How to size a standby generator

A standby generator has to support the required loads at the moment it is needed. Adding the rated kilowatts of all connected equipment and applying a standard margin is not enough. Two facilities with the same running load can behave very differently when motors start, a large block of load is connected or an uninterruptible power supply begins charging. Reliable technical sizing therefore starts with a complete load and duty profile.

Identify the essential loads

List the equipment that must remain available during an outage. This may include production machinery, pumps, ventilation, refrigeration, controls, IT systems, lighting and safety-related systems. Separate essential loads from equipment that can remain off or be connected later.

For each load, record its function, rated kW or kVA, voltage, number of phases, frequency, power factor, starting method, available starting-current data, switching sequence, priority and required operating duration. The total connected load is only the starting point; simultaneous operation and the largest load step are often more important.

Understand kW and kVA

kW is real power and kVA is apparent power. Power factor links both values, but the simple relationship between them does not represent the complete generator-sizing task. Harmonics, phase imbalance, nonlinear loads and transient behaviour can require a different alternator or generator-set selection.

Account for motors and compressors

Motors can draw substantially more current while starting than during normal operation. The effect depends on the motor, starting method, driven equipment and permitted voltage dip. Pumps, compressors, fans, conveyors and refrigeration equipment should therefore be assessed individually. Sequential starting, soft starters or variable-frequency drives may change the required generator size, but their suitability must be confirmed for the application.

Assess UPS and nonlinear loads

UPS systems, rectifiers, chargers and electronic drives may create harmonics and place additional demands on voltage regulation and alternator capability. For data centres, telecommunications and sensitive process controls, the input characteristics of the electronic equipment and its interaction with the generator should be reviewed rather than covered by an arbitrary margin.

Define load steps and switching sequence

Connecting a large block of load can cause a temporary voltage and frequency dip. The acceptable response depends on the equipment being supplied. A staged sequence can connect essential loads first and add lower-priority loads afterwards. Technical sizing should therefore include the maximum individual load step and the required recovery performance.

Match the rating to the duty

A generator used only during a utility outage is not rated in the same way as a set intended for regular prime-power or parallel operation. Ratings such as ESP, PRP, LTP and COP describe different duties. The largest number on a datasheet is not automatically the correct project rating. Intended operating hours, variable or constant load, automatic or manual operation, redundancy and parallel operation must be stated and checked against the current manufacturer documentation.

Include the site conditions

Altitude, ambient temperature, dust, humidity, ventilation and installation location may affect available output and configuration. Indoor installations require adequate combustion and cooling air, heat rejection, exhaust routing and service access. Outdoor projects also require a review of weather protection, drainage, foundations, acoustics and local requirements.

Use a project-specific margin

A margin may allow for future loads, incomplete information or operational flexibility, but one percentage is not correct for every project. Excessive oversizing is not automatically safer. The margin should follow the verified load profile, duty and manufacturer requirements.

Information to send with an enquiry

Provide the load list or requested output, voltage, frequency, phases, largest motors and starting methods, UPS or electronic loads, operating duty, required run time, indoor or outdoor location, preferred enclosure, fuel supply, controls or changeover requirements, site temperature and altitude. German Generator can then size and narrow the available range for the project. Automatic and AMF solutions, synchronisation and single or parallel operation are available; the final configuration is checked against the load profile, site conditions and current manufacturer documentation.