Electric Motors
VFD Energy Savings Calculator
This calculator evaluates estimated power after speed reduction for a single electric motors scenario. It uses original fan power and speed fraction and shows the equation behind the displayed value.
Calculate Estimated power after speed reduction
Check the source and units before calculating.
Preparing the calculation
Separate shaft output power from electrical input power. A source note beside each entry makes the result reproducible.
A prefix error can move the result by several orders of magnitude. For the companion annual energy savings calculation, open Energy Efficiency Savings Calculator.
- Original fan power
- Default example: 15 kW. Enter original fan power in kW.
- Speed fraction
- Default example: 0.8. Enter speed fraction.
Estimated power after speed reduction: calculation method
The arithmetic is based on P₂ = P₁ × speed fraction³. The formula draws on Original fan power and Speed fraction.
The defaults produce 7.68 kW. Replace them with measurements from one operating state.
Calculate estimated power after speed reduction from original fan power and speed fraction. The next worksheet may be VFD Motor Speed Calculator if estimated motor speed is needed.
Compare two operating cases
Change Original fan power from 15 kW to 18 kW with the rest of the inputs held constant. The comparison runs from 7.68 kW to 9.22 kW.
Use a separate scenario when multiple conditions change together.
How to use the result
Interpret Estimated power after speed reduction on the same basis used for the source values. Compare it with starting demand, running current, torque, efficiency, slip, and thermal loading.
A result without its source conditions is difficult to compare or audit. Another relevant worksheet is Occupancy Sensor Savings Calculator.
What the formula leaves out
The cube law applies to suitable variable-torque loads.
Further engineering review can cover starting method, service factor, enclosure, and thermal duty. Use measurements, manufacturer data, or another calculation for effects that can change the decision.
Separate shaft output power from electrical input power. If you also need required nominal storage, continue with Off-Grid Battery Bank Calculator.
A final reasonableness check
Begin by confirming speed fraction at the operating point represented by the other entries. Record whether it came from a meter, nameplate, data sheet, or design assumption.
After calculating estimated power after speed reduction, compare its magnitude with motor, drive, cable, and protective-device limits. An independent order-of-magnitude check can reveal a misplaced decimal or unit prefix.
Document any effect handled outside this page, especially starting method, service factor, enclosure, and thermal duty. That note prevents a later reader from assuming the simple equation covered it.
Questions about VFD Energy Savings
What else is required before final selection?
Use the number as one check, then review nameplate, measured full-load, and actual duty-cycle values.
What precision should move to the next worksheet?
Retain the raw result for comparison and create a separately rounded reporting value if needed.
Can an input legitimately be zero?
Use zero only when it describes the physical condition. It is not a substitute for missing data.