In power systems, there is usually a hazy line between the Real Power and Apparent Power dissipated.

What are these two words and how do they affect your decision to invest in a power equipment such as an Uninterruptible Power System?

• Real Power

This refers to the actual amount of power being used, or dissipated, in a circuit. It is measured in watts (and symbolized by the capital letter P).

• Apparent Power

The Apparent Power refers to the imaginary power in a circuit and is the product of a circuit’s voltage and current, without reference to phase angle. Apparent power is measured in the unit of Volt-Amps (VA) and is symbolized by the capital letter S.

Summarily, Real power which is also known as active power is the true power capable of being transferred by a system to any load while the Apparent Power otherwise known as reactive power is the imaginary power which does not do any useful work but simply moves back and forth in the power system lines.

• How this affects your decision to invest in a UPS

It is possible to appear to have correctly sized a UPS but still overload the UPS simply because the nameplate rating of some UPSs (usually in VA) could introduce some measure of confusion when the actual usable (real) power is not part of the consideration for the prospective load.

Here is what we mean:
Example 1
UPS rating: 1.5kVA

Load: 1kVA

UPS Power Factor: 0.6

Load power factor: 1.0

UPS Real Power: 900W

Load Real Power: 1000W

Example 2
UPS rating: 1.5kVA

Load: 1kVA

UPS Power Factor: 0.8

Load power factor: 1.0

UPS Real Power: 1200W

Load Real Power: 1000W

Considering the above examples, the Apparent power for both UPS is 1.5kVA while the Real Power for UPS1 is 900W and UPS 2 is 1200W; the Apparent power of the Load is 1kVA and the Real Power is 1kW.

For both cases, the VA rating of the load is within the VA rating of the two UPSs.

At face value, both UPSs are appropriately rated to support the load but in operation,

The 1kW rating of the load exceeds the Watt rating of UPS 1, which is 60% of 1.5kVA

And,

The 1kW rating of the load is within the Watt rating of UPS 2 which is 80% of 1.5kVA

Therefore, only UPS 2 is able to power the Load.

Note:

The Real Power (in watts) is the actual power drawn by the equipment.

• Volt-Amp or VA is known as the “Apparent Power”. This is the product of the voltage applied to the equipment and the current drawn by the equipment.
• “VA” is a sum of the resistive and the reactive components of power.
• The ratio of “W” to “VA” is known as the “Power Factor”. Power Factor is usually expressed as a number (0.6, 0.8, 0.9, 1.0, etc) or percentage (60%, 70%, 70%, etc) of the “VA” rating of a power equipment.