In short. UPS power is sized from actual draw, not from the number of servers: an 800 W power supply at 60 % load delivers about 480 W, not 800. Add up servers, switches and storage, divide by a 0.9 power factor to get VA, then add 25 % headroom. Runtime comes from the batteries: 15 minutes covers a clean shutdown, 30 to 60 minutes rides out the usual mains dips. The calculator below works out both.
Calculated with a 0.9 power factor, 0.9 inverter efficiency and 0.8 depth of discharge. Battery capacity is given for a 48 V bus — entry-level single-phase UPS units often use 24 or 36 V, so scale the figure proportionally.
How the numbers are derived
Active load power is the sum of what every device draws. For servers take the real load, not the label on the power supply: a typical dual-socket 2U with a pair of Xeon Silver CPUs and eight drives pulls 350 to 500 W against an 800 W supply rating. Redundant power does not double consumption — the two supplies share the load.
Apparent power in volt-amperes is watts divided by the power factor. Modern supplies with active correction sit at 0.95 to 0.99, but a UPS is sized at 0.9 so that older gear in the same rack still fits. Then 25 % is added on top: for inrush current when everything restarts after an outage, and for growth.
Battery capacity follows from energy: load in watts times runtime in hours, divided by the battery bus voltage, the inverter efficiency and the allowed depth of discharge. That gives amp-hours. Real runtime is always below the calculation on aged batteries — sealed lead-acid loses roughly 20 % of capacity in three years.
Which class of UPS
Line-interactive suits a single server and a network node in an office: it corrects voltage in steps and transfers to battery in 4 to 8 ms, which server power supplies ride through.
Online double conversion is needed once the rack holds two or more servers, has storage, or the load exceeds 3 kW. It delivers an uninterrupted sine wave and protects against frequency deviation — mandatory where a generator is in the loop.
Size the air conditioner separately: it is normally left off the UPS, but a tightly sealed server room without cooling overheats in 15 to 25 minutes. The practical answer is runtime enough for a clean shutdown plus an SNMP-driven stop script.
Next steps
Work out the draw of your own machines: we list rack servers and tower servers with nameplate data on each card, plus storage systems. If the configuration is not chosen yet, the configurator picks one in three steps, and you size the power here. We match UPS models to the result by hand: send the figures to info@itsmart.uz or call +998 91 004 98 58.