The Ergosphere
Thursday, November 10, 2005
 

Unit analysis

A long-overdue post about a very common error.

One reader (who I hope does not mind me quoting him anonymously) wrote:

I was confused by your dislike of the term kh/w and baffled by your assertion that it is a meaningless term....

You seem to be asserting that the term KWH is meaningless and not a defined amount.

I replied:

You've got a relatively common confusion of two different things.  Let me see if I can clear this up.

A kilowatt-hour is a kilowatt TIMES an hour.  A kilowatt in turn is 1000 watts, there are 3600 seconds in an hour, and a watt is a joule per second; a kilowatt-hour is thus 3.6 million joules (the basic unit of energy in the MKS units system).  Kilowatt-hour is abbreviated "kWh".

kW/h would be a kilowatt DIVIDED BY an hour.  This is not a unit of energy, it's a rate of change of power; in most contexts, it's simply wrong.  Some people write "kilowatts per hour" and it's just as wrong [it's just as wrong because it's the same thing].

 




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