Standby power is the correct industry and technical term for a power source that is normally inactive but automatically activates to provide electricity when the primary utility power fails. In a data center context, this is typically achieved through a combination of an Uninterruptible Power Supply (UPS) for immediate, short-term power and a standby generator for long-term outages. The standby system's sole function is to ensure continuity of operations by taking over the electrical load from the failed primary source.

B. Passive: This term describes a component that does not require power to operate (e.g., a heat sink), not a source of electrical power.

C. Active: This term describes the primary power source that is currently in use and supplying electricity, not the backup source.

D. Alternate: While a standby source is an "alternate" source, "Standby" is the more precise technical term for a system specifically designed to activate upon the failure of the primary source.

1. Uptime Institute, "Tier Standard: Topology," (2018). Section 6.3, "Engine-Generator Requirements," describes the role of engine-generators as the standby power source required for Tier III and Tier IV data centers to provide power during a utility outage. It explicitly uses the term "standby" to define this function.

2. Telecommunications Industry Association (TIA), "ANSI/TIA-942-B: Telecommunications Infrastructure Standard for Data Centers," (2017). Annex E, "Data Center Space Planning," and Section 7, "Telecommunications Infrastructure," detail the requirements for redundant power, including standby generators and UPS systems, as essential components for achieving higher availability ratings.

3. National Fire Protection Association (NFPA), "NFPA 110: Standard for Emergency and Standby Power Systems," (2022 Edition). Chapter 3, Section 3.3.4, defines a "Standby Power System" as "A system intended to automatically supply power to selected loads (other than those classified as emergency systems) in the event of a utility power failure." This standard is a foundational document for designing such systems in critical facilities.