the nearest flats. The assessment proposes monthly off-load testing between 10am and 4pm on weekdays, a flue terminating 1.5 metres above the ridge of the adjacent block, and an enclosure with the louvres facing the service road. Permission is granted with conditions on test hours, flue height and a noise verification report after installation. This example is illustrative.

Fuel storage and bunding

Diesel storage above a modest volume at a non-domestic site is regulated by the oil storage regulations, which require a secondary containment system, commonly a bund of sufficient capacity, with fill points and sight gauges contained within it.

For the planning submission, show the tank, the bund, the fill point, the vent and the spill kit position on the site plan. Where the tank is within a groundwater source protection zone or near a watercourse, expect the Environment Agency to comment and plan for an integrally bunded tank rather than a masonry bund.

Deliveries matter too. Say how often the tank is replenished, where the tanker stands, and whether the standing position blocks the service yard or the fire appliance route. That detail belongs in the submission because the highway officer will ask for it.

What to include in the submission

• Site plan and elevations showing the enclosure, flue, louvres, tank and bund at the correct scale.
• A plant schedule with outputs, fuel, emission standard and sound power levels.
• The test regime, stated in hours and times, with a note on whether grid balancing participation is contemplated.
• An air quality assessment scaled to the setting, modelling the test scenario.
• A noise assessment with background measurements at the nearest receptors.
• A statement on which environmental permitting and Clean Air Act requirements apply, and their status.
• Confirmation in the Energy Statement that standby generation is excluded from the carbon reduction calculations, since it provides resilience rather than supply.

That last point is worth making explicitly. Standby plant does not contribute to the energy hierarchy and should not appear as a carbon saving, and an energy statement that includes it as one undermines the rest of the document.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. The full list of report types is at the PlanScope report index. For specialist technical reports, qualified professional input may be required.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.