How to Size a UPS: Power (VA) and Runtime

1. Estimate the load (W)

Best option: measure it with a plug-in power meter while the equipment is under load. If you can't, add up the maximum consumption of the CPU and GPU from the manufacturer specs and add about 50 W for motherboard, RAM and disks. That gives a worst case that is rarely reached.

2. Add a margin

Leave around 25-30% headroom for peaks and future devices. Measured 300 W → size for 400 W.

3. Convert watts to VA

UPS units are sold by apparent power (VA). Divide the real power by the power factor (PF):

VA = W / PF

• PF 0.6: safe, conservative value.
• PF 0.7: reputable brands.
• PF 0.5: very cheap no-name units.

Example: 400 W / 0.6 = 667 VA → buy a 700 VA UPS (or larger).

Many datasheets list both VA and W. Check that the W rating also covers your load.

4. Estimate the runtime

T (min) = (N x V x Ah x Ef) / P x 60

• N: number of batteries.
• V: voltage of each battery (usually 12 V).
• Ah: capacity of each battery.
• Ef: efficiency, around 0.9-0.95.
• P: load in watts. Using the UPS VA rating instead gives a more conservative figure.

Example with 2 x 12 V 7 Ah batteries and a 300 W load:

T = (2 x 12 x 7 x 0.95) / 300 x 60 = 31.9 min

This is optimistic: at high loads batteries deliver less than their rated capacity, and they lose capacity with age. Check the manufacturer runtime chart and plan for a clean shutdown in the first few minutes.