Choosing the correct generator size is essential for reliable business operations. A generator that is too small may experience overload, voltage drop, unstable frequency, overheating, or shutdown. A significantly oversized generator can increase investment cost and spend long periods operating inefficiently at low load.
The correct diesel generator capacity should be determined from the actual electrical load, motor-starting requirements, equipment operating sequence, power factor, required duty, site conditions, and expected future expansion.
A correctly sized generator should be able to support the normal running load and respond to temporary load changes without exceeding the permitted operating limits of the engine and alternator.
Correct sizing helps:
Generator sizing should not be based only on the building type or a general kVA estimate. Two factories of similar size can require completely different generators depending on their machinery and operating processes.
Before calculating generator capacity, prepare a list of every load that may operate from generator power.
For each item, record:
Separate essential loads from nonessential loads. During a utility failure, it may not be necessary to supply every circuit in the facility. Load prioritization can reduce the required generator size and project cost.
Kilowatts represent real power used to perform work. Kilovolt-amperes represent apparent power and include the effect of power factor.
The basic conversion is:
kVA = kW ÷ Power Factor
If the total electrical load is 400 kW at a 0.8 power factor:
400 kW ÷ 0.8 = 500 kVA
Many industrial generator ratings are stated at a 0.8 power factor, but this does not mean every connected load has an actual power factor of 0.8. The real power factor should be obtained from equipment data or site measurements where possible.
If equipment is already rated in kVA, its kVA value should not be converted from kW again.
Determine which systems must remain operational during a utility outage or when the generator is the primary power source.
Essential loads may include:
Removing nonessential loads from the generator supply can significantly reduce the required capacity.
Add the electrical demand of all equipment expected to operate at the same time. Do not automatically add the full nameplate rating of every device if normal operations do not require simultaneous use.
For example:
At an actual power factor of 0.8, the preliminary apparent-power requirement is 500 kVA.
Electric motors can require considerably more current during startup than during normal operation. Pumps, compressors, fans, conveyors, elevators, and refrigeration systems can therefore determine the required generator and alternator size.
Starting demand depends on:
Motor starting should be calculated separately. Adding a general percentage margin does not always provide enough capacity for a large direct-on-line motor.
Not every load needs to start at the same time. Starting the largest motors separately and connecting noncritical loads in stages can reduce the maximum instantaneous demand.
A controller, building-management system, or load-management panel can introduce loads in a planned sequence after the generator reaches stable voltage and frequency.
A reasonable margin can accommodate normal load variation and planned future expansion. However, there is no single percentage suitable for every project.
The required margin depends on:
For the 400 kW example, a 600 kVA generator may be a reasonable preliminary option if the actual power factor is 0.8 and approximately 20% additional capacity is required. The final selection must still be checked against motor starting, transient response, duty rating, and site conditions.
A generator’s prime and standby ratings are different. Standby power is intended for emergency use during utility failures, subject to the manufacturer’s permitted operating conditions. Prime power is intended for longer operation under a variable load.
If the calculated requirement is 500 kVA prime, a generator offering only 500 kVA standby may not be suitable. Quotations should therefore be compared using the same duty rating.
High ambient temperature and altitude can reduce engine and alternator capability. Poor ventilation can also increase operating temperatures and restrict performance.
Before final selection, confirm:
The manufacturer should apply the relevant derating factors when site conditions exceed the standard reference conditions.
Large motors can create high starting current and temporary voltage drop. Their starting method and operating sequence must be included in the generator calculation.
UPS equipment can introduce nonlinear loads, harmonics, and battery-recharging demand. The alternator and excitation system may require additional capacity to maintain acceptable voltage performance.
Elevators, hoists, and cranes have changing loads and significant starting or regenerative characteristics. Their drive type and operating cycle should be reviewed with the equipment supplier.
Welding equipment can create rapidly changing current demand. The generator should be checked for voltage dip and alternator response rather than selected only from average power consumption.
Compressors and pumps often contain some of the largest motors in a facility. Starting several units simultaneously can create a much higher demand than their combined normal running load.
Electricity bills can provide useful information about previous maximum demand, but they may not show motor-starting peaks or identify which loads must operate during a power failure.
For an existing facility, a power-quality analyzer or data logger can record:
Measured data generally provides a more reliable basis than estimating the load from equipment lists alone.
Application type can provide an initial reference, but it cannot determine generator size by itself.
Generator size depends on cranes, pumps, compressors, welding machines, lighting, temporary offices, and whether equipment operates simultaneously. Load sequencing can be especially important.
Industrial facilities should identify essential production lines, motor loads, compressors, HVAC equipment, and equipment that can be disconnected during an outage.
Hospital sizing should separate life-safety, critical, and noncritical loads according to the project requirements. Redundancy, transfer systems, and starting sequence may be as important as total capacity.
Offices, hotels, and shopping centers should evaluate lighting, elevators, pumps, HVAC systems, kitchens, fire systems, and IT equipment. Supplying only essential circuits may substantially reduce generator capacity.
Data-center calculations should consider UPS efficiency, battery recharge, cooling equipment, redundancy requirements, harmonics, and staged loading after a utility failure.
Powerpack offers generator sets for medium and high-power business applications. Selected models include:
A preliminary generator size can be estimated by calculating the running load, converting kW to kVA with the correct power factor, and allowing for project-specific growth. The final capacity must also account for motor starting, load sequence, duty rating, alternator performance, environmental derating, and installation conditions.
For critical or complex projects, provide a detailed load list or measured site data before selecting the final generator.
Need help calculating the correct capacity? Contact Powerpack Generator for project-based generator sizing and a detailed quotation.
