How to Calculate VFD Energy Savings and Payback
Your pump or fan may not need to run at full speed all day. A variable frequency drive (VFD) adjusts motor speed as flow demand changes, which can reduce electricity use compared with throttling valves, dampers, or bypass lines. Calculating potential VFD energy savings and payback can help determine if an upgrade is worth the investment.
How Does a VFD Save Energy?
A variable frequency drive, or VFD, controls the frequency and voltage supplied to an AC motor. This allows the motor to run at the speed required by the application rather than operating continuously at full speed.
Without a VFD, many systems reduce flow through mechanical controls:
- A throttling valve restricts flow from a pump
- A damper restricts airflow from a fan
- A bypass line redirects excess flow
- An inlet guide vane controls air entering a fan
These methods control output, but the motor may continue operating at or near full speed. A VFD adjusts motor speed to match demand, reducing power consumption in many variable-flow applications.
Information Needed to Estimate VFD Energy Savings
A VFD energy savings calculation compares estimated electricity use before and after adding a drive. The calculation requires information on the motor, operating schedule, flow demand, electricity rate, and installation cost.
A spreadsheet or VFD energy savings calculator can make the process easier, but the value of the result depends on the operating data entered.
Typical inputs:
- Motor horsepower or kilowatts
- Annual operating hours
- Electricity cost per kilowatt-hour
- Estimated motor and drive efficiency
- Current flow-control method
- Percentage of operating time at each flow rate
- VFD equipment and installation cost
Operating data may come from a control system, flowmeter, pressure transmitter, power meter, or temporary data logger. This data may be collected as part of an industrial energy audit or through temporary monitoring of the individual system.
How VFD Energy Savings Are Calculated
For centrifugal pumps and fans, the affinity laws describe the relationship between speed, flow, pressure, and power:
- Flow changes in direct proportion to speed
- Pressure changes approximately with the square of speed
- Power changes approximately with the cube of speed
The basic power relationship is:
Power at reduced speed = Full-speed power × (Reduced speed ÷ Full speed)³
If a fan operates at 80% speed, its estimated power requirement under ideal affinity-law conditions is:
0.80³ = 0.512
The fan would use approximately 51.2% of its full-speed power. At 60% speed:
0.60³ = 0.216
The estimated power requirement falls to approximately 21.6%. This cube-law relationship is why relatively small speed reductions can produce substantial variable frequency drive energy savings.
If motor size is listed in horsepower, it can be converted to kilowatts before estimating energy use:
Rated motor output in kilowatts = Motor horsepower × 0.746
Estimating electrical input also requires the motor’s actual load and efficiency. VFD losses should be included when estimating energy use after installation.
Once the estimated power use has been calculated, annual savings can be found with:
Annual energy savings = Current annual kWh − Estimated annual kWh with a VFD
Annual cost savings = Annual energy savings × Electricity rate
How the Operating Profile Affects VFD Savings
Motor size alone does not determine the savings. The estimate must also account for changes in demand during normal operation.
For example, a system may operate:
- 20% of the time at 100% flow
- 50% of the time at 80% flow
- 30% of the time at 60% flow
Calculate energy use at each operating point and combine the results to estimate annual consumption. A pump or fan that runs near full output most of the year may have limited savings potential. A system that frequently operates at reduced flow may present a stronger opportunity.
An assumed flow profile can support an early estimate, but measured operating data will produce a more reliable result.
How to Calculate VFD Payback
Simple payback measures how long the estimated energy savings will take to recover the installed cost.
Payback period in years = Installed VFD cost ÷ Annual energy cost savings
For example, consider a VFD installation that costs $15,000 and is estimated to save $12,000 per year:
$15,000 ÷ $12,000 = 1.25 years
The estimated payback period would be 15 months.
Depending on the application, installed cost may include engineering, electrical work, controls, enclosures, harmonic mitigation, equipment upgrades, startup, and programming. Including the full project cost produces a more realistic payback estimate.
How to Estimate Annual Carbon Reduction
Avoided carbon emissions can also be estimated:
Annual carbon reduction = Electricity saved × Applicable emissions factor
Emissions factors vary by region and electricity source. Facilities in the US can use data from the EPA’s Emissions & Generation Resource Integrated Database to select a factor suited to their location and reporting method.
Carbon reduction results remain estimates unless the calculation uses data specific to the facility’s electricity supply.
When is a VFD a Good Candidate?
VFDs often offer the greatest savings on centrifugal pumps and fans that:
- Operate for many hours each year
- Experience frequent changes in flow demand
- Use throttling valves, dampers, or bypass control
- Run below design flow for extended periods
- Have oversized equipment
Common applications include cooling-water pumps, process pumps, HVAC fans, cooling tower fans, dust-collection systems, and water or wastewater equipment.
When the Estimate Needs a Closer Review
The affinity laws provide a useful starting point, but they do not describe every system accurately. Results may need adjustment for:
- High static-head pumping systems
- Minimum flow or pressure requirements
- Changes in pump, fan, motor, and VFD efficiency
- Multiple pumps or fans operating in parallel
- Harmonics, resonance, or motor-cooling limits
A cube-law estimate may overstate savings if it assumes speed can decrease in direct proportion to flow when the system must still overcome substantial static pressure. An application review can compare the estimate with pump or fan curves, system data, and measured motor load.
See if a VFD Makes Sense for Your System
An energy savings estimate is a good starting point, but every pump and fan system operates differently. DXP supplies variable frequency drives from leading manufacturers, and our team can review your operating conditions, help select the right drive, and support the installation process.
Contact DXP today to discuss your application and potential VFD savings.
