SFP (Specific Fan Power)
The lower the SFP, the more efficient the fan system.
Fan laws
Q∝n, Δp∝n², P∝n³ — reducing speed has a cubic impact on power.
Standards
Power-flow breakdown follows the AMCA 207 fan/transmission/motor/controller system model.
Important
Results are estimates from typical/illustrative relationships. Use manufacturer performance data for final selection.
How to use this configurator
- Enter the duty point: airflow, fan pressure and air density, and say whether the pressure is total or static. Every field lets you switch units inside it.
- Choose how efficiencies are entered: one overall figure (Quick), each stage separately (Advanced), or the motor efficiency looked up from its IE class.
- Set the electricity price, operating hours, annual price increase and grid country, which fills in the emission factor.
- Describe how the fan runs through the year as speed steps with hours, and pick the control method it actually uses.
- Read the power flow, SFP, motor loading and system curve, then compare the fan with a more efficient option on payback, 10-year cost and CO₂.
What this calculates
Power flow from air to grid
The breakdown follows the system model of ANSI/AMCA 207, "Fan System Efficiency and Fan System Input Power Calculation". Air power is divided by the efficiency of each stage in turn — fan, drive (belt or coupling), motor and VFD — to reach the power drawn from the grid:
Air power (W) = Q (m³/s) × Pt (Pa)
Shaft power = Air power / ηfan
Motor output = Shaft power / ηdrive
Motor input = Motor output / ηmotor
Grid power = Motor input / ηVFD
ηoverall = Air power / Grid power
The difference between neighbouring stages is shown as that stage's loss. In Quick mode the single overall efficiency is applied at the fan stage and the other stages count as 100%, so the grid power is right but the split of losses is not. In IE class mode the motor efficiency comes from the IEC 60034-30-1 tables for the chosen rated power, poles and frequency; IE4 and IE5 values are approximated. The chain is worked through step by step in AMCA 207 explained.
Static, velocity and total pressure
Air power always uses fan total pressure. The velocity pressure comes from the outlet velocity and air density:
Pv = ½ × ρ × v²
Pt = Ps + Pv
With the basis set to static pressure, the configurator adds Pv to the value you enter; with total pressure, it subtracts Pv to show the static part. With the outlet velocity at zero both are the same number. Taking one for the other shifts every power figure by the velocity pressure — see static pressure vs total pressure.
Specific Fan Power
SFP = Grid power / Q
= Pt / (ηfan × ηdrive × ηmotor × ηVFD) [W/(m³/s)]
The gauge places the result on the SFP1–SFP6 bands of EN 13779 (up to 500, 750, 1250, 2000 and 3000 W/(m³/s), and above 3000). EN 16798-3 has replaced EN 13779 and classifies SFP differently, so the band is a reference point, not a compliance verdict.
Operating profile and annual energy
A fan rarely spends the whole year at its design point. Each step scales the grid power from the 100% duty point; with a VFD this follows the fan affinity laws:
Q ∝ n Δp ∝ n² P ∝ n³
Annual energy = ∑ P(step) × hours(step)
With any other control method, the power of each step follows that method's curve below instead. The savings compare the profile with running at 100% for the same total hours. To scale one duty point for speed, impeller diameter or density, use the fan laws calculator.
Control method comparison
Electrical power as a share of full-duty power, at an airflow fraction q = Q / Q100:
VFD (variable speed) P/P100 = q³
Inlet guide vanes P/P100 = 0.05 + 0.45q + 0.50q²
Outlet damper P/P100 = 0.45 + 0.55q
On/off cycling P/P100 = q
These are typical, illustrative curves for centrifugal fans, not measurements of your fan. Real savings depend on the fan curve, the system curve and the quality of control, and the on/off model ignores cycling wear. Every method is run through the same operating profile, so the table shows what the control method alone does to the annual bill.
Motor sizing
Minimum motor power = Motor output × (1 + reserve)
Motor loading = Motor output / rated motor power
Motor output is the power the motor delivers at its shaft: after drive losses, before motor and VFD losses. The recommended rating is the next size up on a generic IEC list from 0.18 to 500 kW, and you can pick another. Loading above 100% is flagged as overloaded; below 40% the motor is flagged as oversized, because a lightly loaded motor runs at lower efficiency and power factor. More in how to choose the right motor for a fan.
System curve with a fixed component
Δpsystem = Δpstatic + k × Q²
k = (Pt − Δpstatic) / Q²
k is solved from your duty point. With no fixed component the system curve matches the affinity-law assumption behind the profile and control comparison. A fixed part — for example a duct pressure held by a control loop — makes the required pressure fall more slowly than n² as airflow drops, so savings worked out with P ∝ n³ are overstated; the card shows the pressure error at 80% airflow. Why the fan and system curves decide the real airflow is covered in why a fan does not deliver the expected airflow.
Fan comparison, payback and 10-year cost
P = Air power / ηoverall
Annual energy = P × operating hours
Payback = Price difference / annual cost saving
10-year cost = ∑ Energy × price × (1 + increase)year − 1
CO₂ = Annual energy × emission factor
Both fans deliver the same duty point. Fan A uses the efficiencies set in Inputs, Fan B the overall efficiency entered in the comparison card. The comparison assumes full duty for the operating hours in Inputs, not the speed profile, and the payback is simple — without the price increase or discounting. The emission factor starts from the annual carbon intensity of the selected country's electricity generation and can be overwritten; the 10-year CO₂ keeps it constant.