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.

Btu (th) per Second per Square Inch [Btu(th)/(s·in²)]

The unit Btu (th) per second per square inch, written as Btu(th)/(s·in²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 J). It represents the amount of heat energy transferred through 1 square inch of surface every second.

This unit is commonly used in high-intensity thermal applications, engineering experiments, and solar energy studies where energy flux over small surfaces is significant. Typical applications include:

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  Heat flux from industrial furnaces or radiative heaters

  
  
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  Solar radiation on concentrated panels or small surfaces

  
  
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  Thermal testing of materials under high-intensity conditions

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in Btu(th)/(s·in²), Q is energy in Btu(th), A is area in in², and t is time in seconds.

Conversion to SI units:

While SI units like W/m² are standard, Btu(th)/(s·in²) is useful in U.S. engineering, laboratory experiments, and historical references, providing an intuitive measure of high-intensity heat flux per square inch per second.