A diesel generator may be addressed simultaneously from several perspectives: as a combustion stationary source under the Air Protection Act, as a noise source under hygiene regulations, and as technical equipment that must be correctly placed in project documentation. For larger sources or sensitive locations a dispersion study or noise study may also be required.
For a backup diesel generator, rated electrical output is not the most important factor. For air protection, total rated thermal input, operating hours, and whether backup source regime is correctly stated in the operating permit matter most.
Backup regime and operating hours
A backup diesel generator serves extraordinary situations: electricity outage, technology safety assurance, emergency power for fire, healthcare, waterworks, or data systems. In normal operation it starts mainly during tests, regular trials, service checks, and actual supply failure.
Operating hour count is essential. A backup source running only a few or tens of hours per year has a different impact than equipment used regularly for electricity production, peak shaving, or commercial power regulation.
| Operating regime | Practical significance |
|---|---|
| short regular tests | usually low annual operating hours, but noise during tests may be significant |
| emergency operation during grid outage | irregular operation; safety function is important |
| more frequent regulatory or operational use | may change view of source and its obligations |
| operation above declared hour limit | may change emission determination regime and authority requirements |
According to current Ministry of the Environment interpretation, it is important that the fact the equipment serves as a backup energy source is stated in the operating permit. It is therefore not enough for the operator alone to claim the diesel generator is backup. It must be clear in what regime it is to be operated and how operating hours will be recorded.
Diesel generator emissions: NOx, CO, and TSP
A diesel generator is a reciprocating combustion engine burning liquid fuel, typically diesel or equivalent fuel per equipment specification. During operation it mainly produces nitrogen oxides, carbon monoxide, and total suspended particulates. Depending on engine type, age, load, fuel, and possible end-of-pipe technology, other substances may also be addressed.
From a practical perspective the following are most often tracked:
| Substance | Why it matters |
|---|---|
| NOx | main diesel engine issue from NO2 immission contribution perspective |
| CO | indicator of combustion quality and engine tuning |
| TSP | solid particles in flue gas, significant especially for diesel combustion |
| smoke opacity | operational signal of engine and combustion condition |
| fuel consumption | basis for emission calculation and dispersion study |
For backup sources it is necessary to distinguish whether emissions must be determined by measurement, calculation, or whether under fulfilled conditions there may be no obligation to determine pollution level at all. In its opinion the Ministry of the Environment explains that for backup energy sources operated less than 300 hours per year, relevant specific emission limits under Decree No. 415/2012 Coll. do not apply to reciprocating combustion engines. If no emission limits beyond the decree are set in the operating permit for such a source, the operator has no obligation to determine pollution level by measurement or calculation.
If the backup source is operated more than 300 hours per year, or has a specific emission limit in the operating permit, the situation may differ. In some cases pollution level is determined by calculation, especially linked to operating hours, fuel consumption, emission factors, or supplier guaranteed emission parameters.
Output, power, and source classification
For diesel generators electrical output in kW or kVA is often stated. For air protection, rated thermal input — energy supplied in fuel — decides. It is usually higher than electrical output because engine and generator are not 100% efficient.
Practically important: a diesel generator with electrical output e.g. 800 kW may have rated thermal input well above 0.3 MW. Only generator nameplate electrical output therefore cannot be assessed.
| Parameter | Meaning |
|---|---|
| electrical output | power delivered by generator to grid or technology |
| kVA | apparent generator output; depends on power factor |
| thermal input | energy supplied in fuel; decides air protection classification |
| fuel consumption | important basis for emission calculation and annual balance |
| operating hours | determine actual operation scope and backup source regime |
Reciprocating combustion engines from 0.3 MW total rated thermal input are listed stationary sources under the Air Protection Act. For larger inputs the regime tightens and for sources above 5 MW requirements are broader, including greater emphasis on operating permit, operating records, and authority documentation.
Backup diesel generator noise
Noise is often as important as emissions for a diesel generator. Noise sources include not only the engine but also the exhaust, radiator, fans, air intake and discharge, vibration transmitted to structure, and sometimes noise reflection from facades or technical buildings.
For backup sources the problem may be that tests also run when surroundings are noise-sensitive. A short daytime test usually differs from night start during power outage. For hospitals, residential development, schools, hotels, or office buildings noise assessment may be essential already in the project phase.
Noise is mainly assessed for:
- diesel generator placement inside building or outdoors,
- enclosure or container type,
- exhaust and silencer,
- cooling air intake and discharge,
- test operating regime,
- distance to protected outdoor spaces of buildings,
- day or night operation,
- possible noise reflection from surrounding facades.
A well-designed source therefore addresses the whole acoustic chain, not only the engine. A common mistake is using catalogue equipment noise data without verifying real ventilation discharge direction, exhaust position, wall reflection, or roof placement effect.
Exhaust placement and flue gas path
Exhaust placement matters greatly for pollutant dispersion and for safety and operational functionality. An exhaust led low into a yard, near windows, HVAC intakes, or neighbouring development can be problematic even for a source with low operating hours.
For the exhaust especially height above ground and roof, discharge direction, distance from HVAC intakes, proximity of windows and occupied spaces, surrounding buildings, and possibility of emission accumulation in enclosed space are assessed. For a roof exhaust building aerodynamics, parapets, technical superstructures, and flow around the building may also matter.
