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The short answer
A VFD saves energy by slowing the motor down instead of wasting its output. On a fan or centrifugal pump, power falls with roughly the cube of speed, so running at 80% flow on a drive takes about half the power of running flat out. A damper or throttling valve at the same 80% flow still takes close to 90%. The saving is the gap between those two curves, multiplied by the hours spent at part load and by your electricity rate.
Annual savings = Σ (hours at each flow level × full-load kW × [power today − power with VFD]) × $/kWh Payback (months) = installed drive cost ÷ annual savings × 12
- Part load is the prizeNo time below full flow means no savings — a drive at 100% speed costs 2–4% more than across-the-line
- The baseline mattersInlet vanes already save some energy; outlet dampers and bypass loops save little or none
- Pumps have a floorStatic head caps pump savings well below the pure cube law
Fan and pump affinity laws: why a VFD saves energy
For a centrifugal fan or pump working against friction alone, three relationships hold as speed changes. Flow is proportional to speed. Pressure is proportional to speed squared. Power is proportional to speed cubed. They're called the affinity laws, and they're the reason variable-torque loads are where drives earn their money.
The cube is what makes the numbers so large. Drop a fan to 80% speed and it moves 80% of the air at 64% of the pressure — and it needs only about 51% of the shaft power. At 50% speed the power is about one eighth. Nothing else in a typical plant gives you that kind of return from a single component.
The catch is in the words "against friction alone". The laws describe the fan or pump itself. Your system decides whether they apply in full. A duct system or a closed hydronic loop is almost entirely friction, so savings track the cube closely. A pump lifting water up a building, or holding a fixed discharge pressure, has a large static head that doesn't fall with speed, and the savings are smaller. That's covered in static head, and it's the input most calculators leave out.
Myth
"Slowing the motor 20% saves 20% of the energy."
Reality
On a fan it saves closer to 45–50%. On a conveyor it saves about 20%. The load type decides the curve.
Myth
"A VFD saves energy on any motor."
Reality
A drive only saves energy when the process genuinely needs less than full output some of the time. A motor that must run flat out all day uses slightly more energy on a drive, because of the drive's own losses.
| Speed | Flow | Pressure / head | Shaft power (ideal) |
|---|---|---|---|
| 100% | 100% | 100% | 100% |
| 90% | 90% | 81% | 73% |
| 80% | 80% | 64% | 51% |
| 70% | 70% | 49% | 34% |
| 60% | 60% | 36% | 22% |
| 50% | 50% | 25% | 13% |
| 40% | 40% | 16% | 6% |
Reading the table: these are ideal figures for a friction-only system. In practice, motor and drive efficiency both drop a little at very light load, which is why the calculator adds a fixed drive loss and why we'd treat anything below about 30% speed with caution.
How this calculator works
Most free VFD savings calculators make two assumptions that flatter the result: that the motor runs at full power all day without a drive, and that power falls with the pure cube of speed on a drive. Both are true for some systems and badly wrong for others. This calculator makes each of those assumptions an input you can see and change.
1. Full-load input power
From horsepower: kW = HP × 0.746 × load % ÷ motor efficiency
From measurement: kW = √3 × volts × amps × power factor ÷ 1000
2. Annual hours = hours/day × days/week × 52.14
3. For each of the ten output levels q (100% … 10%):
Power today = control-method curve at q (see table below)
Power with VFD = q × (s + (1 − s) × q²) ÷ drive efficiency (fans and pumps; s = static head share)
= q ÷ drive efficiency (constant-torque loads)
4. Savings = Σ hours at q × kW × (power today − power with VFD) × $/kWh
What it includes
- The part-load curve of your current control method, not just "full speed"
- Static head for pumps, which lowers the savings on lift and pressure systems
- Straight-line power for constant-torque machines
- Drive losses, so running at 100% on a drive costs a little more than across-the-line
- Motor loading, because very few motors actually run at nameplate
What it leaves out
- Demand (kW) charges — often a real extra saving, see beyond kWh
- Changes in motor and pump efficiency at very low speed
- Soft-start, maintenance and process benefits
- Harmonic mitigation, filters, or bypass contactors in the installed cost
- Rebates, unless you subtract them from the drive cost yourself
Checking it against the simple method: choose Full speed, no flow control, set static head to zero and drive efficiency to 95.2%, and this calculator gives the same answer as the classic Σ share × speed³ × 1.05 estimate. Everything else here is a correction to that baseline.
