| Speed (% of full) | Hours per year | Power with VFD |
|---|---|---|
| 30.9 kW | ||
| 15.8 kW | ||
| 6.7 kW |
- Energy today
- 162,000 kWh/yr
- Energy with VFD
- 91,794 kWh/yr
- Saving
- 70,206 kWh/yr (43 %)
- Cost saving
- $8,425 per year
- Simple payback
- 0.9 years
The short answer
On a centrifugal fan or pump, power falls roughly with the cube of speed (the affinity laws), so running at 80 % speed needs about 51 % of full-speed power. A VFD saves the difference between that and what the motor draws at constant speed with a damper or valve, minus about 3 % drive losses.
Formula
Power at speed n (fraction) ≈ P_rated × n³ ÷ η_driveSaving (kWh/yr) = Σ hours × (P_constant − P_vfd)Payback (years) = installed cost ÷ annual saving
How to use it
- Enter the motor’s input power at full load.
- Enter the power the motor draws today with throttling, as a % of full load.
- Split the running hours across the speeds the VFD would run at.
- Add your electricity price and the installed cost of the drive.
Assumptions
- Pure cube law, which holds for systems with little static head (fans, circulating pumps). Systems with high static head save less.
- Drive efficiency 97 %. Motor efficiency changes at low load are ignored.
Frequently asked questions
How much energy does a VFD save?
On variable-flow fans and pumps, typically 20–50 % compared with damper or valve control. On constant-load machines, or pumps with mostly static head, the saving can be close to zero.
Why does speed matter so much?
Flow follows speed, pressure follows speed squared and power follows speed cubed. Cutting speed by 20 % cuts power by about half.
Does a VFD save energy on a conveyor?
Little, because conveyor torque is roughly constant and power falls only in proportion to speed. The benefit there is soft starting and speed control.