On Monday morning, a manufacturing plant is preparing to start its first shift.
The machines are ready. Operators are taking their positions. The control system is waiting for the start signal.
At the same time, a municipal building is receiving visitors, medical equipment is operating in a hospital, and automated systems in a logistics centre are managing thousands of shipments.
Then the lights go out.
For the first few seconds, people simply look at one another. After a minute, someone calls the grid operator. After five minutes, the plant manager starts calculating the losses. After an hour, the outage is no longer a technical inconvenience. It becomes an operational, financial and sometimes even a public safety problem.
This is when the practical question appears:
Should the facility use a diesel generator, a gas generator or a hybrid backup power system?
There is no universal answer. The right choice depends on the required output, backup duration, load profile, fuel availability, installation location, emissions requirements, noise restrictions, service conditions and the level of interruption the facility can tolerate.
A generator operating in an industrial plant near London will have very different duties from a backup source installed in a public office, hotel, school or data centre.
In one facility, the priority will be many hours of operation under high load. In another, low noise and reduced local emissions may be more important. Somewhere else, the most suitable solution may combine a generator, battery energy storage and photovoltaic generation.
The technology should follow the actual operational problem.
What should a backup power system actually do?
The most common mistake is made before anyone chooses a technology. The investor starts with the question:
Which generator should we buy?
The more important question is:
What exactly should happen when the grid supply fails?
Does the entire facility need to keep operating? Is it enough to maintain emergency lighting, security systems and the server room? Must production continue without interruption? Or is the generator only required to give the facility enough time to shut down a technological process safely?
These are completely different requirements.
In an office building, a generator may supply lifts, access control, ventilation systems, the server room, CCTV and selected lighting circuits. In a manufacturing plant, the list of critical loads is usually much longer. It may include motors, compressors, pumps, furnaces, automation systems, production lines and equipment that cannot simply be switched off halfway through a cycle.
Public buildings have another responsibility: protecting the people inside them.
A power failure in a sports hall, school, shopping centre or public administration building may require emergency lighting, fire protection systems, smoke extraction, alarms, CCTV and communication systems to remain operational.
That is why generator selection begins with a load assessment, but does not end there. The design should also consider starting currents, nonlinear loads, load sequencing, power quality and the required autonomy period.
A 500 kVA generator may operate perfectly in one facility and be entirely unsuitable in another. The number on the rating plate does not tell us whether the generator will handle the real operating conditions.
Is a diesel generator still the best choice?
For many applications, it is.
Diesel technology remains one of the most established ways of providing backup power for industrial facilities, technical infrastructure and sites with demanding reliability requirements.
The reason is practical.
Diesel engines are available across a wide range of power ratings, tolerate heavy loads well and can operate for long periods when the installation is properly designed and maintained.
Fuel can be stored on site, and its availability does not depend on the continuity of the gas network.
That independence can be decisive.
A gas generator usually relies on the gas distribution network. If the same event affects both the electricity supply and the gas infrastructure, the generator may lose its fuel source precisely when it is needed most.
A properly designed diesel installation with an on-site fuel reserve gives the operator greater control over the duration of the backup period. This is particularly important in remote facilities, industrial plants, telecommunications infrastructure, water treatment facilities and logistics centres.
Diesel generators are also well suited to situations where an outage may last for many hours. A production plant, cold store or pumping station needs a source that can continue operating long after a short battery reserve has been exhausted.
However, diesel generators require correct operating conditions.
A generator that starts several times a year for a few minutes at very low load is not being tested under realistic conditions. Short, unloaded test runs may contribute to incomplete combustion and the build-up of deposits in the engine.
A test should confirm more than the fact that the engine starts.
The complete system should be checked under load. The operator should verify voltage stability, frequency response, load acceptance, automatic transfer operation and the behaviour of the generator when the load changes.
A generator that starts beautifully with no meaningful load attached has passed only the easiest part of the exam.
When does a gas generator make more sense than a diesel generator?
A gas generator may be a suitable option where lower local emissions, reduced noise and access to a stable gas supply are important.
This can be relevant in urban facilities, hotels, office buildings and sites located close to residential areas or other users. Noise, exhaust gases, fuel deliveries and on-site fuel storage may be more difficult to manage in a densely built environment than on an industrial site located outside the city.
Gas generators are also considered in buildings that already have a suitable gas connection and may benefit from combined heat and power generation.
If the unit produces both electricity and useful heat, it can play a role beyond conventional emergency power. This may be attractive for facilities with a stable demand for hot water, process heat or space heating.
There is an important condition, though: the heat must actually be used.
Cogeneration makes sense when the thermal output corresponds to the facility’s real operating profile. Installing a gas unit simply because gas is generally perceived as a cleaner fuel, without analysing the heat demand, can result in poor utilisation and unnecessary investment.