Estimating your savings, step by step
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Step 1 — Identify the load
Is it a fan or blower, a centrifugal pump, or a machine whose torque stays flat with speed — a conveyor, mixer, extruder, or positive-displacement pump? This decides which curve applies and is the single biggest factor in the answer.
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Step 2 — Find out how flow is controlled today
Walk the system. Look for an outlet damper, inlet guide vanes, a throttling valve on the pump discharge, or a bypass line back to the suction or tank. If none of those exist and the process simply gets whatever the motor delivers, the baseline is full speed. See control methods compared.
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Step 3 — Get the motor's real input power
The nameplate gives you horsepower and efficiency. Better still, clamp the running amps and measure the voltage at the motor starter on a normal day, then use the measured volts & amps option. Many motors are oversized and run at 60–80% of nameplate, and a measurement catches that.
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Step 4 — Build the load profile
This is the input that matters most and the one most people guess. The best sources, in order: building-automation or SCADA trend logs of airflow, flow or damper position; the valve or damper position recorded over a week; and, failing that, a conversation with the operator about how the system behaves by season and time of day. The ten percentages must add up to 100.
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Step 5 — Use your blended electricity rate
Divide a recent monthly bill's total by its total kWh. That captures energy charges, riders and taxes. Demand charges are separate — if yours are significant, the savings shown here are conservative.
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Step 6 — Price the whole installation
The drive, any line reactor or filter, the enclosure, wiring, commissioning, and your labour. Then subtract any utility rebate. That's the number that belongs in the payback field.
Control methods compared
A drive's savings are always measured against something. The table shows input power as a share of full-load power at each flow level, using the part-load curves this calculator applies. The damper and throttling-valve curves are typical of published part-load data for those methods; your equipment will sit somewhere near them, not exactly on them.
| Flow | Full speed / bypass | Outlet damper or throttling valve | Inlet guide vanes | VFD, friction-only system | VFD, 40% static head | VFD, constant torque |
|---|---|---|---|---|---|---|
| 100% | 100% | 100% | 100% | 103% | 103% | 103% |
| 90% | 100% | 95% | 89% | 75% | 82% | 93% |
| 80% | 100% | 89% | 79% | 53% | 65% | 82% |
| 70% | 100% | 84% | 69% | 35% | 50% | 72% |
| 60% | 100% | 78% | 60% | 22% | 38% | 62% |
| 50% | 100% | 73% | 53% | 13% | 28% | 52% |
| 40% | 100% | 67% | 45% | 7% | 20% | 41% |
VFD columns include a 3% drive loss, which is why they read 103% at full flow.
Outlet damper and throttling valve
The fan or pump runs at full speed and a restriction downstream burns off the excess pressure. Power falls a little as flow drops, because the machine rides back up its curve, but most of the energy is still being spent. These are the baselines where a drive saves the most.
Inlet guide vanes
Vanes pre-swirl the air entering the fan wheel, which reduces the work the wheel does. They're genuinely better than a damper, so the drive's advantage is smaller. Vanes also stick and drift with age — a drive replacing seized vanes often saves more than the curve suggests.
Variable frequency drive
The drive lowers the motor's speed until the fan or pump makes exactly the flow the system needs. No pressure is thrown away, so power follows the affinity laws down — limited only by static head and the drive's own small losses.
Bypass and recirculation: a pump that dumps excess flow back to its suction or tank runs at a constant operating point, so its power barely changes with the flow the process actually uses. For savings purposes it behaves like full speed, and it's often the best retrofit candidate in the building.
