Fan Motor Heat Gain Calculator
Every watt a fan motor draws ends up as heat somewhere. Find how much reaches your airstream or room, and how much it warms the air.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Heat added
- 2,993BTU/h
- Air temperature rise
- 2.31°F
About 2,993 BTU/h (877 W) of heat, warming the air by 2.3 °F.
- Heat added
- 877W
- Motor electrical input
- 0.877kW
- Shaft power
- 1hp
Show the working
- Shaft power = rated output × load1 × 100 % = 1 hp = 0.746 kW
- Electrical input = shaft power ÷ efficiency0.746 ÷ 0.85 = 0.877 kW
- Heat = all electrical input × 3,412 BTU/h per kW2,993 BTU/h
- Temperature rise = heat ÷ (1.08 × density ratio × CFM)2,993 ÷ (1.08 × 1,200) = 2.31 °F
Where the heat goes
A motor turns electricity into shaft power plus losses. Both end up as heat: the losses where the motor is, and the shaft power where the fan or driven load is. So the heat added to the air depends on what is in the airstream:
- Motor and fan both in the air: all the electrical input, shaft power ÷ efficiency
- Motor outside, fan in the air: the shaft power only
- Motor in the air, driven load outside: the losses only, input minus shaft power
Numbers used
Shaft power is rated output × load. Electrical input is shaft power ÷ efficiency (the DOE motor-load relationship). One horsepower is 745.7 W and one watt is 3.412 BTU/h (NIST), so one shaft horsepower is about 2,544 BTU/h. The temperature rise of the air is heat ÷ (1.08 × CFM), corrected for elevation.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Motor rated output
- 1 hp
- Motor load (share of rated output)
- 100 %
- Motor efficiency at that load
- 85 %
- Where are the motor and the fan?
- Motor and fan both in the airstream (all input becomes heat)
- Airflow past the motor or fan (optional)
- 1,200 CFM
- Site elevation
- 0 ft
Result
- Heat added
- 2,993 BTU/h
- Heat added
- 877 W
- Motor electrical input
- 0.877 kW
- Shaft power
- 1 hp
- Air temperature rise
- 2.31 °F
About 2,993 BTU/h (877 W) of heat, warming the air by 2.3 °F.
Assumptions
- All electrical energy a motor uses ends up as heat: the losses where the motor is, and the shaft power where the fan or driven load is.
- 1 hp = 745.7 W and 1 W = 3.41214 BTU/h (NIST); 1.08 sensible factor scaled for elevation.
Limitations
- Motor efficiency falls at light load; use the efficiency at the actual load.
- Variable-speed drives add their own losses where they are installed.
Frequently asked questions
How much heat does a 1 hp blower motor add?
With the motor and fan in the airstream at full load and 85 % efficiency, about 2,990 BTU/h — roughly a quarter of a ton of cooling load.
Why does motor efficiency matter if all the energy becomes heat?
It decides where the heat ends up. A less efficient motor draws more power for the same shaft output, so more heat is released where the motor sits.
Does an ECM blower add less heat?
It adds heat equal to the power it draws. Because it usually draws less power for the same airflow, it adds less heat.
Sources
- Determining Electric Motor Load and Efficiency — U.S. DOE. Equation 1: three-phase input kW = V × I × PF × √3 ÷ 1,000 (worked example 469.7 V, 37 A, PF 0.763 → 22.9 kW); Equation 2: full-load input kW = hp × 0.7457 ÷ efficiency.
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
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