How to use the VFD energy savings calculator
Fans and centrifugal pumps throttled with a damper or valve still draw close to full-load power, so reducing flow that way saves far less energy than most operators expect. With a variable frequency drive the shaft power follows the affinity laws and drops with the cube of speed, so the same reduced flow is delivered for a fraction of the input power. This calculator compares the part-load draw of the existing control method with the draw under VFD control and converts the difference into annual kWh and dollars.
Part-load power for the existing control is approximated from published fan part-load curves: outlet damper or throttling valve uses 0.55 + 0.45 x flow ratio, inlet vane uses 0.20 + 0.80 x flow ratio, and constant-speed full-load operation uses 1.0. The VFD side uses flow ratio x (s + (1 - s) x flow ratio squared), where s is the static head fraction, and includes a 96% drive efficiency. Systems with a large static lift therefore save less than a pure friction loop.
Actual savings depend on the annual load profile, minimum drive frequency, motor efficiency at reduced speed and any demand-charge change. Treat the output as a first-pass feasibility estimate, and confirm it with metered power readings and the manufacturer pump or fan curve before committing to a retrofit.
Frequently asked questions
Only for systems where friction loss dominates. A pump working against significant static head does not see power fall with the cube of speed, so enter the static head fraction to correct the estimate.
Drive losses of roughly 3 to 4%, reduced motor cooling and efficiency at low speed, harmonic mitigation, shaft grounding against bearing currents, and a sensible minimum speed setting all need review.