What a standby generator is for
A standby generator is installed to keep selected equipment running when the normal utility supply fails. It sits idle most of the time, starts automatically or on command when power is lost, and carries the load until the utility returns. Typical reasons to have one include protecting refrigerated storage, keeping process lines from spoiling a batch, holding emergency lighting and life-safety systems, and maintaining control and communications equipment.
Not all generators are meant for the same job. A unit rated for standby service is expected to run for the limited hours of an outage, not for months at a time. Sites with no grid at all need a different class, prime power generators, designed for continuous use. This article focuses on backup duty and what to settle before choosing one.
The information here is general. Generator selection and installation involve electrical codes, fuel storage rules and air quality permits that vary by location, so work with a qualified engineer or supplier.
Decide what must keep running
The most useful first step is not a generator size but a list. Walk through the facility and sort loads into three groups:
- Life safety and legal requirements. Emergency lighting, alarm systems, fire pumps and similar equipment, which may have specific code requirements.
- Critical operations. Equipment whose loss would cause serious damage, safety problems or large financial loss, such as refrigeration, critical process controls and data systems.
- Everything else. Loads that can wait for the utility to return.
Backing up the whole plant is possible but usually much more expensive than backing up the critical group. Many facilities connect only essential panels to the generator, which also keeps the required size down.
Where a short interruption can disrupt electronics or controls, a generator alone does not help because it takes seconds to start. Such systems often rely on uninterruptible power equipment to bridge the gap until the generator is ready to carry the load.
How the pieces fit together
A standby system is more than an engine and an alternator. Typical parts include:
- Prime mover. The engine, usually reciprocating, that burns the fuel.
- Alternator. Converts the engine’s rotation into electrical power.
- Controls. Monitor the system, start it on loss of power and protect it from faults.
- Automatic transfer switch. Disconnects the facility from the utility and connects it to the generator, and back again afterward. It prevents the generator from feeding power back onto the utility lines, which is dangerous for utility workers.
- Fuel system. Tank, piping, filters and sometimes a day tank.
- Cooling, exhaust and ventilation. Engines need large amounts of air and a safe path for exhaust.
- Enclosure. Indoor room or weatherproof housing, sometimes with sound attenuation.
Sizing in outline
Sizing starts with the total running demand of the loads you decided to back up, expressed in kilowatts, plus an allowance for future growth. It then needs adjustment for what happens at the moment of start-up, since motors draw several times their running current as they accelerate. Generators are rated in kilovolt-amperes (kVA) and kilowatts (kW), related by the power factor. For example, a hypothetical 100 kVA unit at a power factor of 0.8 delivers about 80 kW. Loads with poor power factor draw more current for the same useful power.
Other sizing factors include the sequence in which loads are started, harmonic loads from electronic drives, altitude and temperature, which reduce the output of many engines, and voltage dip limits for sensitive equipment. Careful generator sizing, including starting versus running demand, is best backed by a professional load study, which helps avoid a unit that is too small to start the largest motor or too large to run well at light load.
Fuel choices
Common fuel options for standby engines each have trade-offs.
| Fuel | Points in its favor | Points to weigh |
|---|---|---|
| Diesel | Widely used, stores on site, strong starting reliability record | Fuel ages and needs testing and rotation; tank rules and spill controls apply |
| Natural gas | No on-site fuel delivery; low storage burden | Depends on pipeline supply, which may be affected by the same events that cut power |
| Propane | Stores on site, keeps well | Tank size limits runtime; vaporization can be a factor in cold weather |
| Dual-fuel | Some engines can use two fuels | Added complexity |
Runtime is a major driver. Ask how long an outage might realistically last in your region and how fast fuel can be resupplied. On-site storage is sized for a number of hours at a given load, not simply a full tank. Fuel stored for long periods needs periodic care, and environmental rules often cover tanks and spill containment.
Placement and permits
Generator location affects everything from noise to safety. Engines produce exhaust that must be vented away from air intakes and occupied spaces. They also need access for fuel delivery and maintenance, adequate clearances, fire protection and protection from flooding. Local rules may limit noise, emissions and the number of testing hours per year, and a permit is often required. Foundations, grounding systems and wiring should be handled by licensed professionals.
Testing and care
A standby generator that has not been tested is a hope rather than a system. Exercising it on a schedule under load, checking batteries, coolant, oil, fuel quality and the transfer switch, and recording the results are the heart of good practice. The manufacturer’s manual sets the intervals, and the importance of regular generator maintenance becomes obvious the first time a long outage tests an engine that has not run in a year. Starting batteries are among the most common reasons a standby set fails to start, so they get special attention.
Facilities that plan around disaster recovery equipment often fold generator testing into broader emergency exercises, so operators know what to do and which loads will return first.
A short decision list
- List loads by priority and measure their demand.
- Choose the duty class: standby for outages, prime for continuous use.
- Select fuel based on supply security and storage rules.
- Size for running load, start-up demand and growth with professional input.
- Plan placement, permits, exhaust and fuel storage.
- Commit to scheduled testing and maintenance before buying.
Outcomes during a real outage depend on many variables, and no generator is a guarantee, but careful planning makes a good result much more likely.