Energy Currency

One surprisingly difficult thing to learn when studying energy is all the different measurement units that can be  – and are – frequently applied.

Just as one Euro can be converted to about 1.3 U.S. dollars, energy units can also be freely converted.  For instance, one kilowatt-hour is equivalent to 3,412 British Thermal Units (Btu) of energy.  Some of the more common measurement units of energy are listed below:

  • Joules
  • British Thermal Units (Btu)
  • Watt-Hours
  • Kilowatt-hours (kWh)
  • calories
  • Calories (this is 1,000 small-c calories, and what we refer to in food energy content)
  • Horsepower-hours
  • Quads
  • Ton-Hours (Tons are units of heat, not just weight.)

All of these units can be converted to the others.  Horsepower-hours can be converted to Calories, for instance, or kWh.

At some point I will double back and explain what some of these units mean.  For example, 1 calorie is the energy needed to raise the temperature of 1 gram of water by 1 degree Centigrade.

Energy and Power are intimately linked, but are quite different.   Energy is equal to power multiplied by time.  The fact that Watt-Hours is a unit of energy suggests, correctly, that Watt is a unit of power.  Power retains its colloquial meaning of intensity, generally speaking.  So, for instance, a stereo system with a 100 Watt output can play louder than a 50 Watt counterpart.  Some of the more common measurement units of power are listed below:

  • Joules per Second
  • British Thermal Units per Hour (Btuh)
  • Watts
  • Kilowatts
  • Horsepower
  • calories or Calories per Hour
  • Tons (thermal tons, not weight)

Here too, the units are readily converted one to another.  For example, tons of cooling can be readily converted to Btuh.

Aside from not confusing power and energy terms, the fledgling student of energy must also contend with some rather perverse naming practices.  Consider for instance, that kWh and Btu both refer to energy, while kW and Btuh refer to power.  That “h” jumping around can be surprisingly confusing when you are also trying to master the concepts of energy and energy conservation.

So be careful out there…


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