Energy & Sustainability
Variable Frequency Drive (VFD) Energy Savings
Calculate energy savings when installing a Variable Frequency Drive (VFD) on a pump or fan running at partial load, applying the affinity laws.
Units:
The affinity laws state that pump/fan power varies with the CUBE of speed. Reducing speed from 100% to 80% saves 49% of power theoretically. Real savings are typically 30–70% depending on the load profile. VFDs have their own efficiency loss (typically 2–4%).
P₂/P₁ = (n₂/n₁)³ [Affinity law for centrifugal machines]
Input
Motor rated power iFull-load rated power of the motor driving the pump or fan in kW.
kW
Average operating speed iAverage operating speed as percentage of full speed. For pumps with throttle valves: often 70–85%.
% of full speed
Annual operating hours iOperating hours per year.
h/year
VFD efficiency iEfficiency of the VFD itself. Typical: 95–98%.
%
Energy price iIndustrial electricity tariff in €/kWh.
€/kWh
VFD installation cost iTotal cost of VFD purchase, installation and commissioning in €.
€
Result
Power without VFD (full speed) [kW]
—
Power with VFD (at avg speed) [kW]
—
Annual energy saving [kWh/year]
—
Annual cost saving [€/year]
—
Payback period [months]
—
CO₂ reduction [kg CO₂/year]
—