What Is The Difference Between Ballast Factor and Ballast Power Factor?

Fluorescent Tube Lights

Reading fluorescent ballast and HID ballast specifications can make your head hurt. Case in point: ballast factor vs ballast power factor.

Ballast Factor Affects Light Output

Ballast factor is a number normally between 0.70 and 1.2. This number is a multiplier applied to the rated initial lumens of a light bulb. The result tells you how many lumens are actually delivered from the bulb/ballast system. In some cases the result will be fewer lumens than the lumen rating for the bulb alone (Low Ballast Factor). In other cases, the result will be roughly the same lumens as the bulb alone (Normal Ballast Factor), or more lumens than the bulb alone (High Ballast Factor).

To add to the confusion, a ballast can have different ballast factors depending on the type of light bulb connected to the ballast. Example: a 32 watt T8 bulb may have a ballast factor of 0.89 and a 17 watt T8 bulb connected to the same ballast may have a 1.06 ballast factor.

Ballast factor tells you how much light (lumens) you will get from a specific bulb/ballast combination. Because low ballast factor ballasts use less power than normal or high ballast factor ballasts, they may be used where lower energy costs are the goal of a lighting retrofit. In new construction, high ballast factor ballasts may be used because the higher bulb/ballast light output will allow fewer fixtures to be installed, for example, in a large open office.

Power Factor Affects the Electric Utility

Ballast Power Factor measures how “efficiently” a ballast uses its power. Expressed as a percentage, 0%  to 100%, power factor is the ratio of power used by a ballast compared to the total power supplied by the utility: Power Factor = Watts/Volt-Amperes. In most cases the power factor is less important to the owner of the ballasts than to the electric utility. Low power factors cause the current delivered by the utility to the building owner to be out of phase with the voltage. When this happens, the utility has to supply more volt-amperes. Large commercial or industrial customers may be charged an additional fee if their power factor is too low.

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Dave Burtner

Dave has been active in the lighting industry since 1994. Formerly a member of the Illuminating Engineering Society of North America and certified by the National Council On Qualifications for Lighting Professionals, Dave now writes blog posts, lighting tips and provides lighting product assistance for the Topbulb website.