Watt per Square Meter [W/m²]

The unit watt per square meter (W/m²) measures heat flux density or radiant flux, representing the amount of energy transferred per unit area per unit time. One watt equals 1 joule per second, so 1 W/m² corresponds to 1 joule of energy passing through 1 square meter every second.

This unit is widely used in physics, engineering, meteorology, and solar energy studies to quantify energy transfer rates across surfaces. Examples of applications include:

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  Solar radiation: measuring sunlight incident on a surface

  
  
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  Heat transfer: quantifying conduction or convection per unit area

  
  
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  Building energy analysis: evaluating heat gain or loss through walls and roofs

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in W/m², Q is energy in joules, A is area in square meters, and t is time in seconds.

For example, solar constant at Earth’s surface is approximately 1361 W/m², indicating the power received per square meter from the Sun outside Earth’s atmosphere.

W/m² is the standard SI unit for energy flux density, replacing older units like cal/cm²·s or Btu/ft²·h, and is essential for thermal engineering, climatology, and renewable energy calculations.

Calorie (IT) per Hour per Square Centimeter [cal(IT)/(h·cm²)]

The unit calorie (IT) per hour per square centimeter, written as cal(IT)/(h·cm²), measures heat flux density or thermal power per unit area using the International Table calorie (1 cal(IT) = 4.1868 J). It represents the amount of heat energy transferred through 1 cm² of surface in 1 hour.

This unit is commonly used in solar energy studies, building physics, and small-scale thermal experiments where energy transfer over extended periods and small areas is measured. Typical applications include:

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  Solar radiation received on small surfaces

  
  
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  Thermal testing of materials over time

  
  
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  Calorimetry experiments

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(IT)/(h·cm²), Q is energy in calories, A is area in cm², and t is time in hours.

Conversion to SI units:

Although SI units like W/m² are standard, cal(IT)/(h·cm²) remains useful in small-scale experiments, historical solar radiation studies, and building physics references, providing an intuitive way to quantify energy transfer per hour per unit area.