Power Factor (PF) measures how efficiently electrical power is converted into useful work output. PF = Real Power (W) ÷ Apparent Power (VA). Range: 0 to 1.0. Low PF (<0.5) means the driver draws more current than necessary, causing higher transmission losses and potential fines from utilities. Standards: ENERGY STAR requires PF≥0.7 for residential and ≥0.9 for commercial LED products. EU Ecodesign (EU 2019/2020): PF≥0.9 for drivers >25W. Always verify PF when procuring LED luminaires—low PF drivers increase electricity costs and may not meet building codes.