Static head: why pumps save less than fans
A pump's system curve has two parts. Friction head falls with the square of flow, just like the affinity laws expect. Static head does not fall at all — it's the height the water has to be lifted, or the minimum pressure a booster set has to hold at the top of a building, or the pressure a boiler feed pump has to overcome. At zero flow the pump still has to make that pressure.
System head at flow q: H(q) = Hs + (H100 − Hs) × q² Pump power with a VFD: P(q) ≈ q × (s + (1 − s) × q²) where s = Hs ÷ H100
With no static head this collapses back to the cube law. With 40% static head, running at 60% flow takes about 38% of full power instead of 22%. The drive still wins against a throttling valve, but by a much smaller margin, and it can't turn the pump down below the speed at which it just matches the static head — below that, flow stops.
Low static head (0–25%)
Closed chilled-water and hot-water loops, condenser-water loops, most circulating pumps. The water ends where it started, so the head is almost all friction plus the differential-pressure setpoint. Savings track the cube law closely.
High static head (30–60%+)
Domestic water boosters, transfer and lift pumps, cooling-tower make-up, boiler feed, and anything holding a fixed discharge pressure. Enter the static share honestly — this is where simple calculators overstate savings the most.
Finding your static share: divide the pressure (or height) the pump must hold at no flow by the pressure it makes at design flow. A booster making 80 psi at design that must hold 35 psi at the top floor has a static share of about 44%.
Constant-torque loads: when energy isn't the reason to buy
Conveyors, mixers, extruders, positive-displacement pumps and screw compressors need roughly the same torque at every speed. Power is torque × speed, so power falls in a straight line: half speed, about half power. That still sounds useful, but there's a catch. On a conveyor, running slower usually means moving less product — the energy per tonne barely changes. Real savings appear only where the machine was genuinely running faster than the process needed.
That's why the calculator shows modest numbers on these loads, and why we'd rarely justify a drive on a conveyor by kWh alone. The better reasons are usually:
- Soft starting — no across-the-line inrush, no belt slip, no shock to gearboxes and couplings
- Matching speed to the process — throughput, product quality and fewer jams
- Replacing mechanical speed control — eddy-current clutches, variable-pitch sheaves and hydraulic couplings waste energy and need maintenance
- Positive-displacement pumps — here speed control is flow control, and replacing a relief-valve bypass with a drive can save a great deal
Sizing note: constant-torque loads need a Heavy Duty drive rating and often a separately powered motor blower for sustained low-speed running. Use the VFD sizing calculator to get the frame right.
Five savings estimates, worked end to end
Every figure below comes from this calculator with the inputs shown, so you can reproduce each one above.
Example 1 — 50 HP air-handler supply fan with an outlet damper
Application: VAV supply fan in an office building, 16 hours a day, 7 days a week. Premium-efficiency motor measured at about 85% load. Flow controlled by an outlet damper.
Full-load input: 50 HP × 0.746 × 0.85 ÷ 0.945 = 33.6 kW Hours: 16 h × 7 d × 52.14 = 5,840 h/yr Profile: 100% 5 · 90% 10 · 80% 20 · 70% 25 · 60% 20 · 50% 15 · 40% 5 Rate: $0.12/kWh Cost today (outlet damper) $19,503/yr Cost with VFD $9,119/yr Annual savings 86,537 kWh = $10,384/yr (53%) Installed cost $9,000 → payback 10.4 months
Result: a strong case. Notice that the hours at 100% flow actually lose a little on the drive — 303 kWh a year — and that the 70% and 60% rows deliver more than half the savings. That's typical: the money is in the middle of the profile.
Same fan with inlet guide vanes instead of a damper: savings drop to $7,130 a year and payback stretches to 15.1 months. Still worthwhile, but the baseline changed the answer by a third.
Example 2 — 25 HP chilled-water pump with a throttling valve
Application: secondary chilled-water pump on a closed loop, running 24/7. Motor at 80% load, 93.6% efficient. Flow trimmed by a balancing valve; about 20% of design head is the differential-pressure setpoint.
