What is the most energy-efficient way to obtain 10 liters of water at 50 degrees Celsius?

Answer

Heating a fixed amount of water to a certain temperature, in either of these two ways, requires the same amount of energy – the law of conservation of energy (first law of thermodynamics) has something to do with this. Theoretically, one method is not more energy efficient than the other, because in any case you will have to add a fixed amount of energy to the water to get it to that temperature.

The only way in which you can save energy is to ensure that energy losses (for example through transport of energy, evaporation of water, etc.) are kept to a minimum. Assuming that the hot water comes from the tap and not from the whistling kettle, this mainly means that the hot water pipes must be well insulated, and that the boiler must be properly adjusted.

Because there is always hot water in the pipes that loses some heat anyway (which is then lost), it is just a bit more interesting to keep the water in the pipes (and in the boiler) at a fairly low temperature. For reasons of energy efficiency, it is better to lower the boiler temperature and take more water from the boiler than to set the boiler higher and take less water from it. This is because a constant amount of hot water remains in the pipes and boiler, and the lower its temperature, the less heat is lost therein if it comes to a standstill in the pipes after the water has been taken off.

As a final answer, I must still recommend the first: it is better to heat the 10 liters immediately to 50 degrees. But the reasons are almost purely constructional in nature.

By the way, if the water in a cooking pot is heated on a fire, always put a lid on it: this way the heat is much better preserved in the pot and it will have to be on the fire up to 1/3 less time before it is hot . A considerable saving, and here too simply by limiting the heat losses.