UPS systems are most commonly rated in Volt-Amps (VA), and this has a lot to do with how electrical power is consumed by the devices they are designed to protect. VA is a measure of ‘apparent power’, calculated by multiplying the voltage supplied to a device by the current drawn by the device. This apparent power rating is crucial because it indicates the maximum load a UPS can theoretically support. This rating is based on an ideal scenario where current and voltage are perfectly in sync, which is rarely the case with complex electronic devices like computers, servers and networking equipment.
The actual ‘real power’, measured in Watts, is usually a secondary feature because it depends on the ‘power factor’ (PF) – the ratio of real power to apparent power. The power factor accounts for the difference caused by the ‘out of phase’ condition in many electronic devices, where the current and voltage are not perfectly aligned, and hence, the real power (Watts) is less than the apparent power (VA). However, this doesn’t mean Watts are less important. In fact, they are crucial for determining the actual power a UPS can deliver to the devices. Yet, since the power factor can vary among devices, manufacturers often focus on VA ratings as a more stable and uniform measure. The onus is then on buyers to ensure that both the VA and Watt ratings of the UPS are suitable for their specific requirements.