UPS Sizing for Server Rooms: kVA, kW, Runtime, and the Math

Step 1: Calculate Total Load (Watts)

Add nameplate wattage of every device that will be on UPS. Typical enterprise loads:

• 1U rackmount server: 300–500W
• 2U server: 500–900W
• Top-of-rack switch (48-port PoE+): 150W base + up to 1440W PoE load
• Storage array (2U SAN, 12 bays): 400–700W

Step 2: Convert Watts to kVA

UPS is rated in kVA (apparent power), servers draw watts (real power). Use a power factor of 0.9 for modern servers, 0.7 for mixed loads.

kVA needed = Total Watts ÷ Power Factor ÷ 1000

Example: 8,000W load ÷ 0.9 = 8,889 VA → need at least 10 kVA UPS (round up, leave headroom).

Step 3: Runtime

Size battery bank for graceful shutdown time + DG startup time. Typical target: 15 min at full load. For sites with no DG backup, go 60–90 min. Battery runtime is not linear — halving load more than doubles runtime due to Peukert's law.

Topology: Line-Interactive vs Online Double-Conversion

• Line-interactive (APC SMT, Eaton 9PX-Lite): cheaper, fine for offices, switches over in ~4ms
• Online double-conversion (APC Smart-UPS SRT, Eaton 9PX, Vertiv GXT5): zero transfer time, clean sine wave, required for servers/SAN

Our Standard Picks (India)

• Up to 3 kVA — APC Smart-UPS SRT 3000
• 5–10 kVA — Eaton 9PX 6000/10000
• 10–20 kVA — Vertiv Liebert GXT5 10/20K
• 20+ kVA — Vertiv Liebert iTA2, Eaton 93PS