Why companies rate UPS in VAs and PSUs in Watts

1. Introduction2. Apparent Power, Real Power and Reactive Power 3. Explaining and calculating the power factor (PF)4. Sizing PSU and UPS units5. View All Pages

People with little or no technical expertise are not the only ones to fall victim as many who studied computers, electronics and other related subject often believe that VAs are equal to Watts; this is actually true, but only for DC circuits. When talking about any AC circuit, there are three types of power to consider : Apparent (or Complex) power, Real (or True or Effective) power and Reactive (or Magnetic) power.

For DC circuits power is calculated by using the definition:

This definition is not valid for AC circuits because the vast majority of power loads will cause a phase shift between voltage and current (related reading : AC-DC Voltage; What’s the difference?).

A very similar definition is used to calculate Apparent power :

Apparent power has little to no meaning for residential and business users, but it is absolutely necessary for sizing any and all AC power equipment ranging from your household safety fuses and simple UPS systems to immense transformers and power generators.

So what about real power? Real power is, much like the name suggests, the actual amount of power used by your equipment and it is commonly used to calculate the thermal loading generated by the equipment.

For AC circuits real power is calculated by using the following definition :

Real power is usually all that residential and business users care about because that is the amount of power you purchase from the utility company.

Reactive power is something not widely known and rarely ever used because it usually only matters to engineers designing and sizing electric power transmission and distribution systems. Any inductive and/or capacitive load which will cause a phase shift between the current and voltage waveforms will cause reactive power to be drawn by the equipment, even though the equipment will not actually use it. Reactive power moves no energy, which is why it is often referred to as the “imaginary” power.

It can be calculated by using the following definition :

To summarize, apparent power is the total amount of power that will move through your equipment and therefore it is critical to size all wiring, circuit breakers and any other equipment according to it, yet residential and business users will not be charged based on their apparent power but by their real power consumption. Real power is the effective power used by your equipment and moves energy. Reactive power moves no energy but will still be the cause of a higher, useless current.

For most part of the world only large businesses and industrial consumers are being penalized if reactive power exceeds a certain portion of their total power consumption.

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