Fan Efficiency Calculator — Static, Total, Wire-to-Air & FEI Benchmark
Fan efficiency tells you how much of the electricity becomes moving air — and it is now regulated (EU ErP 327/2011, US DOE / AMCA 208 Fan Energy Index). Enter the duty point and the power, and get static, total and wire-to-air efficiency, the FEI-style ratio against a reference fan, and the annual cost of the inefficiency.
No signup · Works on mobile · Formulas shown below · Updated 2026-08-23
Fan Efficiency from Duty Point & Power
η = Q × p ÷ W. Static uses static pressure; total adds the velocity pressure at the outlet.
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.
Definitions
Fan static efficiency ηs = Q × ps ÷ Wshaft Fan total efficiency ηt = Q × pt ÷ Wshaft, pt = ps + ½ρvoutlet² Wire-to-air efficiency ηw2a = Q × p ÷ Welectrical = ηfan × ηmotor × ηdrive × ηtransmission Fan Energy Index (AMCA 208) FEI = FEPreference ÷ FEPactual// FEP = fan electrical power; FEI ≥ 1.0 required by ASHRAE 90.1-2019 / IECC 2021 / many DOE rules ErP 327/2011 (EU) minimum ηtarget = f(fan type, power) — e.g. backward-curved ≥ ~62 % static at 1 kW, axial ≥ ~40 %
Regulators moved from fan efficiency (a single number that punishes small fans) to FEI (a ratio against a reference fan at the same duty point), so a small fan can comply as long as it is well matched. Both are about the duty point: the same fan can be 70 % efficient at its design point and 30 % when throttled.
What good looks like
Fan + motor
Typical wire-to-air static efficiency at design point
EC airfoil plug fan, 500 mm, 3 kW
60–68 %
EC backward-curved, 250 mm, 0.5 kW
45–55 %
AC backward-curved external rotor, 250 mm
30–40 %
EC axial 450 mm condenser fan
35–45 % (static), 55 % total
AC axial 450 mm condenser fan
20–30 % static
Forward-curved AC blower, fan-coil
15–25 %
Compact DC axial 120 mm
10–20 %
Cross-flow fan
8–15 %
Belt-drive industrial centrifugal 15 kW, IE3 motor
55–70 %
Every efficiency point lost becomes heat: a 2 kW fan at 40 % overall efficiency dumps 1.2 kW into the air it moves — often the largest single heat load in an AHU in winter-free-cooling mode.
Worked example
Given: 8,000 m³/h (2.22 m³/s) at 600 Pa static, EC plug fan with 500 mm outlet, measured input 2.3 kW.
Static air power = 2.22 × 600 = 1,333 W → ηs,w2a = 1,333 ÷ 2,300 = 58 %
Outlet velocity 11.3 m/s → pv = 77 Pa → ηt,w2a = 2.22 × 677 ÷ 2,300 = 65 %
Reference fan (66 % total, 88 % motor) would draw ≈ 2,590 W → FEI ≈ 1.13 — compliant with margin.
Annual: 2.3 kW × 6,000 h = 13,800 kWh ≈ $1,660. A forward-curved alternative at 35 % would cost ≈ $2,750 — the EC fan pays its premium in under two years.
Frequently Asked Questions
How do I calculate fan efficiency?
Efficiency = air power ÷ input power. Air power (W) = airflow (m³/s) × pressure (Pa). Use shaft power for fan efficiency and electrical input for wire-to-air efficiency. In imperial units: efficiency = CFM × in. w.g. ÷ (6,356 × BHP).
What is a good fan efficiency?
Peak fan (aerodynamic) efficiency: 75–85 % for airfoil/backward-curved centrifugal, 60–75 % for axial, 45–60 % for forward-curved. Wire-to-air including the motor: 55–68 % for EC plug fans, 30–45 % for small EC axial fans, 15–25 % for forward-curved AC blowers.
What is FEI (Fan Energy Index)?
A metric from AMCA 208 comparing the electrical power of a fan at a duty point with that of a reference fan at the same point. FEI = reference power ÷ actual power; FEI ≥ 1.0 is required by ASHRAE 90.1-2019, IECC 2021 and several US state codes for fans above 1 HP. Higher is better.
What is the difference between static and total efficiency?
Total efficiency credits the kinetic energy of the air leaving the fan (velocity pressure); static efficiency does not. For ducted fans whose outlet velocity is recovered downstream, total is the fair measure; for plug fans discharging into a plenum, static is what you actually get.
Does ErP apply to my fan?
EU Regulation 327/2011 applies to fans with input power 125 W – 500 kW placed on the EU market, with minimum efficiency grades by fan type and power. Fans integrated into other ErP-regulated products are assessed with that product. Longwell fans for the EU market carry the ErP declaration; see our certifications page.
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