Energy, cost, and emissions savings from a VFD on a fan or pump.
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Motor & Operating Parameters
Average Flow with the VFD (% of design)
80%
40%60%80%100%
Energy Savings Results
Constant-Speed kW Input
—
kW
VFD kW Input at Average Flow
—
kW
Annual kWh Saved
—
kWh/yr
Annual $ Saved
—
$/yr
Simple Payback
—
years
CO₂e Avoided
—
lb/yr
Flow vs. Power with the VFD
Flow
Shaft Power (% of design)
VFD kW Input
kWh/yr
$/yr
Shaft power on a system curve that keeps the fixed pressure share: P/Pdesign = q × [f0 + (1 − f0) × q²]. With f0 = 0 this is the ideal cube law (80% flow → 51% power, 60% → 22%). VFD kW input adds motor and drive losses.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This calculator estimates the energy a variable frequency drive (VFD) saves on a fan or pump motor. Enter the motor horsepower and how heavily it is loaded, the motor and drive efficiencies, the share of design pressure the system holds fixed, the electricity rate, operating hours, installed drive cost, and grid emission factor, then set the average flow. The tool returns constant-speed and VFD input kW, annual kWh and dollars saved, a simple payback, and the avoided CO₂e.
The flow-versus-power table shows how steeply power falls as flow drops, and how a fixed static or differential-pressure setpoint flattens that curve compared with the ideal cube law. It is a fast screening tool for justifying a drive retrofit before detailed analysis.
q is the average flow as a fraction of design and f0 the share of design pressure the system holds fixed (duct-static or pump differential-pressure setpoint, typically 20–40%). The power fraction comes from the system curve H = H0 + (Hdesign − H0)·q² with constant fan/pump efficiency; f0 = 0 gives the ideal affinity (cube) law. 0.7457 kW/HP is the mechanical horsepower. The baseline assumes the constant-speed motor draws its design load for all the hours (constant volume, or throttling that saves little); riding a forward-curved fan curve or inlet vanes would lower the baseline and the savings. Motor efficiency falling at low load and cubing one average flow instead of an hour-by-hour profile are not modeled, so treat the result as a screening figure.
The default 0.83 lb CO₂e/kWh is the U.S. national average total output emission rate from EPA eGRID2022 (827.5 lb/MWh). Enter your eGRID subregion rate for a local figure; marginal (non-baseload) rates, often used for avoided emissions, are higher — 1,412.5 lb/MWh nationally.
Frequently Asked Questions
How does a VFD save energy?
A constant-speed fan or pump runs at full speed and throttles or recirculates when the system needs less flow. A VFD slows it instead. In a system with no fixed pressure, shaft power falls with the cube of flow (the affinity law): 80% flow needs about 51% power and 60% flow about 22%. Most HVAC systems hold a duct-static or pump differential-pressure setpoint, so this tool keeps that fixed share of the design pressure: with the default 30%, 80% flow needs about 60% of design shaft power and 60% flow about 33%.
How is the motor input kW calculated?
Shaft kW at design flow is nameplate horsepower × 0.7457 × the motor load (the share of nameplate the motor actually draws). Without a VFD the input kW is that shaft kW divided by the motor efficiency. With a VFD the shaft kW is multiplied by the part-load power fraction and divided by both the motor efficiency and the drive efficiency, so the drive’s own 2–4% loss counts against the savings. Annual kWh saved is the difference times the operating hours.
How is simple payback computed?
Simple payback equals the installed drive cost divided by the annual dollar savings, where annual savings is kWh saved times the electricity rate. The calculator reports payback in years. It does not discount future savings, so for a longer-horizon view that accounts for the time value of money, use a payback and ROI tool with NPV.
Is the cube law always exact in practice?
No. The cube law assumes a system curve dominated by friction and ignores fixed static pressure, minimum-speed limits, and motor efficiency falling off at low load, so it overstates savings. This tool adds the fixed-pressure share (set it to 0 to see the pure cube law) and the drive loss, but it still holds fan and motor efficiency constant and uses one average flow rather than an hour-by-hour flow profile. Treat the result as a screening estimate and confirm with the actual system curve, control sequence, and drive data.
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