Gas does not eliminate the need to assess emissions either. Requirements depend on the engine type, output, installation category, location and applicable legislation. The specific unit and installation must be assessed instead of relying on a general label such as eco-friendly generator.
The gas infrastructure itself must also be checked.
The project should verify the available pressure and flow rate, the gas shut-off system, protection devices, ventilation, exhaust routing and the behaviour of the installation during a pressure drop.
Starting performance is another important factor. The generator must start within the required time, stabilise its electrical parameters and accept the load correctly. If the gas installation cannot provide the necessary conditions during peak demand, the presence of a gas connection alone does not guarantee reliable backup power.
Is a gas generator actually cheaper to operate?
Not necessarily.
Fuel cost is only one element of the total cost of ownership. The analysis should also include the gas connection, pressure regulation equipment, safety systems, ventilation, exhaust installation, servicing, inspections and any structural modifications required at the facility.
A gas generator may be economically attractive where it operates regularly and achieves a high utilisation rate. If it is used only during grid failures and otherwise remains on standby, the economic advantage may look very different.
The operating profile matters.
For a backup unit running only a few dozen hours per year, fuel may represent a relatively small part of the total cost. Purchase, installation, service and readiness maintenance may be more significant. For a generator operating every day, fuel efficiency becomes much more important.
Energy and fuel prices can also change faster than the infrastructure surrounding the generator. A backup power system is usually purchased for many years of operation. The decision should therefore be based on realistic operating scenarios rather than a temporary price difference between diesel and gas.
Can a hybrid system replace a generator?
A hybrid system combines several sources of energy, for example a battery energy storage system, photovoltaic generation, a diesel or gas generator and an energy management system.
In this configuration, the battery can take over the load immediately after the grid fails. The generator starts later or operates at a more efficient load level. Solar generation can reduce fuel consumption during daylight hours, while the control system determines which source should supply the facility at a given moment.
Does that sound complicated?
It is.
But complexity can be justified when it solves a real operational problem.
Imagine a facility with a continuous demand of 100 kW and occasional short peaks.
A 250 kVA diesel generator may spend most of its operating time at a low load. A battery can cover short peaks, while the generator operates for longer at a more stable and efficient load level.
In another facility, large motors may require a significant amount of power for only a few seconds during start-up. A battery can support the starting sequence, reducing the need to select the generator solely around the most demanding short-term event.
A hybrid system can also reduce the number of generator starts. This is useful where the generator must be ready for a prolonged outage, but does not need to run every time the grid experiences a brief interruption.
The battery may provide several minutes of uninterrupted supply while the system assesses the situation. If the grid returns, the generator may not need to start at all. If the outage continues, the generator takes over as the long-duration source.
Is a hybrid system always the most sustainable option?
There is no automatic guarantee.
A hybrid system can reduce fuel consumption and generator running hours, but its actual performance depends on the control strategy, battery capacity, weather conditions and the facility’s load profile.
If the battery is too small, its contribution may be negligible. If it is too large, the investment may be difficult to justify. If the energy management system is poorly configured, the individual components may work against one another and create a system that is difficult to operate and diagnose.
Battery life must also be considered.
A battery energy storage system is not a permanent component. The design should take into account cell degradation, operating temperature, cooling, monitoring, fire protection and eventual replacement.
In commercial and industrial facilities, a hybrid installation should be treated as a complete energy system. Connecting batteries to a generator and adding an inverter is not enough. The installation needs a coherent control architecture, correctly coordinated protection and testing in all relevant operating modes.
The system should be able to answer practical questions:
What happens if the grid fails at night?
What happens if the battery is partially charged?
What happens if the generator fails to start?
What happens if solar production suddenly drops?
What happens if one of the communication links between the control components is lost?
If the project has no clear answer, the hybrid concept is not finished.
Which solution is suitable for a manufacturing plant?
In a manufacturing plant, a diesel generator is often the first technology considered, particularly where the site requires high output, long autonomy and independence from the gas network.
The final decision should be based on the production process.
If a power failure can damage materials, interrupt a furnace cycle, spoil a batch, stop a cooling system or require several hours of restarting procedures, the generator must be selected for the process as a whole rather than for the average power demand.
Starting currents need particular attention.
An electric motor, compressor or pump may temporarily require several times more power during start-up than during normal operation. If the generator is selected only by adding together the nominal ratings of the connected equipment, voltage dips or failed starts may occur.
A hybrid system may be helpful in this type of environment. The battery can support starting sequences and short load peaks, while the generator provides long-duration power.
In other situations, a conventional generator with the right rating and a carefully designed load-sequencing system may be the better solution.
The technology should come from the load analysis, not from fashion.
Which backup power source works best in a public facility?
Public facilities have to consider more than power output.
