Know both UPS limits
A UPS usually has both kVA and kW ratings. The kVA rating describes apparent power capacity, while the kW rating describes real power capacity.
Modern IT equipment often has high power factor, but the UPS still must be checked against both limits. Exceeding either one can overload the unit.
Build a load estimate
Add the wattage of servers, storage, network equipment, monitors, controls, and any supporting devices connected to the UPS. Use measured power where possible, not only power supply nameplate maximums.
Then add a growth allowance and check runtime requirements. A UPS sized only for today may become undersized after one rack expansion.
- Measure actual load where possible
- Check kW and kVA capacity
- Reserve capacity for future equipment
- Confirm battery runtime at the planned load
Avoid common mistakes
Do not size a UPS only from the biggest label on each server power supply. Redundant supplies and real operating load can make nameplate-only sizing misleading.
Also verify input circuit capacity, bypass requirements, maintenance access, cooling, and monitoring before final selection.