BTU per hour (BTU/h) equals approximately 0.293 watts, the standard rating unit for air conditioners, heaters, and furnaces in North America. A typical window air conditioner rated at 10,000 BTU/h can cool a 400–500 square foot room, making this unit essential for HVAC system sizing. One BTU/h equals about 0.2931 watts.

Calories per second (cal/s) equals exactly 4.184 watts, used primarily in thermodynamics and physics to express heat transfer rates. While not a common everyday unit, it bridges thermal and mechanical power calculations in scientific research. One calorie per second equals exactly 4.184 watts or about 0.00561 horsepower.