Variable Frequency Drive Savings Calculator
A Variable Frequency Drive (VFD) slows a motor to match the actual demand of its load instead of running flat-out and throttling the output with dampers or valves. On variable-torque centrifugal loads like fans and pumps, that speed reduction unlocks the cube law -- power scales with the cube of speed, so a small drop in speed yields a dramatic drop in power. This calculator takes a motor's horsepower, efficiency, operating hours, electricity price, and the average speed it will run at with a VFD, then reports the full-speed power, the power at the reduced speed, the annual energy with and without the VFD, and the dollar savings. For the baseline energy use of the motor itself, see our Industrial Motor Energy Calculator, and for another high-payback industrial efficiency measure, see our Compressed Air System Calculator.
Nameplate rated horsepower of the motor driving the fan or pump.
The ratio of mechanical output power to electrical input power. Premium-efficiency motors typically run 92-96%.
Annual hours the motor runs. 6,000 hours/year is roughly equivalent to continuous operation ~16.4 hours/day.
Typical industrial electricity rates are generally lower than residential rates; adjust to your actual blended $/kWh.
A common time-weighted average speed for variable-torque loads (fans, pumps) previously controlled by throttling or dampers before VFD installation.
(motor horsepower × 0.746) ÷ (motor efficiency ÷ 100)
full-speed power (kW) × (average operating speed with VFD (%) ÷ 100)³
full-speed power (kW) × operating hours per year
power at average VFD speed (kW) × operating hours per year
annual energy without VFD (kWh) − annual energy with VFD (kWh)
annual energy savings (kWh) × electricity price ($/kWh)
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
This calculation uses the cube law (affinity laws), which applies to variable-torque centrifugal loads like fans and pumps. It does NOT apply to constant-torque loads like conveyors, positive-displacement pumps, or cranes, where VFD savings are much smaller and calculated differently.
How we calculate this →The cube law is what makes VFDs one of the highest-ROI investments in industrial efficiency: cutting a centrifugal fan or pump to 70% speed cuts its power draw to roughly 34% of full-speed power (0.7 cubed), not 70%. On a 50 HP motor running 6,000 hours a year, that's the difference between 243,240 kWh and just 83,460 kWh annually -- nearly $16,000 a year in savings at $0.10/kWh. This is exactly why VFDs pay for themselves so quickly on variable-torque loads like fans and pumps, but offer little benefit on constant-torque loads like conveyors.
How Variable Frequency Drive savings are calculated
This calculator ties the energy a VFD saves on a centrifugal fan or pump to five inputs: the motor's rated horsepower, its efficiency, the hours it runs each year, the electricity price, and the average speed it will operate at once the VFD is installed. Six quantities tie the calculation together.
Full-Speed Power (kW) = (Motor Horsepower (HP) × 0.746) ÷ (Motor Efficiency (%) ÷ 100). The 0.746 factor converts one mechanical horsepower into kilowatts, and dividing by efficiency (as a decimal) accounts for the energy lost as heat inside the motor. At 50 HP and 92% efficiency, that is (50 × 0.746) ÷ 0.92 = 40.54 kW of electrical input power at full speed.
Power at Average VFD Speed (kW) = Full-Speed Power (kW) × (Average Operating Speed with VFD (%) ÷ 100)³. This is the cube law -- the affinity laws of fluid mechanics -- which says that for a centrifugal fan or pump, the power required scales with the cube of the impeller speed. Reducing speed to 70% cuts power to 0.7³ = 0.343 of full-speed power, not 70%. At 40.54 kW and 70% speed, that is 40.54 × 0.343 = 13.91 kW.
Annual Energy Without VFD (kWh) = Full-Speed Power (kW) × Operating Hours per Year; at 40.54 kW and 6,000 hours, that is 40.54 × 6,000 = 243,240 kWh. Annual Energy With VFD (kWh) = Power at Average VFD Speed (kW) × Operating Hours per Year; at 13.91 kW and 6,000 hours, that is 13.91 × 6,000 = 83,460 kWh.
Annual Energy Savings (kWh) = Annual Energy Without VFD (kWh) − Annual Energy With VFD (kWh); at 243,240 kWh and 83,460 kWh, that is 243,240 − 83,460 = 159,780 kWh saved annually. Annual Cost Savings ($/year) = Annual Energy Savings (kWh) × Electricity Price ($/kWh); at 159,780 kWh and $0.10/kWh, that is 159,780 × 0.10 = $15,978 per year.
The crucial caveat: the cube law applies specifically to variable-torque centrifugal loads -- fans, pumps, and blowers where flow is created by a rotating impeller. It does NOT apply to constant-torque loads like conveyors, positive-displacement pumps, cranes, or hoists, where the torque demand stays roughly constant regardless of speed and power scales roughly linearly with speed instead of with its cube. On those loads, VFD savings are much smaller, calculated differently, and in some cases may not exist at all beyond the VFD's own losses. The average operating speed is the single most decision-relevant input: because power scales with the cube of speed, even modest speed reductions produce outsized savings, which is exactly why VFDs on throttled fan and pump systems pay back so quickly. Two notes on the model. First, it assumes the motor was previously running at full speed with output throttled by dampers or valves -- the classic VFD retrofit case -- and that the average speed entered is a representative time-weighted average of the actual load profile. Second, it does not subtract the VFD's own inherent losses (typically 2-5% of throughput), which slightly reduce the net savings but are almost always far outweighed by the cube-law savings on variable-torque loads. Data sources: Affinity laws (cube law) from fluid mechanics and pump/fan engineering standards (ISO, ASHRAE); VFD efficiency data from manufacturer specifications and IEEE standards; industrial load profiles from DOE and facility energy audits; electricity rates from EIA industrial data; VFD payback analysis from utility rebate programs and industrial case studies.