How to choose the VA rating for a power supply

This question usually means picking a UPS with enough VA to power a computer or device power supply. The label on a power supply shows its maximum output, not what it actually draws. Follow these steps.

1. Find the real load. Use a plug-in watt meter or your PC's power reading. If you cannot measure, estimate that a gaming PC with a 750 W power supply often draws 300 to 500 W under load.
2. Convert to wall power. Power supplies are not perfectly efficient. Divide the output load by efficiency. A 400 W load at 90 percent efficiency draws about 444 W from the wall.
3. Add other devices. Include the monitor, router, modem and other items on the same UPS.
4. Add headroom. Multiply by 1.25. For 444 W plus 60 W of other devices, that is about 630 W.
5. Check the UPS watt rating. Pick a UPS whose watt rating is at least that number.
6. Convert to VA. VA = watts ÷ power factor. If the UPS has a power factor of 0.6, a 630 W load needs about 1,050 VA. At 0.9, it needs about 700 VA.

Quick guide

• Light desktop (150 to 250 W): 600 to 850 VA.
• Typical desktop (250 to 400 W): 1000 to 1200 VA.
• Gaming PC (400 to 700 W): 1500 VA or more.
• Home server or NAS: 600 to 1000 VA, depending on drives.

Other things to check

• Waveform: choose a pure sine wave UPS for power supplies with active power factor correction.
• Runtime: check the manufacturer's chart.
• Outlets: the right plug type and number of battery-backed outlets.

Transformers

For a power supply that uses a transformer, VA is the rated voltage times the rated current. Choose a transformer with a VA rating above the load's apparent power, with a margin of 20 percent or more.