Fan HP & Energy Cost Calculator

Brake horsepower, motor size, annual energy cost, and VFD savings
Fan Inputs
Results
Air HP
—
horsepower
Brake HP (BHP)
—
horsepower
Motor Size
—
HP NEMA
Input kW
—
kilowatts
Annual Energy
—
kWh/yr
Annual Cost
—
$/year
VFD Savings Estimate
VFD kW at Speed
—
kilowatts
VFD Annual Cost
—
$/year
Annual Savings
—
$/year
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About This Calculator

This calculator turns a fan's airflow and pressure into the numbers that drive equipment selection and operating cost. Enter the airflow in CFM, the total static pressure the fan must develop, and the fan and motor efficiencies, and it returns air horsepower, brake horsepower, the next standard NEMA motor size, electrical input in kilowatts, and the annual energy and cost based on your run hours and energy rate.

It also estimates the savings from a variable frequency drive. Because fan power falls with the cube of speed, trimming the average speed cuts energy use far faster than it cuts airflow, so the tool compares full-speed cost against the reduced VFD draw and reports the annual savings. Use it to sanity-check fan submittals, compare constant-speed and VFD operation, and put a dollar figure on static-pressure reductions.

Formula & Method
Air HPAHP = CFM × TSP ÷ 6356
Brake HPBHP = AHP ÷ ηfan
Input powerkW = BHP × 0.7457 ÷ ηmotor
Annual costCost = kW × hours × rate
VFD powerkWvfd = kW × (speed)³

The constant 6356 converts CFM and inches of water column to horsepower at standard air density, and 0.7457 is the exact kilowatts per horsepower. The VFD estimate applies the fan affinity law, where power varies with the cube of speed (P ∝ N³). The tool also reports specific fan power, SFP = kW × 1000 ÷ CFM in W/CFM, and flags high values against ASHRAE 90.1 fan-power guidance. Selections follow standard ASHRAE and NEMA engineering practice, with the motor sized to the next NEMA frame at or above 1.15 × BHP.

Frequently Asked Questions
How is fan brake horsepower calculated from CFM and static pressure?
Air horsepower is airflow times total static pressure divided by 6,356: AHP = CFM times in.w.c. divided by 6356. Brake horsepower (BHP) is the air horsepower divided by the fan's mechanical efficiency, so a less efficient fan needs more shaft power for the same airflow and pressure. The constant 6,356 comes from converting CFM and inches of water column into horsepower at standard air density.
How does the calculator convert horsepower to kilowatts?
Input electrical power in kilowatts equals brake horsepower times 0.7457 divided by motor efficiency. The 0.7457 is the exact kilowatts per horsepower conversion, and dividing by motor efficiency accounts for losses between the wires and the shaft. Annual energy is that kilowatt draw times the operating hours per year, and annual cost is the energy times your dollar-per-kWh rate.
How much can a VFD save on fan energy?
Fan power follows the affinity law, falling with the cube of speed, so power is proportional to speed cubed. Running a fan at 80 percent speed draws about 0.8 cubed, or roughly 51 percent, of full-speed power, a savings near 49 percent. The calculator multiplies the full-speed kilowatts by the speed ratio cubed to estimate the reduced draw, cost, and annual savings.
What motor size does the tool recommend?
The tool selects the next standard NEMA motor size at or above 1.15 times the brake horsepower, giving roughly a 15 percent service margin so the motor is not loaded to its limit. It also reports specific fan power in watts per CFM and flags high values against ASHRAE 90.1 fan-power guidance so you can check the design against energy-code expectations.
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.