Before selecting a power generation system for a particular application, it is important to determine the requirements of the electrical service and load. That will allow the generator and transfer switch to be properly sized.

A standby power system can be configured to take all, or a portion, of the electrical load in the event of a power interruption. If it is configured to assume the entire load, the calculation of size must encompass all the elements of the electrical load within the facility. If it is configured to assume only the critical portion of the electrical load, then the requirements of the critical load must be determined, and that load must be isolated from the non-critical portion onto a circuit, or bus, of its own. The standby power system will then be connected to that bus, with the generator and transfer switch ready to connect and supply power to the critical load when needed.

Critical loads are commonly comprised of emergency lighting systems, key machinery, and / or data processing systems that are deemed essential. The number and mix of these items will determine the requirements of the critical load. Certain items like air conditioning systems (unless providing critical cooling to certain portions of the facility) are often considered non-essential, since they draw large amounts of power and are merely an inconvenience when not operating.

Certain types of devices such as electric motors, UPS (uninterruptible power supply) systems, electronic lighting ballasts, etc. may require special consideration as part of the load because they require more power for startup. The higher starting requirements for electric motors, and the special needs of non-linear loads may necessitate additional starting kVA (kilovolt amps) via a larger alternator. Consult with your electrical engineer or contractor to ensure that your generator selection meets your needs.