Is it more economical to use cold or hot water? I have an electric kettle to boil water for tea/coffee that switches off automatically when water boils.
I can either pour cold or warn water from a sink. Warm water is not hot enough, so it will need to be boiled anyway, but for shorter time (smaller difference to a boiling temperature) and thus lower electricity.
Assuming, that I am not in a hurry (so time difference is not important), I am wondering what will be cheaper for me:

to pour cold water (that is cheaper) and boil it longer (using more electricity), or
to pour warm water (that is more expensive) and boil it shorter (less electricity)?

Cold water costs me 9,85 PLN/m3 and warm water costs me (9,85 + 25,88 =) 35,73 PLN/m3.
Electricity costs 0,3195 PLN/kWh.
Unfortunately, I do not know how much electricity does my kettle consume.
Am I able to calculate how should I boil water for my tea/coffee?
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.