VFD Speed vs Power Savings Calculator
Power reduction from slowing a fan with a VFD.
Enter your values
Enter the full-speed power in kW.
Enter the new speed in % of full speed.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- New power draw
- 5.12 kW
- Power saved
- 4.88 kW
- Percent saved
- 48.8%
- New airflow (approx)
- 80% of full CFM
Quick answer
Fan power drops with the cube of speed reduction, so even small speed cuts on a VFD save significant energy.
P2 = P1 x (N2/N1)^3
At a glance
| What it does | Power reduction from slowing a fan with a VFD. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Full-speed power, New speed |
| Main output | New power draw |
| Formula | P2 = P1 x (N2/N1)^3 |
| Cost | Free — no sign-up, no download |
How to use the VFD Speed vs Power Savings Calculator
- 1Enter the fan's power draw at full speed.
- 2Enter the reduced VFD speed as a percent of full speed.
- 3Read the new power draw and percent saved from the cube-law relationship.
Inputs explained
- Full-speed power(kW)
- Enter the full-speed power in kW.
- New speed(% of full speed)
- Enter the new speed in % of full speed.
Worked example
Using the values the calculator loads with:
Inputs
- Full-speed power10 kW
- New speed80 % of full speed
Results
- New power draw5.12 kW
- Power saved4.88 kW
- Percent saved48.8%
- New airflow (approx)80% of full CFM
Frequently asked questions
Is this exact for all fans?
It is a good estimate for centrifugal and axial fans in typical duct systems, ignoring drive and motor efficiency changes at part load.
Why do VFDs save so much?
Because power falls with the cube of speed, a 20% speed cut can save roughly 50% of the fan's energy.
What do I need to enter into the VFD Speed vs Power Savings Calculator?
Just 2 values: full-speed power, new speed. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the VFD Speed vs Power Savings Calculator work out the answer?
Fan power drops with the cube of speed reduction, so even small speed cuts on a VFD save significant energy. It applies the formula P2 = P1 x (N2/N1)^3 and shows the working so you can check each step by hand.
Is the VFD Speed vs Power Savings Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.