What is ballast factor? Ballast factor is something the Lighting Gods devised to make sure we must keep using our brains! It is the calculation of the lumen output of a luminaire in combination with a ballast against the reference ballast and luminaire when designed and manufactured.
There are typically three ballast factors; low (.78), normal (.88) and high (1.2). For example, a standard T-8 fluorescent tube that is rated at 2,000 lumens would on a low ballast factor produce 1,560 lumens (2,000x .78). This is important to know as the ballast affects the light output of the lamp it is used with and must be taken into consideration for the deployment and purpose of the light.
This also affects the energy use. The same 32-watt tube would only consume 24.96 watts with low ballast factor while also producing less light. This is important to calculate effective ROI for a lighting retrofit.
Knowing Your Ballast Factor:
Good luck! Many personnel responsible for maintenance and facilities will not have this knowledge. In commercial real estate properties, sell and management contracts switch many times over with the engineering staff turning over, too. This turnover is lower in corporate facilities and public facilities, but still exists.
A rule of thumb to know your ballast factor is that interior lights and fixtures are typically at the low or normal factor, while exterior, high bay or garage fixtures are at the high factor. Another way to know is by checking the ballast in the fixture. While I do not recommend checking the 500 fixtures in your parking garage, a spot check of four or five fixtures spread throughout the garage would be a start. If the ballast is the same, then you can assume that factor is uniform throughout.
LEDs and Ballast
The Lighting Gods went into overtime when it comes to LEDs and ballast. Remember the ratios and calculations used above to determine lumen output and energy use? With LEDs, that is out the window. The formulas above work for fluorescent lighting and ballast, but LEDs are affected completely different by ballast. You must refer to the manufacturer’s “spec sheet” to determine how the ballast factor will affect lumens and wattage. The best part is that not many manufacturers list this on their sheets and it differs between manufacturers.
That is why I always recommend going with Ballast Bypass LEDs vs. Ballast Compatible. You will pay a little more in installation, but will have accurate lighting and wattage numbers. Ballast can add as much as 50-60 percent to the wattage of an LED light.