How to size a UPS for your network rack

The number printed largest on a UPS box is its VA rating, and it is the least useful number for deciding which one to buy. What you actually care about is how long the rack stays up, and whether the UPS can carry the load at all.

Work out the real load first

Add up the continuous draw of everything in the rack, in watts, not VA:

• A 24-port PoE switch with twelve access points at roughly 8 W each: about 150 W including the switch itself.
• An NVR with four surveillance drives: about 60 W.
• A router and a small PBX appliance: about 30 W.

That is roughly 240 W. Now add headroom, because a UPS running at its limit has a short, unhappy life. Size for about 70% of the UPS’s rated wattage, so you want a unit rated near 350 W.

VA is not watts

Manufacturers quote VA, and the relationship depends on the power factor. Older line-interactive units often assume 0.6, so a 600 VA unit delivers only about 360 W. Modern units quote 0.9 or higher. Always find the watt rating on the datasheet before you compare prices; two units with the same VA can differ by a third in real capacity.

Then choose runtime

Runtime is a battery question, not a VA question. At 240 W:

• A typical 600 VA / 360 W unit gives roughly 5 to 8 minutes. Enough to ride out a flicker and shut things down cleanly. Not enough to keep trading.
• Doubling the battery capacity roughly doubles the runtime, but the curve is not linear at high loads; a heavily loaded UPS drains disproportionately fast.

Decide what the runtime is for. Surviving a brief outage and a clean shutdown is a different purchase from keeping the cameras recording through a long one.

Line-interactive or online

Line-interactive is right for most offices and counters. It is cheaper, quieter and efficient, and it switches to battery in a few milliseconds, which every piece of IT equipment tolerates.

Online, or double-conversion, runs everything through the inverter continuously, so there is no transfer time at all and the output is clean regardless of what the mains is doing. Worth it for a rack on an unstable supply, or where a generator with a rough waveform sits upstream.

Batteries are the consumable

Sealed lead-acid batteries last about three to five years, less in a hot room. A UPS that has never had its battery replaced is a UPS that will fail at exactly the wrong moment. Budget for replacements and test the runtime once a year, under real load, before you need it.

Tell us what is going in the rack and we will size it with you.