How do you size a UPS system for critical loads?

UPS sizing requires calculating total critical load in kVA (including power factor), adding margin for future growth (typically 20-25%), and selecting runtime based on backup power requirements or generator start time. Consider load power factor and crest factor for non-linear loads. Battery capacity determines runtime: size batteries for autonomy plus recharge requirements. Account for temperature derating and battery aging. Redundant configurations (N+1, 2N) improve reliability for critical facilities.