Guides
A server room UPS is chosen by load, phase, topology, runtime and where it will stand. Most server rooms use single-phase rack or tower UPS; larger rooms move to three-phase. The decisions are the same as in a data centre, at a smaller scale.
It depends on the load. Eaton's UPS basics paper puts single-phase UPS where computing power is less than 20,000 VA and three-phase UPS for high kVA applications. A server room near that line should plan for growth: Eaton advises estimating needs for the next three to five years when choosing.
Online (double conversion) for anything critical; line-interactive is acceptable for small, non-critical loads on clean power. Schneider Electric's White Paper 79 notes that in the range below 5000 VA both designs are available and the choice depends on the installation, while above 5000 VA line-interactive has been impractical. Eaton describes line-interactive UPS as preferable for small server and network loads in facilities with relatively trouble-free utility power. The full comparison is in types of UPS.
Enough to carry the load until the generator takes over, or to shut down cleanly if there is no generator. Eaton puts a typical UPS battery at five to 15 minutes and notes that external battery modules or cabinets can add up to several hours at full load. Estimate it with the runtime calculator, then confirm with the maker's runtime table for the exact UPS and battery.
A design choice more than a technical one. Eaton notes that the largest UPS are not made in rack form, and that between rack-mounted and freestanding UPS neither approach is inherently superior; some sites consolidate into racks, others keep rack space for servers. Weight and battery access matter in practice: battery modules are heavy and need room to slide out.
When the load grows past what single-phase UPS handle comfortably, or when redundancy and runtime call for a larger system. Eaton places three-phase UPS in high kVA applications with high compute densities, such as data centres and large buildings that distribute power over long distances. A server room approaching that size benefits from the same planning as a data centre: measured load, a growth margin, redundancy and a maintenance programme; see UPS sizing.
When the UPS says so, or on schedule. Schneider Electric's SRT manual says to replace the battery at least every 5 years and immediately when the UPS indicates replacement is needed, and lists heat, high humidity, poor utility power and frequent short discharges as causes of shorter life. Replacement batteries for server-room UPS are listed in a replacement UPS battery catalogue; for UPS service, see UPS services.
Add up the load in VA, add margin for growth, and choose a UPS at or above that; Eaton's method multiplies the total by 1.2. The UPS sizing guide walks through it.
For small, non-critical servers on clean power, often yes. For anything critical, or on a generator, choose online double conversion.
Long enough to reach the generator, or to shut down cleanly. Eaton puts a typical battery at five to 15 minutes.
On a small UPS like the Smart-UPS On-Line SRT, at least every 5 years, and immediately when the UPS calls for it.
Sources, checked 2026-10-04:
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