Full-load input: 25 HP × 0.746 × 0.80 ÷ 0.936 = 15.9 kW Hours: 24/7 = 8,760 h/yr Static head: 20% Profile: 90% 10 · 80% 20 · 70% 25 · 60% 25 · 50% 15 · 40% 5 Rate: $0.11/kWh Cost today (throttling valve) $12,572/yr Cost with VFD $6,391/yr Annual savings 56,192 kWh = $6,181/yr (49%) Installed cost $5,500 → payback 10.7 months
Result: a strong case. Closed loops are close to ideal for a drive. Commission it with a differential-pressure sensor at the far end of the loop, not at the pump, or much of the saving disappears into an unnecessarily high setpoint.
Example 3 — 40 HP domestic water booster, and what static head does
Application: booster pump holding pressure for an eight-storey building, 24/7. Motor at 90% load, 94.1% efficient, throttled today. About 40% of the pump's design head is lift and minimum residual pressure.
Full-load input: 40 HP × 0.746 × 0.90 ÷ 0.941 = 28.5 kW
Hours: 24/7 = 8,760 h/yr
Profile: 100% 10 · 90% 20 · 80% 25 · 70% 20 · 60% 15 · 50% 10
Rate: $0.10/kWh
Static head 40% Static head 0%
Cost today (throttling valve) $21,701 $21,701
Cost with VFD $15,371 $12,560
Annual savings $6,330 (29%) $9,141 (42%)
Installed cost $7,500 → payback 14.2 months 9.8 months
Result: still worthwhile, but not the number a simple calculator gives. Ignoring static head would overstate this project's savings by about 44%. If a proposal for a booster set shows cube-law savings, ask where the static head went.
Example 4 — 30 HP single-speed cooling tower fan
Application: induced-draft cooling tower fan, running 18 hours a day, 7 days a week at full speed with no other control. Motor at 90% load, 93.6% efficient.
Full-load input: 30 HP × 0.746 × 0.90 ÷ 0.936 = 21.5 kW Hours: 18 h × 7 d × 52.14 = 6,570 h/yr Profile: 100% 10 · 90% 10 · 80% 15 · 70% 15 · 60% 15 · 50% 15 · 40% 10 · 30% 10 Rate: $0.11/kWh Cost today (full speed) $15,552/yr Cost with VFD $5,794/yr Annual savings 88,706 kWh = $9,758/yr (63%) Installed cost $6,000 → payback 7.4 months
Result: an excellent case — with a caveat. If the fan cycles on and off on condenser-water temperature today, it doesn't really run 6,570 hours at full speed. Enter only the hours it's actually on, or the savings will be overstated. Drives on towers also end the gearbox wear caused by frequent across-the-line starts.
Example 5 — 20 HP conveyor, where energy doesn't carry the case
Application: belt conveyor, two shifts, five days a week. Motor at 75% load, 93% efficient. The line needs full speed most of the time and runs slower during changeovers.
Full-load input: 20 HP × 0.746 × 0.75 ÷ 0.93 = 12.0 kW Hours: 16 h × 5 d × 52.14 = 4,171 h/yr Profile: 100% 40 · 90% 20 · 80% 20 · 70% 10 · 60% 10 Rate: $0.10/kWh Cost today (full speed) $5,019/yr Cost with VFD $4,502/yr Annual savings 5,174 kWh = $517/yr (10%) Installed cost $4,000 → payback 7.7 years
Result: not an energy project. The drive is still often the right call here — for soft starting, speed matching and less wear on the belt and gearbox — but justify it on those grounds, not on kWh. See constant-torque loads.
Savings the kWh number doesn't show
The calculator deliberately counts energy only, because it's the one benefit that can be estimated from a few inputs. On a real project these usually add to the case — sometimes by more than the energy itself.
Demand charges
Many commercial and industrial tariffs bill peak kW separately from kWh. If the fan or pump runs below full flow during your billing peak, the drive trims demand too. If it runs flat out on the hottest afternoon, it won't — which is why we leave demand out rather than guess.
Soft starting
An across-the-line start draws six to eight times full-load current and hits the belts, couplings and gearbox with a torque spike. A drive ramps up smoothly, which cuts mechanical wear and can let you avoid supply upgrades on weak feeders.
