Free Energy Savings Tool

Fan Speed Savings Calculator — VSD / EC Part-Load Payback

Most fans are sized for the worst hour of the year and then run flat-out for all the others. This calculator uses the fan laws and your real duty profile to show what a variable-speed drive, an EC fan or a simple two-speed control would save — in kWh, money, CO₂ and years to payback.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Part-Load Energy & Payback

Power at part load = full-load power × (speed ratio)³ (+ drive losses). Enter how many hours per year the fan runs at each speed.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How the savings are calculated

Power at part speed W = W_full × (N/N_full)³ × (1 + drive loss)
Annual energy = Σ W(speed) × hours(speed)
Saving = W_full × hours − annual energy with control
Payback = upgrade cost ÷ annual cost saving

The cube law is the reason this works: at 80 % speed a fan needs 51 % of the power, at 60 % speed 22 %, at 40 % speed 6 %. The duty profile is the part people guess wrong — if you can, log the fan for a week (a clamp meter on one phase, or the BMS trend) before committing to the numbers. Our motor sizing tool converts amps to kW.

Real installations see 10–20 % less than the theoretical figure because some system pressure is fixed (filters at minimum flow, pressurised rooms), motor efficiency falls at light load, and VSDs add 2–5 % losses. The calculator already includes the last two; the first is why we recommend 80 % rather than 70 % for "normal" operation when you are unsure.

Which control option?

Option	Typical cost	Turn-down	Losses	Best for
EC fan (integrated electronics, 0–10 V / PWM / Modbus)	New fan	10–100 %	≈ 2 %, motor η 85–92 %	Replacing AC fans ≤ 6 kW; retrofit AHU, condenser, FFU, fan wall
VSD on existing AC induction motor	$150–$300 per kW	20–100 %	3–5 % + motor part-load loss	Larger motors (> 5 kW), belt-driven fans
Two-speed motor (Dahlander) or tapped winding	Low	50 / 100 %	None	Simple day/night or summer/winter
Electronic / transformer speed controller (AC voltage)	$30–$150	30–100 %	Motor runs hotter, η drops	Small external-rotor AC fans only
Staging (switching fans off in a fan wall)	Controls only	Steps	None	Multiple-fan systems

Worked example

Given: 7.5 kW AHU supply fan, 3,500 h/yr, profile 15/45/30/10 % at 100/80/60/40 % speed, $0.15/kWh, $1,800 VSD installed.

Full speed all year: 7.5 × 3,500 = 26,250 kWh
With VSD (3 % loss): 100 % → 7.73 kW × 525 h = 4,056; 80 % → 3.96 kW × 1,575 h = 6,233; 60 % → 1.74 kW × 1,050 h = 1,824; 40 % → 0.51 kW × 350 h = 180 → 12,293 kWh
Saved: 13,957 kWh ≈ $2,090 per year, 5.6 t CO₂
Payback: 1,800 / 2,090 = 0.9 years

Frequently Asked Questions

How much energy does a VSD save on a fan?

Typically 30–60 % of annual fan energy when the fan runs at part load most of the time, because fan power falls with the cube of speed. A fan at 80 % speed uses about half the power; at 60 % about a fifth.

What is the payback period of an EC fan upgrade?

Usually 1–3 years for fans that run more than 4,000 hours a year at variable load (AHUs, condensers, data centers, refrigeration), and under a year for 24/7 duty at high tariffs. Fans that genuinely run at full speed all the time pay back more slowly — for those, motor efficiency (EC vs AC) is the lever.

Do the fan laws overestimate VSD savings?

They can, when part of the system pressure is fixed (duct static setpoint, pressurised room, dirty filter) rather than proportional to flow squared. Expect real savings 10–20 % below the pure cube-law figure; our calculator includes drive and motor part-load losses but assumes a pure system curve.

Can I put a VSD on any fan motor?

Standard three-phase induction motors, yes (check insulation class and bearing currents on motors above 30 kW). Single-phase capacitor motors and shaded-pole motors cannot be run from a standard VSD — replace them with an EC fan instead.

Link to or Cite This Tool

This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:

<a href="https://www.longwellfans.com/resources/fan-speed-savings-calculator/">VFD / Fan Speed Energy Savings Calculator</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). VFD / Fan Speed Energy Savings Calculator. https://www.longwellfans.com/resources/fan-speed-savings-calculator/

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