Joule per Second per Square Meter [J/(s·m²)]

The unit joule per second per square meter, written as J/(s·m²), is a measure of heat flux density or radiant flux. Since 1 joule per second equals 1 watt, this unit is equivalent to watt per square meter (W/m²). It represents the amount of energy transferred or radiated through 1 square meter of surface every second.

This unit is widely used in physics, thermodynamics, meteorology, and solar energy studies. Examples include:

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  Measuring solar radiation incident on the Earth’s surface

  
  
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  Evaluating heat transfer through walls, roofs, and other surfaces

  
  
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  Calculating power density in radiant heating or cooling systems

  
  

Mathematically, the heat flux is expressed as:

where q is the flux in J/(s·m²), Q is energy in joules, A is area in m², and t is time in seconds.

For example, solar constant outside Earth’s atmosphere is approximately 1361 J/(s·m²), indicating the power received per square meter from the Sun.

Using J/(s·m²) or W/m² provides a standardized, SI-compliant measure for energy transfer per unit area, replacing older units like cal/cm²·s or Btu/ft²·h in modern engineering and scientific applications.

Kilocalorie (IT) per Hour per Square Meter [kcal(IT)/(h·m²)]

The unit kilocalorie (IT) per hour per square meter, written as kcal(IT)/(h·m²), measures heat flux density or thermal power per unit area using the International Table kilocalorie (1 kcal(IT) = 4186.8 J). It represents the amount of heat energy transferred through 1 square meter of surface in 1 hour.

This unit is often used in building engineering, HVAC, and solar energy studies to quantify energy transfer through walls, roofs, or solar collectors. Typical applications include:

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  Heat gain or loss through building surfaces

  
  
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  Solar energy incident on rooftops or flat surfaces

  
  
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  Design of heating and cooling systems

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in kcal(IT)/(h·m²), Q is total heat in kilocalories, A is area in m², and t is time in hours.

Conversion to SI units:

Although SI units such as W/m² are now standard, kcal(IT)/(h·m²) remains useful in legacy building and thermal engineering literature, providing an intuitive measure of energy transfer over time in kilocalories.