Ballast Efficacy Factor. Relative light output (ballast factor) divided by input power (watts). Used to measure the level of efficiency of similar ballast models. For example, the OSRAM SYLVANIA QT2X32IS which has a ballast factor of 0.90 and input watts of 59 (BEF=1.53), is more efficient than competitors' electronic ballasts with ballast factor of 0.875 and input watts of 62 (BEF=1.41).