Maintenance
Dampers, vanes and throttling valves wear, stick and drift. Removing or locking them open takes a maintenance item off the list, and lower average speed means longer bearing and seal life on the fan or pump.
Control and comfort
A drive holds a pressure or flow setpoint far more precisely than a damper. In buildings that means steadier comfort and quieter duct systems; in process it means consistent product and fewer upsets.
Displacement power factor
A standard PWM drive presents a displacement power factor of about 0.95 or better to the supply at any motor load, where a lightly loaded motor on its own can sit well below 0.8. Harmonics lower the true power factor, so check the tariff before counting this.
Noise
Fan noise falls steeply with speed. A fan at 70% speed is noticeably quieter, which matters in occupied buildings and near property lines.
Payback, rebates and making the case
Simple payback — installed cost divided by annual savings — is the number most maintenance and facilities budgets are approved on, and it's what the calculator reports. As a rule of thumb, fan and pump retrofits with a meaningful part-load profile often pay back within one to three years. Much longer than that and it's worth checking the inputs, or looking at a smaller drive.
Check for a utility rebate first
Many electric utilities offer prescriptive incentives for drives on HVAC fans and pumps, often paid per horsepower, plus custom incentives for larger process projects. The DSIRE database (dsireusa.org) lists incentive programs by state. Subtract the rebate from the installed cost before calculating payback — it can halve it.
Look past year one
A drive typically serves well over a decade. The 5-year figure in the calculator is savings minus the installed cost over five years, before maintenance and demand benefits. For capital requests that need it, the same annual saving drops straight into an NPV or IRR model.
Codes may decide for you. Energy codes based on ASHRAE 90.1 require variable-speed or variable-flow control on many new and replaced HVAC fans and hydronic pumps above size thresholds that vary by edition. On those jobs the question isn't whether a drive pays back, but which one.
Six mistakes that inflate savings estimates
1 — Comparing against full speed when there's a damper
Assuming the motor draws full power all day ignores the energy the damper or vanes already save.
Instead: pick the control method that's actually installed.
2 — Using the cube law on a booster pump
Static head doesn't fall with speed, and on lift and pressure systems it can be half the total.
Instead: enter the static head share and let the system curve do the work.
3 — Taking nameplate horsepower as running power
Oversized motors are the norm. A 50 HP motor doing 35 HP of work saves 30% less than the nameplate suggests.
Instead: clamp the running amps, or set the load percentage.
4 — Guessing the load profile optimistically
"It runs at half speed most of the time" is the most common and least checked claim in any savings proposal.
Instead: use a week of trend data or damper position logs.
5 — Counting hours the motor is off
A cycling cooling-tower fan or a pump on a float switch doesn't run 8,760 hours a year.
Instead: enter run hours, not calendar hours.
6 — Forgetting the drive's own losses
At full speed a drive costs 2–4% more energy than across-the-line. A motor that rarely turns down can end up worse off.
Instead: keep the drive efficiency in the model, as this calculator does.
Drives built for fan and pump savings
Variable-torque applications are where drives pay back fastest, and most families have a model tuned for them. We stock new and tested refurbished drives with a 24-month warranty on every part and repair and same-day shipping on in-stock items from Raleigh, NC.
HVAC and building services
Drives with fan and pump macros, PID, sleep and wake-up, and BACnet or Modbus built in — the features that turn the savings above into savings on site.
- Energy-optimising V/Hz modes
- Sleep mode at low demand
- Fire and override modes
ABB VFDs
The ACH580 and ACH550 are common choices for HVAC fans and pumps. The ACH580 keeps energy-saving counters on the keypad, so the savings above can be checked after commissioning.
- Energy-saving counters (ACH580)
- Large installed base
- Clear Normal Duty ratings
Invertek
Optidrive Eco and the HVAC range for variable-torque building services; E3 and P2 for general and heavy-duty machinery.