User safety, noise, visual impact, ventilation, fuel storage and the ability to carry out maintenance without disrupting the building all matter.
A school, public office, library or sports facility may not need every load to remain energised during an outage. It may be enough to separate and protect priority circuits. These can include fire protection systems, emergency lighting, access control, CCTV, server rooms, communication systems, pumps and selected ventilation equipment.
In this scenario, a battery energy storage system can provide immediate support to critical circuits while a generator supplies the facility for a longer period.
A gas generator may also be worth considering in a densely built environment, provided that the gas infrastructure offers the required pressure, flow and reliability.
The installation location is crucial.
A generator installed in an underground car park, on a roof or close to occupied rooms requires a detailed assessment of noise, vibration, ventilation and exhaust routing. The design must also allow technicians to access the equipment safely during inspection and repair.
Diesel installations require additional planning for fuel storage and fuel quality control. Fuel kept on site for long periods must be stored under appropriate conditions and included in a regular maintenance and monitoring programme.
The generator should be treated as part of the building’s safety infrastructure, not as an oversized appliance that can be placed wherever there happens to be a free corner.
What matters more: generator output or runtime?
Both are important, but they answer different questions.
Output determines which loads the generator can supply at the same time. Runtime determines how long it can supply them without refuelling or support from another energy source.
A high-output generator with a small fuel tank may stop after only a few hours. A smaller generator with a large fuel reserve will not solve the problem if it cannot handle the connected loads.
The design should therefore define at least three operating scenarios:
A short interruption.
An outage lasting several hours.
A prolonged loss of grid power.
For each scenario, the facility should determine which loads remain active, how much power is required and whether refuelling during operation is possible.
In strategically important facilities, fuel must remain available even when roads are blocked, deliveries are delayed and local infrastructure is operating under emergency conditions.
The same principle applies to batteries. Their autonomy should be calculated under the actual expected load, including the influence of temperature, ageing, reserve capacity and the required end-of-life performance.
A battery that provides fifteen minutes on the day of commissioning may provide something very different several years later if degradation has not been included in the design.
What mistakes are most common when selecting a backup power source?
The first mistake is sizing the generator based solely on the facility’s connection capacity.
Connection capacity does not necessarily represent the actual operating demand. It also does not describe motor starting currents, nonlinear loads or the sequence in which equipment will be energised.
The second mistake is overlooking the automatic transfer system.
A generator may have sufficient output, but if the automatic transfer switch does not operate correctly, the power will not reach the required circuits. The entire system must be tested as one installation: grid supply, control system, generator, switchgear, protection and loads.
The third mistake is failing to test under load.
An engine starting without load does not confirm that the system is ready to operate under real conditions. Load acceptance, voltage stability and frequency recovery should be verified during commissioning and periodic testing.
The fourth mistake concerns the installation location.
A generator requires airflow, cooling, exhaust routing, fuel access and sufficient space for maintenance. Equipment pushed into an unsuitable corner can create problems during every inspection for years afterwards.
The fifth mistake is treating service as an optional extra.
A backup power system should have a maintenance schedule, a testing procedure, spare parts availability and clearly assigned responsibility for responding to alarms and failed starts.
The sixth mistake is ignoring the human factor.
A sophisticated system is only as reliable as the people who know how to operate it. Someone must understand the control panel, acknowledge alarms, initiate manual procedures, coordinate refuelling and know when the system should be taken out of service.
Can a diesel generator, gas generator and battery system be combined?
Technically, yes.
Whether it makes sense depends on the size and operating profile of the facility.
A multi-source system may be justified in a large site where the battery provides an immediate response, the gas generator operates over a longer period and the diesel generator serves as an independent reserve in case the gas supply is interrupted.
Such a configuration requires advanced control systems and precisely defined priorities.
The system must know when to use stored energy, when to start the gas generator, when to switch to the diesel unit and how to respond if one of the sources becomes unavailable.
Every additional component also creates another point requiring inspection, testing, documentation and specialist knowledge.
A more complex system can provide greater resilience against a single failure, but only if it has been designed properly and can be maintained by people who understand how the complete architecture works.
Sometimes two well-integrated sources are more reliable than three poorly coordinated ones.
How can you check whether the selected source meets the requirements?
The design stage should cover electrical installation requirements, fire protection, noise, emissions, fuel, ventilation, control systems and maintenance.
The applicable European emissions requirements must be checked for the specific engine and application. Regulation (EU) 2016/1628 establishes requirements relating to gaseous and particulate pollutant emission limits and type approval for certain internal combustion engines used in non-road mobile machinery. The exact obligations depend on the engine, power category, application and classification of the equipment.
Requirements from one equipment category should not automatically be transferred to another. A stationary generator, a transportable generator and an engine used in a specific mobile application may be subject to different requirements.
Technical documentation is equally important.
