A UPS should be sized from the load it will support, not simply from the number of devices in the rack.
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.
Make an equipment list
List every device that will be connected to the UPS. Include servers, switches, storage, networking equipment and any other equipment requiring backup.
Determine power consumption
Use measured consumption where available. Manufacturer specifications can provide useful planning information, but maximum ratings may be higher than normal operating consumption.
Add the loads
Add the expected real power requirements of the equipment. The resulting figure provides a starting point for UPS sizing.
Allow headroom
Do not select a UPS that is expected to operate continuously at its maximum rating. Sensible spare capacity can also allow for future equipment additions.
Consider runtime separately
Capacity and runtime are related but are not the same design question. A correctly sized UPS may still require additional battery capacity if a longer runtime is required.
- List all protected equipment
- Determine actual or expected consumption
- Add the loads
- Check both watts and VA
- Allow sensible capacity headroom
- Determine required runtime
- Check physical installation requirements
The final UPS selection should always be checked against the manufacturer's specifications for the specific model.
Confirm the exact product specification, supported configuration and compatibility before ordering. Platform generation, firmware, licences and optional components can change what a product supports.