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?

3 Answers 3


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.


Unfortunately, I do not know how much electricity does my kettle consume.

We also do not know how much electricity your kettle uses but there are devises that can tell you.

Get a Kill A Watt Electricity Usage Monitor and plug your kettle into it. Do some tests over a few days.

The first morning do a test with cold water in the kettle and record the electrical usage to bring the water to a boil.

The next morning do the same test with hot water from the tap in the kettle and record the usage.

Take both numbers and calculate the costs - 0,3195 PLN/kWh - for the amount of Kw you used for each test.

Now you know the cost difference.

  • 2
    actually the electricity is the easiest part, that can be calculated when you know the starting temperature. the big unknown is the real cost of warm tap water, including waste.
    – ths
    Jan 18, 2021 at 17:08

An exact calculation would depend of the temperatures of your cold and warm tap water, and the amount of water you need to let out until it runs warm.

Generally, warm tap water is cheaper than electrically heating it, assuming your water is actually heated by a diffeeent method (gas, central heating, district heating, etc.)

But to get warm water from the tap, you generally need to let it flow for a while, and this means you loose a large amount of hot water in the drains and pipes. This is of course grossly inefficient.

So it depends on your exact situation; if you've just used warm water so it flows warm immediately, i'd fill the kettle from the warm tap, otherwise from the cold one.

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