Standby Generator Basics for Data Centers: Selection, Sizing, and System Design

Your UPS Buys You Minutes. Your Generator Buys You Hours. Here’s How to Choose the Right One.

By Molly Gross, Principal, Power Solutions, LLC

For any facility that cannot afford extended downtime, a standby generator is as essential as the UPS itself. This whitepaper explains how generators work, what types are available, and how to choose the right one.

UPS systems provide minutes of backup power — enough for a generator to start. But for data centers, hospitals, campuses, and manufacturing floors that need to sustain operations through extended outages, a standby generator is a non-negotiable part of the power protection system. This whitepaper covers the three most common generator types (standby, prime, continuous), explains how the generator system works with the UPS and ATS, and walks through the key factors for selecting the right generator for a data center application: load requirements, fuel type, Tier classification, environmental considerations, and ATS integration. Power Solutions provides standby generator sales, rentals, and service.

Whitepaper Summary

Q: What is the difference between a standby, prime, and continuous generator?

A: Standby generators are for emergency use when the primary power source fails. Prime generators can serve as a primary power source with unlimited run time. Continuous generators supply constant loads but are not rated for overloads — rarely used in data center applications.

Q: What Tier of data center requires a prime generator vs. a standby generator?

A: Tier I and II data centers typically use standby generators. Tier III and IV data centers require prime generators capable of sustaining the load for unlimited hours.

Q: How does a standby generator work with a UPS?

A: When the UPS detects an outage, it immediately provides battery power. The ATS detects the outage, commands the generator to start, and switches the power source. Once the UPS confirms stable generator power, it transfers the load off battery.

Q: What is an Automatic Transfer Switch (ATS) and why is it required?

A: The ATS is the control mechanism that monitors both utility and generator power and automatically switches the power source when the utility fails. It is an essential component of any standby generator system.

Q: What factors determine the right generator size for a data center?

A: Key factors include total connected load (with future growth considered), power factor, Tier classification requirements, fuel type, environmental constraints, run time requirements, and integration with existing UPS and ATS infrastructure.

Download this whitepaper.