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VFD Energy Savings Calculator

See what a drive saves before you buy one.

Describe how the motor runs today. We compare its annual energy cost with the same duty on a variable frequency drive, then show the savings, the payback and the drives we stock for it.

Start the estimateSend trend data for review

  • Built on the affinity laws

    Fans and pumps follow the cube law. Trimming speed by 20% can cut shaft power close to half.

  • Payback in months

    Add the installed drive cost and we return simple payback and five-year net savings.

  • Checked by an engineer

    Send trend data or a nameplate photo and we return a firm number at no charge.

Estimate your VFD energy savings

Five short steps. Results update as you type. The defaults match a 25 HP HVAC fan on an outlet damper, running 16 hours a day.

Step 1 of 5

Application

Start with a preset. Adjust anything that does not match your equipment.Start with a preset and adjust what does not match.

Load type
How flow is controlled today

Step 2 of 5

Motor

Use the nameplate, or measured volts and amps if you have them.

Calculate power from

Most motors run at 75–90% of nameplate. Use the motor’s actual loading if you know it.Most motors run at 75–90% of nameplate.

Step 3 of 5

Running hours

Typical weekly duty. We convert it to hours per year.Typical weekly duty, converted to hours per year.

5,840 operating hours / year

Step 4 of 5

Load profile

Share of operating time at each output level. Fans track airflow, pumps track flow, conveyors track speed. The ten values must total 100%.Share of time at each output level. Must total 100%.

Share of operating time at each output level

Total 100% · profile is valid

Step 5 of 5

Costs

Use a blended electricity rate: total bill ÷ total kWh.

Installed cost is the drive plus installation, minus any rebate. Modern PWM drives lose 2–4% as heat.

Fan and pump affinity laws: why a VFD saves energy

For a centrifugal fan or pump working against friction alone, flow, pressure and power all fall with speed, power fastest of all. That cube is why trimming speed saves so much.

  • Flow follows speed: at 80% speed a fan or pump moves 80% of the air or water.
  • Pressure or head follows speed squared: at 80% speed it is 64%.
  • Shaft power follows speed cubed: at 80% speed it needs about 51% of full power.

Power₂ = Power₁ × (Speed₂ ÷ Speed₁)³

Ideal for friction-only systems. Static head and drive losses reduce the real saving, and the calculator above includes both.

Affinity laws at reduced speed

Ideal, friction-only system

Speed FlowFlow Pressure / headHeadShaft powerPower
100%100% 100%100% 100%100%
90%90% 90%90% 81%73%
80%80% 80%80% 64%51%
70%70% 70%70% 49%34%
60%60% 60%60% 36%22%
50%50% 50%50% 25%13%
40%40% 40%40% 16%6%

Below about 30% speed, motor and drive efficiency fall off, so treat very low-speed savings with caution.

Where VFD energy savings come from

The calculator compares your current control method with variable-speed operation at the same load profile, hour by hour.

  • The baseline matters: dampers, vanes and bypass loops each waste power differently.
  • Part load creates the savings; at full speed a drive adds 2–4% losses.
  • Pumps have a floor: static head limits how far speed can drop.

Annual savings = Σ (hours × full-load kW × power difference) × $/kWh

Payback (months) = installed cost ÷ annual savings × 12. Full-load input = HP × 0.746 × load ÷ efficiency.

Your inputs: HVAC fan · outlet damper · 25 HP · $0.12 / kWh

Updates with your inputs

Output Time kW todaykW now kW with VFDkW VFD kWh saved / yrkWh/yr
100% 5% 17.0 17.6 −154
90% 10% 16.1 12.8 1,926
80% 20% 15.2 9.0 7,210
70% 25% 14.2 6.0 11,980
60% 20% 13.3 3.8 11,096
50% 15% 12.4 2.2 8,901
40% 5% 11.4 1.1 3,006
Total 100% 43,966

Rounded values. Levels with 0% of the time are hidden. At 100% output the drive’s own losses make the saving slightly negative.

Three worked examples, three answers

The same model, three typical duties. Load any of them into the calculator and adjust from there.

  • Rooftop exhaust fan · 25 HP

    $5,276 / yr

    Outlet damper today, 16 h/day. Payback 8.0 months on a $3,500 installed drive.

  • Cooling tower fan · 50 HP

    $16,864 / yr

    Full speed today, 18 h/day through the season. Payback 4.6 months on a $6,500 installed drive; biggest win in the set.

  • Booster pump · 15 HP

    $2,722 / yr

    40% static head limits the turndown. Payback 19.8 months on $4,500 installed, better with a rebate.

Variable frequency drives we stock for this duty

Matched to the 25 HP example. New and IAC-certified refurbished units ship same day from Raleigh, NC on orders placed by 4:00 PM Eastern, with a 24-month warranty.

Browse all HVAC drives

VFD energy savings questions, answered

Call (877) 727-8757 to talk through your numbers with an engineer.

Will a VFD save energy on every motor?

No. Savings come from running below full speed for part of the operating time. A motor that always runs at full load gains little, and drive losses can slightly increase energy use.

What if my load profile does not total 100%?

The results pause until the ten values add up to 100%. The total under the chart shows how much to add or remove; “Reset to application preset” puts the preset’s profile back.

How should I enter the electricity rate?

Use a blended rate: the total of a recent bill divided by its total kWh, so energy charges, riders and taxes are included. Demand charges are billed on peak kW and are not part of this estimate.

What does installed drive cost include?

The drive, any filter or reactor, wiring and labour, minus any utility rebate. Leave it blank to see savings without payback.

How does pump static head affect savings?

Static head is pressure the pump must make even at zero flow, such as lift or a minimum setpoint. It does not fall with speed, so pumps with high static head save less than the cube law suggests and cannot slow as far.

Do rebates change the payback?

Yes. Many utilities pay per horsepower for VFDs on HVAC fans and pumps. Subtract the rebate from the installed cost; the DSIRE database at dsireusa.org lists programs by state.

Want a firm number?

Send trend data or a nameplate photo. An IAC engineer reviews the duty cycle, checks the estimate and returns a quote, free of charge.

Send trend dataRequest a quote

(877) 727-8757 · sales@iac.us.com

What to send

  • A nameplate photo, or HP, voltage and FLA
  • A week of trend data or BMS export, if you have it
  • How flow is controlled today and the hours it runs

Related tools and guides

Keep going from the estimate: size the drive, check a drive that will not start, or compare new and refurbished options.

Method references: US DOE motor and pump sourcebooks, Hydraulic Institute guidance, ASHRAE 90.1, IEC 61800-9-2, IEC 60034-30-1, NEMA MG 1 Part 31 and IEEE 519.