Choosing and sizing a UPS

1. Calculate the actual UPS load

List the router, switch, NAS, NVR and other devices connected to battery-backed outlets. Measure the mains-side AC load while disks start and camera night lights are on. If you measure the entire AC load of a PoE switch, its cameras and APs are already included. Do not add them a second time.

A power supply’s rated output is its capacity, not the device’s usual consumption. If measurement is unavailable, use a justified manufacturer maximum and record the uncertainty.

2. Example of watt and VA sizing

Assume a 180 W load and a measured or justified estimated load power factor of PF 0.9. Apparent power is 180 / 0.9 = 200 VA. Adding 25 % growth allowance gives minimum capacities of 225 W and 250 VA. Both limits must be met; this does not yet establish 15 minutes of battery runtime.

The load’s PF is not the ratio of the UPS’s rated watts to VA. If PF is unknown, measure it with a suitable meter or confirm the connected equipment’s specifications.

3. Select runtime separately

Use the chosen UPS manufacturer’s runtime curve at the actual 180 W load. A 25 % capacity margin does not increase runtime by 25 %. If the future load grows, check runtime at that load too. Battery age, temperature and condition affect actual results.

Example target: the NAS needs 4 minutes for a controlled shutdown, and you want to wait through a 3-minute outage before starting shutdown. Also allow the safety margin specified by the manufacturer; do not schedule shutdown for the estimated moment the battery runs out.

4. UPS type and connections

Count battery-backed Schuko/IEC outlets separately from surge-only outlets. Check the supplied data and installation cables, battery replacement, space and ventilation.

5. Use the calculator and verify the installation

Enter the load, PF, growth allowance and runtime target in the calculator. Transfer the same values to the product selector. If no result is found, adjust only a real requirement or request site-specific sizing; do not reduce values merely to make a product appear.

1. Verify backups and test the shutdown connection according to the manufacturer’s instructions during a maintenance window.
2. Confirm that the NAS shuts down before battery exhaustion and that any network path needed for shutdown remains operational.
3. Test restoration of mains power and service startup. Record the load, battery age and shutdown times.

Eaton: UPS sizing guide; Synology: UPS compatibility.

Continue with a practical implementation: NAS and UPS: controlled shutdown and a restorable backup.