The Raritan Blog

Active Power and Apparent Power

James Cerwinski
July 15, 2009

Apparent Power is the Total Power Flowing

When electrical power in an AC system flows to a load (a computer, motor, lighting, cooling, etc.) all of the power is not necessarily absorbed and used to perform useful work – a portion of the power may be reflected back to the power source (power company).

The total power flowing is known as the “apparent power” and is measured as the product of the voltage and current (V * I).. For example, if 208 volts and 5 amps are measured – the apparent power is 1040VA (VA means volt-amps – the measurement unit of apparent power).

See how intelligent rack PDUs automatically calculate power usage for you. 

Active Power is the Portion of Total Power put to Useful Work

The portion of power that is absorbed and used by the load is known as the “active power” and is always equal to or less than the apparent power. For example, if 90% of the apparent power in the above example is used by the load, the active power is 936W (W means watts – the measurement unit of active power).

The ratio of the active power to the apparent power is known as the power factor and is a number between 0.0 and 1.0. The power factor in the above example is 936/1040 = .90.

Most modern computers incorporate power factor corrected (PFC) power supplies – and their power factor is .95 or better. This means 95% of the power flowing (the apparent power) is performing useful work and 5% or less of the total power is being reflected back to the power company.

Active Power is Important since it is the Primary Measurement for Billing

Utility companies charge for active power but will penalize companies with poor power factors. Poor power factor is usually a system of large inductive loads such as motors – not modern computers.


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