How is the “size” of UPS determined?
Typically, a UPS has a VA rating. The VA rating is the maximum number of Volts x Amps it can deliver. The VA rating is not the same as the power drain (in Watts) of the equipment. This would be true if the load was only resistive or the circuit were DC, not AC. Computers are predominantly non-resistive. A typical Power Factor (Watts / VA) for some computers may be as low as 0.6, which means that if you record a drain of 100 Watts, you need a power source with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the specific equipment. Moreover, there’s really no way to determine these numbers without measuring them. Some UPS can continue to deliver power if the VA rating is exceeded, but not once the supply power is lost. A UPS should never be run above its power rating. It is good practice never draw more than 75% of its VA rating.
