Method: For suitable centrifugal pumps and fans, the affinity laws approximate power as proportional to the cube of rotational speed: P₂ / P₁ = (N₂ / N₁)³ The theoretical reduced-speed power is therefore: P₂,theoretical = P₁ × (N₂ / N₁)³ The calculator then allows for VFD electrical losses: P₂,input = P₂,theoretical ÷ VFD Efficiency Annual energy use is: Energy = Input Power × Operating Hours Annual energy savings are: Savings = Existing Energy − VFD Energy Annual cost savings are: Cost Savings = Energy Savings × Electricity Tariff This affinity-law model is most appropriate for variable-torque centrifugal pump and fan applications where system resistance is predominantly frictional. Real systems may save less where significant static head or static pressure exists, where flow does not vary directly with speed, or where the existing system already operates with efficient control. Motor efficiency may also change at reduced speed and load. For investment-grade analysis, use measured operating profiles, pump or fan curves, system curves, motor efficiency data and actual VFD performance. This method should not be applied directly to constant-torque loads such as many conveyors, positive-displacement pumps and similar equipment.