UPS capacity and battery runtime must be calculated separately. This example shows a practical planning method without pretending that a generic runtime figure applies to every UPS.
Practical example: protecting a small server rack
Capacity and runtime are separate UPS sizing questions.
Scenario: A rack contains a server, NAS, managed switch and router. Their illustrative planning load totals 860 W.
Adding 25 percent design headroom gives approximately 1,075 W. The UPS must support both the required watt load and VA load. Runtime is then checked separately against the manufacturer's runtime data for the exact UPS and battery configuration.
Decision: First confirm capacity, then decide whether the objective is only to bridge short interruptions, allow an orderly shutdown or maintain operation for a longer period. Each objective produces a different battery requirement.
Common mistakes and selection checklist
Common mistakes
- Selecting a UPS from VA alone without checking its watt rating.
- Assuming the advertised maximum runtime applies to the actual rack load.
- Connecting equipment without considering startup load, growth or the UPS topology required by the application.
What happens if you get it wrong?
The UPS may overload, provide far less runtime than expected, or fail to achieve the intended objective of bridging an outage or allowing an orderly shutdown.
Selection checklist
- Measure or estimate the real watt load of the equipment being protected.
- Check both the UPS watt and VA capacity.
- Add sensible design headroom for growth and transient load.
- Use the manufacturer's runtime data for the exact model and load.
- Define whether the goal is ride-through, graceful shutdown or extended operation.
Example protected load
| Equipment | Planning watts |
|---|---|
| Server | 420 W |
| NAS | 160 W |
| PoE switch | 220 W |
| Router and fibre equipment | 60 W |
| Total | 860 W |
Add capacity headroom
Using 25 percent planning headroom gives 1,075 W. The selected UPS must support at least this watt load and the required VA load. Eaton emphasises checking both ratings because UPS units with the same VA rating can have different watt capacities.
Runtime is model-specific
Battery runtime depends on load, battery capacity, battery age, temperature and optional battery modules. Use the runtime chart for the exact UPS model at the planned load.
Plan shutdown behaviour
Decide whether the UPS must bridge short interruptions, allow graceful shutdown, or support prolonged operation. Those are different battery requirements.