According to Eley et al. (1993),
One of the most important ballast parameters for the lighting designer/engineer is the ballast factor. The ballast factor is needed to determine the light output for a particular lamp-ballast system.
Ballast factor is a measure of the actual lumen output for a specific lamp-ballast system relative to the rated lumen output measured with a reference ballast under ANSI test conditions (open air at 25 °C [77 °F]). An ANSI ballast for standard 40-watt F40T12 lamps requires a ballast factor of 0.95; the same ballast has a ballast factor of 0.87 for 34-watt energy saving F40T12 lamps. However, many ballasts are available with either high (conforming to the ANSI specifications) or low ballast factors (70 to 75%). It is important to note that the ballast factor value is not simply a characteristic of the ballast, but of the lamp-ballast system. Ballasts that can operate more than one type of lamp (e.g., the 40-watt F40 ballast can operate either 40-watt F40T12, 34-watt F40T12, or 40-watt F40T10 lamps) will generally have a different ballast factor for each combination (e.g., 95%, <95%, and >95%, respectively).
Ballast factor is not a measure of energy efficiency. Although a lower ballast factor reduces lamp lumen output, it also consumes proportionally less input power. As such, careful selection of a lamp-ballast system with a specific ballast factor allows designers to better minimize energy use by "tuning" the lighting levels in the space. For example, in new construction, high ballast factors are generally best, since fewer luminaires will be required to meet the light level requirements. In retrofit applications or in areas with less critical visual tasks, such as aisles and hallways, lower ballast factor ballasts may be more appropriate.
To avoid a drastic reduction in lamp life low ballast factor ballasts (<70%) should operate lamps in rapid start mode only. This is particularly relevant for 32-watt F32T8 lamps operated at high frequency.
Finding the ballast factor for lamp-ballast combinations may not be easy, as few ballast manufacturers provide this information in their catalogs. However, if the input power for a particular lamp-ballast system is known (usually found in catalogs) an estimate of the ballast factor is possible. [See Eley et al. 1993 for details on estimating ballast factors.]
The efficiency of a fluorescent lamp ballast changes depending on the type of lamp operated. Similarly, lamp efficacy is affected by ballast technology: the same lamp will perform differently when operated by a heater cutout ballast than it will when operated at high frequency. As a consequence, the only meaningful comparison between lamps or ballasts is the lamp-ballast system efficacy. The system efficacy can be calculated as follows:
System Efficacy (lumens/watt) = * Number of Lamps * Ballast Factor