title_body: Is it more economical to use cold or hot water?
upvoted_answer: From a physics point of view, over 85%  of the energy of heating 20 C or 50 C water and boiling it at 100 C goes into actually boiling it. In fact, comparing the heat required  for equal amounts (say, 1/2 liter) of water starting at 20 C vs 50 C, the energy difference is 62 kJ or 0.017 kWh, thermal.  Electric kettles are very efficient, so figure 0.020 kWh electrical difference.
  Don't waste the water.