Fan systems can be configured with a wide variety of options. The number of combinations and the lack of scaling laws makes testing and rating of fan system energy performance an impractical task. While estimates can be made using component peak efficiencies, these calculations are often applied inconsistently or without part load performance. Method to standardize the calculations under general operating conditions is presented which applies to fan systems containing any combination of regulated poly-phase induction motors, v-belt transmissions, and pulse-width modulated variable frequency drives. This presentation provides detailed review of the underlying assumptions, the calculation procedure, and examples of those calculations.