Erg per Hour per Square Millimeter [erg/(h·mm²)]

The unit erg per hour per square millimeter, written as erg/(h·mm²), measures energy flux density or power per unit area using the cgs system. An erg is defined as the energy required to move 1 dyne over 1 centimeter, or 1 erg = 10⁻⁷ joules. Dividing by hour and mm² provides the rate of energy transfer per small area over time.

This unit is typically used in laboratory-scale physics, radiation experiments, and surface energy studies where small areas and low energy transfers are involved. Applications include:

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  Measuring radiation incident on tiny surfaces

  
  
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  Calorimetry experiments on small samples

  
  
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  Micro-scale energy transfer studies in materials science

  
  

Heat or energy flux can be expressed mathematically as:

where q is energy flux in erg/(h·mm²), E is energy in ergs, A is area in mm², and t is time in hours.

Conversion to SI units:

While largely of historical or specialized use, erg/(h·mm²) provides a convenient cgs-based measure for micro-scale, low-intensity energy transfer in scientific experiments.

Kilowatt per Square Meter [kW/m²]

The unit kilowatt per square meter (kW/m²) measures heat flux density or radiant flux, representing the amount of energy transferred per unit area per unit time, scaled to kilowatts. One kilowatt equals 1000 watts, so 1 kW/m² = 1000 J of energy passing through 1 square meter every second.

This unit is widely used in solar energy, fire safety engineering, and high-intensity thermal applications to quantify power delivered per area. Typical applications include:

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  Solar concentrators and photovoltaic panels: assessing the intensity of sunlight on a surface

  
  
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  Fire testing: measuring heat flux from flames or radiative heaters

  
  
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  Industrial processes: calculating energy input in furnaces or reactors

  
  

Mathematically, heat flux is expressed as:

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

For reference, direct sunlight at noon on a clear day provides roughly 1 kW/m² at the Earth’s surface.

kW/m² is an SI-derived unit and is convenient for large-scale energy calculations, replacing older units like Btu/ft²·h or cal/cm²·s. It provides a straightforward way to assess power per unit area in engineering and environmental applications.