The Power Factor and Efficiency plays a major role in what a power supply can draw.
VA = Consumption / Efficiency / Power Factor
For example, if the PC hardware consumes 300 watts from the PSU, the PSU efficiency is 80% and has a power factor of 0.95, this works out as follows:
- VA = 300 / 0.8 / 0.95
- VA = ~395
Unfortunately, these VA figures really only apply to a UPS that produces a true sign wave. The vast majority of consumer UPS units provide a modified sign wave including in that above review, which means that a power supply that has power factor correction to achieve a power factor of 0.95 may only be able to achieve the reduced power factor caused by the UPS' modified sign wave while on battery. If this is let's say 0.65, then the same 300 watt power draw would work out as follows:
- VA = 300 / 0.8 / 0.65
- VA = ~577
The power factor also varies from make to make while supplied with a modified side wave, so this is the reason some PSU makes need a much higher UPS VA rating, due to their poor power factor while running on a modified sign wave. So like others mentioned, it is recommended to get a UPS with double the VA figure of the PC's PSU to cover for efficiency and power factor.
Most peripherals such as a monitor, router, external HDD, etc. don't have power factor correction, so their power factor is typically 0.65. So let's say consumption of the additional items is 100 watts, the actual draw would be 154VA even with a pure sign wave UPS. This also needs to be taken into account when deciding on a UPS.
If the equipment does exceed the UPS rating, as long as the power does not fail or fluctuate while this happens, it should not cause a problem. Most UPS' units will beep if its power rating is exceeded or at least display a message in its software if it was a brief surge. However, if the power does go out or go over/under-voltage while the UPS rating is exceeded, generally the UPS will disconnect the output, i.e. act as if it was just a dumb powerstrip.