- Compact footprints
- Single-phase input on smaller frames
Not sure which frame? Savings are calculated from horsepower, but drives are sized on current. Take your motor's full-load amps to the VFD sizing calculator to get the rating right before you order.
Want us to check the numbers?
Send the motor nameplate, a description of how the system is controlled today, and any trend data you have. One of our application engineers will review the estimate, size the drive and quote from stock. There's no charge.
Frequently asked questions
How much energy does a VFD save?
On fans and centrifugal pumps that spend a lot of time below full flow, 20–60% of the motor's energy is typical. The saving depends on how flow is controlled today, the share of hours at reduced output, and for pumps, how much of the head is static. A motor that must run at full output all day saves nothing and uses 2–4% more because of drive losses.
Is a VFD worth it on a motor that runs at full speed?
Not for energy. At 100% speed the drive adds a small loss, so energy use rises slightly. It can still be worth it for soft starting, reduced mechanical wear, or process control, but the payback has to come from those benefits.
Why does the calculator show less savings than other calculators?
Most free calculators assume the motor draws full power all day without a drive, and that pump power falls with the pure cube of speed. If you already have a damper, inlet vanes or a throttling valve, or your pump has significant static head, both assumptions overstate the savings. This calculator models the control method you have and the static head you enter.
What is the affinity law for fans and pumps?
For a centrifugal fan or pump in a friction-only system, flow is proportional to speed, pressure to speed squared, and power to speed cubed. At 80% speed a fan needs about 51% of full power; at 50% speed about 13%.
How do I find the load profile for my fan or pump?
The best source is a week or more of trend data from a building-automation or SCADA system showing airflow, flow, or damper and valve position. Without trends, record the damper or valve position at different times of day and season, or ask the operator how the system behaves. The percentages across all output levels must total 100.
What is static head and why does it reduce pump savings?
Static head is the pressure a pump must produce even at zero flow, such as the height water is lifted or a minimum pressure setpoint. It does not fall with speed, so on booster, lift and transfer pumps a VFD saves less than the cube law predicts, and the pump cannot be slowed below the speed that just matches the static head.
Does a VFD save energy on a conveyor?
Only where the conveyor genuinely runs faster than the process needs. Conveyors are constant-torque loads, so power falls in a straight line with speed rather than with the cube. Energy savings are usually modest, and drives on conveyors are justified mainly by soft starting, speed matching and reduced wear.
What electricity rate should I use?
Use a blended rate: the total of a recent bill divided by its total kWh. That includes energy charges, riders and taxes. Demand charges are billed on peak kW and are not included in this calculator, so if they are significant your actual savings may be higher.
Are there rebates for installing a VFD?
Many electric utilities offer incentives for VFDs on HVAC fans and pumps, often paid per horsepower, and custom incentives for larger process projects. The DSIRE database at dsireusa.org lists programs by state. Subtract any rebate from the installed cost before calculating payback.
How accurate is a VFD savings estimate?
An estimate built on measured motor power and trend-logged load profiles is usually close enough for a budget decision. One built on guessed hours and a nameplate rating can easily be off by 30% or more. For a firm number, measure the motor's input power at several operating points and log the load profile before and after.
Standards and sources
- US DOE, Improving Fan System Performance and Improving Pumping System Performance — sourcebooks covering part-load behaviour of dampers, vanes, throttling valves and adjustable-speed drives.
- Hydraulic Institute — pump system guidance, including affinity rules and system curves with static head.
- ANSI/ASHRAE/IES Standard 90.1 — energy standard for buildings; sets variable-flow requirements for many HVAC fans and hydronic pumps.
- IEC 61800-9-2 — energy efficiency indicators and IE classes for drives and power drive systems.
- IEC 60034-30-1 — efficiency classes of line-operated AC motors (IE codes).
- NEMA MG 1, Part 31 — requirements for motors fed from adjustable-speed drives.
- IEEE 519 — harmonic control in electric power systems; relevant when adding drives to an existing supply.
Estimates on this page are for budgeting and screening. For projects that depend on a guaranteed saving, measure before and after, and follow your utility's measurement and verification requirements.