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.

CHU/Hour/Square Foot

The unit CHU/hour/square foot is used to measure Cooling Heat Units per hour per square foot, commonly applied in air conditioning and refrigeration calculations. It indicates the amount of cooling energy, in CHUs, that passes through or is required for one square foot of a surface area over an hour. This measurement helps engineers and designers assess cooling loads, select appropriate air conditioning systems, and ensure thermal comfort in buildings. A higher CHU/hour/square foot value shows greater heat gain, meaning more cooling is needed, while a lower value indicates better insulation or lower cooling requirements. It is essential in designing energy-efficient HVAC systems and optimizing equipment sizing, helping reduce operational costs and maintain desired indoor temperatures. By using CHU/hour/square foot calculations, building designers can balance comfort, energy efficiency, and sustainability effectively.