GuidePower & facilities
Sizing a UPS for a server room: a practical method
By Inara Crest Engineering 2 min read
Add up the real load, check both watts and VA, allow headroom and decide how much runtime you actually need.
An undersized UPS shuts down at the moment you need it; an oversized one wastes money and battery capacity. Sizing well comes down to four steps: measure the load, convert it correctly, add headroom and decide on runtime.
1. List every load the UPS must protect
Include servers, storage, network switches, the firewall, the internet router and any monitoring equipment. Leave out laser printers and heaters — their surge draw can overload a UPS.
Use measured power where possible, from a metered power distribution unit or the server’s management interface. Nameplate ratings show the maximum the power supply can deliver and usually overstate real consumption.
2. Check watts and VA
UPS units are rated in both volt-amperes (VA) and watts (W). Your total load must fit within both ratings. To estimate VA from watts, divide by the power factor of the load; modern server power supplies are typically around 0.9 or higher.
3. Add headroom
Run the UPS at no more than about 75–80% of its rating. This leaves room for growth and avoids overloads during start-up surges.
4. Decide on runtime
Runtime depends on battery capacity and load, and manufacturers publish runtime charts for each model. Decide what the UPS is for:
- Bridging to a generator — a few minutes may be enough, provided the generator starts reliably.
- Riding through short outages — 15–30 minutes covers most brief interruptions.
- Clean shutdown — enough time for servers to shut down automatically, which requires the UPS to signal them over the network or USB.
Longer runtimes usually mean external battery packs, which add cost, weight and maintenance.
5. Choose the topology
Standby and line-interactive units are economical for desktops and small network cabinets. Online double-conversion units continuously regenerate clean power and are the usual choice for server rooms with sensitive equipment or unstable mains supply.
6. Plan for battery life
Sealed lead-acid batteries are rated at around 25 °C and wear out significantly faster in hot rooms. Expect to replace them every few years, test them periodically and keep the UPS in a cooled space.
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About the author
Inara Crest Engineering
Engineering team
Written by the Inara Crest Technologies engineering team, who design, install and support software, network, security, power and data centre systems for organisations in Kenya and East Africa.