The Difference Between Ballast Factor and Ballast Power Factor

Reading fluorescent ballast and HID ballast specification sheets 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 fluorscent or HID 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. If you are driving a lamp that is rated at 1000 lumens with a ballast that has a ballast factor of 0.95, the light fixture (not including a diffuser or louver) will emit 950 lumens. 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, even at the cost of lower light levels.

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. On a per-fixture basis, the energy costs will be higher, but this is offset by the need for fewer fixtures overall.

Special Case - Ultraviolet Bulbs

Ultraviolet bulbs represent a special case. While the concept is the same with ultraviolet and germicidal ballasts in terms of ballast factor, energy savings is almost never a priority here, especially for UVC bulbs. With UV bulbs, optimizing the UV output is the primary purpose of using ultraviolet light, so having a lower ballast factor ballast will result in lower overall UV output, which can make any type of ultraviolet lamp less efficient for the application.

This is one reason why it is always recommended to use specialty UV ballasts to drive UVA, UVB, or UVC lamps. The ballast factor and other specifications are optimized for these bulbs to deliver the expected rated output. While other ballasts can also be used, including regular CFL and fluorescent ballasts, they may under drive the lamp resulting in a lower level of irradiation.

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.