Power Factor: the Straight Shot

by Jeff Robbins, LC, MIES  (from the Fall 2012 LDL Newsletter)


Power Factor is just one of the many criteria utilities use when evaluating lighting products, and is one of the more misunderstood.  For instance, a minimum power factor of .9 is required for luminaires to qualify for the LDL LED List.  Power factor is defined by the industry as the power input in watts divided by the product of the ballast/driver input voltage and ballast/driver input current (measured in amps). The power factor of an electrical system is a number between 0 and 1 -- the ratio of the real power flowing to the load to the apparent power in the circuit.

 The equation is Real Power (Watts) divided by Apparent Power (Volt-Amps).

The power factor of a load, (such as a fluorescent ballast or LED driver) determines how effectively the input power of the circuit is converted into usable power by that load. Any load with a low power factor uses more current than a load with a high power factor. Since electrical bills are based on metered watts, not volt-amps, the closer the system comes to a power factor of 1.0, the more the bill will reflect true billable electrical usage.

High current content also increases the energy lost in the distribution system, requiring heavier gauge wires, larger conduit, etc. Because of the higher equipment costs and the wasted energy, utilities may elect to penalize customers whose electric load has a low power factor, (typically anything less than 0.9). 

EXAMPLE 1: An LED PAR38 lamp rated at 18 watts (real power), when measured actually draws 20 volt-amps, (apparent power).  The power factor for that lamp and driver combination is 18w / 20VA, which equals 0.9, and is considered to be high.

EXAMPLE 2: A CFL lamp rated at 18 watts (real power), when measured, actually draws 30 volt-amps, (apparent power). The power factor for that lamp and ballast combination is 18w / 30VA, which equals 0.6, and is considered to be low.

NOTE: Power factor is often confused with Ballast Factor, which compares the lumen output of a lamp and ballast combination to that of a reference ballast rated at 1.0.