It should include electrical diagrams, generator ratings, engine data, control-system information, installation conditions, fuel requirements and maintenance instructions.
If the source is intended to supply a public or industrial facility, the documentation should support commissioning, acceptance testing, inspections and future maintenance.
Paperwork does not generate electricity, but missing paperwork can certainly prevent a system from being accepted and placed into service.
Diesel, gas or hybrid: how should the final decision be made?
A diesel generator will usually be the most suitable option where the main priorities are:
- long runtime;
- high output;
- independence from the gas network;
- on-site fuel storage;
- operation in demanding conditions;
- established service support.
A gas generator may be worth considering where the facility has a stable gas connection, local emissions and noise are important considerations, and the unit can operate regularly or form part of a cogeneration system.
A hybrid system may be the right choice where the facility requires an immediate response, has a variable load profile, wants to reduce fuel consumption or plans to integrate renewable energy sources.
It also requires more extensive design work, control systems and operational expertise.
The best backup power source does not have to be the most technologically advanced one. It has to be properly matched to the problem it is expected to solve.
Start with practical questions.
How long can the facility operate without the grid?
Which loads are genuinely critical?
Will fuel be available during a prolonged outage?
How reliable is the gas supply?
How frequently will the generator operate?
Who will perform the tests?
Who will approve refuelling?
Who will respond if the generator fails to start?
Only after these questions have been answered does it make sense to compare specific technologies, output ratings and configurations.
How should the technology be matched to the actual facility?
The choice between diesel, gas and hybrid power should be based on the facility’s load profile, required autonomy, available infrastructure and operating environment.
| Criteria | Diesel generator | Gas generator | Hybrid system |
|---|---|---|---|
| Best suited for | Industrial sites, critical infrastructure and facilities requiring long runtime | Urban buildings, facilities with a stable gas supply and cogeneration applications | Sites with variable loads, battery storage and renewable generation |
| Runtime | Long, depending on the fuel reserve | Long, provided that the gas network remains available | Short from the battery, extended after the generator starts |
| Independence from infrastructure | High — fuel can be stored on site | Dependent on the continuity of the gas supply | Dependent on the configuration and availability of each source |
| Response to grid failure | Very fast after automatic start-up | Fast, provided that gas pressure is sufficient | Immediate through the battery; the generator can start later |
| Large loads and starting currents | Very good when correctly sized | Good, depending on the engine and gas installation | Very good when the battery and generator are properly coordinated |
| Operation in demanding conditions | Established technology for industrial and outdoor applications | Requires stable gas infrastructure and suitable installation conditions | Effective when the control system and protection are correctly designed |
| Local emissions | Requires exhaust management and compliance with applicable emissions requirements | Often lower local emissions, depending on engine technology | Can reduce generator runtime and fuel consumption |
| Noise | Depends on enclosure, acoustic treatment and installation location | Often suitable for urban applications, but still requires a project-specific assessment | Battery operation is quiet; the generator still requires acoustic control |
| Service and operation | Established service networks and broad parts availability | Requires engine service and inspection of the gas installation | Most complex: batteries, inverter, generator and energy management system |
| Investment cost | Usually predictable, particularly at higher output ratings | May increase due to the required gas infrastructure | Often the highest because of battery storage and advanced controls |
| Main risk | Poor fuel quality, inadequate load testing or insufficient ventilation | Loss of gas pressure or interruption of the gas supply | Undersized battery, incorrect control logic or future battery replacement costs |
| Main advantage | Independence, high output and long autonomy | Lower local emissions and potential cogeneration | Immediate response, flexibility and reduced fuel consumption |
Diesel provides independence and long runtime. Gas can reduce local emissions and support cogeneration. Hybrid systems can combine the immediate response of battery storage with the long-duration capability of a generator.
Every option can work very well.
Every option can also be poorly selected.
That is why the decision should be based on a load analysis, clearly defined priority circuits, verified installation conditions, realistic testing procedures and a full cost-of-ownership calculation.
Before purchasing, it is worth reviewing the diesel generators and backup power solutions available from ElectroQuell, including generators for industrial applications and configurations tailored to the requirements of specific facilities. If purchasing a unit is not the right option at a particular stage, a generator can also be rented for temporary power supply during an outage, installation upgrade, event or project.
For more practical content about generators, energy independence and power continuity, follow ElectroQuell on LinkedIn.
Sources and reference documents
- ISO 8528-1:2018 — Reciprocating internal combustion engine driven alternating current generating sets
- IEC 60364-5-56:2018 — Low-voltage electrical installations and safety services
- Regulation (EU) 2016/1628 — Emission limits and type approval for internal combustion engines
When the grid fails, the generator will not have time to reflect on its technological identity. It will simply have to start ... preferably at the exact moment when everyone else in the building is still standing in the dark, trying to decide who should call the electrician.