For a diesel generator it is appropriate to address the exhaust already in the project. Subsequent stack raising, exhaust relocation, or flue path modification is often technically and financially unpleasant.
Measurement capability must also be considered in design. If measurement obligation could arise in a specific regime, the exhaust must be technically accessible and the measurement location must allow representative sampling. For backup sources this may not always be the main topic, but for larger or operationally significant equipment it should not be forgotten.
When a dispersion study may be needed
A dispersion study assesses how source emissions manifest in surrounding air. For diesel generators mainly NOx, i.e. contribution to nitrogen dioxide immissions, and depending on circumstances particulate matter are addressed.
According to current Ministry of the Environment opinion, the obligation to submit a dispersion study does not apply to reciprocating combustion engines and gas turbines up to 5 MW total rated thermal input inclusive if they burn gaseous or liquid fuel, serve as backup energy sources, and operating hours do not exceed 300 hours per calendar year. At the same time, after the legal change effective 1 March 2025 the regional authority may request a dispersion study with regard to local conditions.
Local conditions may include:
| Local condition | Why it may matter |
|---|---|
| complex relief or enclosed yard | worse pollutant dispersion |
| nearby residential development | sensitive receptors at short distance |
| HVAC intakes nearby | risk of flue gas re-entrainment |
| multiple sources on site | emission accumulation |
| worse immission situation in locality | higher area sensitivity to further contribution |
| low exhaust | worse dilution and flue gas dispersion |
If the backup source does not meet statutory exemption conditions — e.g. higher input, higher operating hours, or not a true backup regime — a dispersion study may be standard required documentation. In its opinion the Ministry of the Environment states that for backup sources not meeting exemption conditions a dispersion study must be required.
Permitting documentation
For a backup diesel generator documentation should be prepared so it is clear what source is being permitted and in what regime it will operate. Insufficient documentation often leads to supplementation by the regional authority, building authority, regional public health authority, or CEI.
Usually especially diesel generator technical data, rated electrical output, rated thermal input, fuel type, expected operating hours, manufacturer emission parameters, fuel consumption, exhaust height and placement, silencer data, equipment acoustic parameters, and operating regime description must be prepared.
For larger or sensitive projects professional documents may also be needed:
| Documentation | When typically addressed |
|---|---|
| professional source classification assessment | with unclear input, regime, or source aggregation |
| dispersion study | on legal, authority, or sensitive location requirement |
| noise study | near protected spaces or night operation |
| operating permit | for listed stationary source |
| operating records | to demonstrate operating hours, fuel, and source regime |
| basis for designer | when designing exhaust, silencers, enclosure, and placement |
For data centres, hospitals, or waterworks facilities it matters that permitting documentation matches the actual safety function of the source. Emergency backup for grid outage is assessed differently from a source regularly operated for economic load optimisation.
Operating records and regime control
Operating records are key for a backup source mainly because they demonstrate operating hour count. If the source is to be assessed as backup, operating regime must be provable. Project assumption alone is not enough.
Records should allow retrospective demonstration of when the generator ran, how long, whether it was a test, service, fault, grid outage, or other reason, what fuel was consumed, and whether limits set in the operating permit were exceeded.
In practical terms: if the permit states the source is to be operated at most 300 hours per year, this must be operationally monitorable. Exceeding hour count may change operator obligations, especially regarding emission limits and pollution level determination.
Most common mistakes with backup diesel generators
For diesel generators the difference between electrical output and thermal input is often underestimated. The designer states generator output but air protection needs engine thermal input. Another common mistake is general labelling "backup source" without clear operating hour definition and without stating this regime in the operating permit.
Problems also arise with exhausts. Low discharge into a yard, proximity of windows, or HVAC intakes can be problematic even with limited operation. For noise, exhaust, cooling, and reflection from surrounding objects are often forgotten. For emissions the operator sometimes assumes that if the source is backup, no permitting documentation is needed at all. That may not be true.
Summary
A backup diesel generator is technically simple equipment but can be surprisingly sensitive for permitting. Its rated thermal input, fuel, actual backup regime, operating hours, exhaust placement, noise, and local conditions decide. For some sources emission measurement may not be necessary, but operating permit, operating records, noise assessment, or dispersion study may still be required.
Special attention is appropriate for sources at hospitals, data centres, waterworks, industrial sites, and objects near residential development. There high operational reliability requirements often meet air protection and noise protection requirements.
Send us the diesel generator datasheet, rated electrical output, thermal input, fuel consumption, expected operating hours, exhaust placement, building situation, and any authority requirement. We will verify source classification and propose whether operating permit, dispersion study, noise study, operating records, or further permitting documentation is needed.
Factual basis of the article
The article is based mainly on these sources:
- Act No. 201/2012 Coll., on air protection,
- Decree No. 415/2012 Coll., on permissible pollution levels and their determination,
- Government Regulation No. 272/2011 Coll., on protection of health from adverse effects of noise and vibration,
- Ministry of the Environment opinion on selected questions of permitting backup energy sources.
The 2025 Ministry of the Environment opinion explicitly addresses pollution level determination for backup energy sources, the 300 operating hours per year threshold, the regime up to 500 operating hours as a rolling three-calendar-year average, the obligation to state backup regime in the operating permit, and when a dispersion study may be required for backup sources.