Drawing No. EH–TH–014 // Thermal Engineering & HVAC
Fan Power Calculator
Reviewed August 2026
Find fan air power, shaft power and electrical input power from airflow rate and pressure rise, accounting for fan and motor efficiency.
What problem does this solve?
Fan energy consumption is set by the useful air power it delivers (flow × pressure) divided down through two efficiency losses: the fan's own aerodynamic efficiency (converting shaft power to air power) and the motor's electrical efficiency (converting electrical input to shaft power). This calculator finds all three power levels from airflow, pressure and the two efficiencies.
Inputs
Results
Background
Pair [W] = Q [m³/s] × ΔP [Pa]. This is the useful hydraulic/pneumatic work done moving air against the system's resistance — the theoretical minimum power needed if the fan and motor were both 100% efficient.
Shaft power = Pair / ηfan, accounting for aerodynamic and mechanical losses inside the fan itself. Electrical input power = shaft power / ηmotor, accounting for the motor's electrical-to-mechanical conversion losses. Overall efficiency = ηfan × ηmotor.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
Well-designed centrifugal fans commonly reach 65–80% peak efficiency; axial fans can reach 75–85% for well-matched applications. Efficiency varies significantly with operating point on the fan curve — running far from a fan's best-efficiency point (too much or too little flow for its design) can cut efficiency substantially.
Since fans often run continuously or for very long hours in HVAC systems, even a few percentage points of efficiency difference between two fan/motor combinations compounds into a meaningful annual energy cost difference — this is part of why fan and motor efficiency standards (and variable-speed control, see the Fan Laws Calculator) matter so much in HVAC energy codes.
Not separately — those losses would need to be folded into the fan or motor efficiency values entered, or accounted for as an additional multiplier. Direct-drive fans avoid belt losses entirely; VFD losses are typically small (a few percent) but non-zero.