Electric water boiler - should you keep it on all the time or turn it on as needed?

Heating Trends, last updated on November 14, 2022

Will the storage electric water heater consume less electricity if it is switched on as needed or kept constantly switched on? We reveal the final answer to this question with a few practical tips and examples.

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Electric water boiler - should you keep it on all the time or turn it on as needed?

The topic of using a storage electric water heater is often mentioned on forums, so completely wrong answers can often be read. The most common answer to this question will be - keep the boiler constantly on. Proponents of this use of boilers say that the explanation lies in the basic laws of physics, ie that less electricity is needed to reheat water than is needed to heat the total volume of the boiler. While this is true, in practice the answer is completely different. Whether the water in the boiler is heated during a lower tariff or a higher tariff, it is more economical to turn it on as needed. Of course, the monthly cost of heating water will be much lower if you have a dual-tariff meter and turn on the boiler at night.

Examples in practice

Let us show two examples of the use of an electric instantaneous water heater with a capacity of 50 L, with a declared power of 2000 W. There are two people in a household.

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Boiler always on

The boiler is constantly on and turns on every time you turn on the hot water. This means that after each hand wash or use in the kitchen, the boiler will consume electricity to compensate for the temperature difference of the accumulated hot water. The moment the first member of the household decides to take a shower, the water in the boiler is heated and ready. During the shower, the boiler will be constantly running and heating new water that has replaced the used, heated water. Also, the boiler will work after showering until it reaches the set temperature again. Then, after a while, another member of the household will take a shower. The boiler will work again to compensate for the difference in water temperature and reheat the total amount of water even though there is no need for it. For the rest of the day, although there is no need for the total capacity of the boiler to be heated - the boiler will consume electricity after each hand wash or minor use of hot water in the kitchen. Also, in this case there is an unnecessary waste of energy. Since no water heater is perfectly insulated, heat energy is lost, which means that the water heater will be lit for the rest of the day and especially at night to compensate for electricity losses. This will be especially pronounced if the space is not heated at night and the temperature of the space in which the boiler is located is lower. A larger temperature difference between the heated water in the boiler and the room temperature also means higher heat losses.

If the boiler consumes about 6 kWh per day during the higher tariff, the monthly cost of heating hot water will be 180 kn.

Switching on the boiler as needed

If you turn on the water heater before showering to fully heat the water to the maximum temperature, you will provide enough water for the shower of two members of the household and additional hand washing for the rest of the day. The boiler will heat the total amount of water in an hour and a half (depending on the quality and condition of the boiler) and this is the total consumption of electricity that day. The boiler will not turn on unnecessarily for every hand wash and you will avoid heat losses, ie the boiler will not consume electricity to compensate for the losses.

Users with a two-tariff meter should heat the water in the boiler late in the evening or immediately in the morning. The cost will then be twice as low.

If you heat the boiler during a higher tariff, the boiler will consume 3 kWh per day, and the monthly cost of heating hot water will be 90 kn, or twice less than in the previous example. By heating the water during the lower tariff, you achieve 50% lower costs for the time of turning on the boiler in the lower tariff. If you additionally heat the water during the day, the boiler will not have to heat the total volume and the monthly cost will be drastically lower than in the first example.

In households where more people use hot water, hot water must always be available for showering, hand washing, washing dishes and household maintenance - it will probably be easier to keep the water heater on at all times. However, even in this case, it is recommended to heat the boiler to the maximum temperature during the lower tariff, and during the day to reduce the temperature to one that is comfortable for showering (45 ° C, mixing with cold water).

For complete comfort, someone will keep the water heater constantly on so they don’t have to think about turning on the water heater. Sometimes you are in a hurry and want to take a quick shower, but to heat the water you need to wait. Decide for yourself if you want to pay the extra cost of comfort.

Additional smart solutions

In electronic equipment stores (even in ordinary supermarkets) it is possible to buy an analog time switch (around 30 kuna) or a digital time switch (around 70 kuna) which is plugged into an outlet and the boiler into a switch. Using the time switch (digital timer) you can determine that the boiler is turned on at 5-7 o'clock in the morning. The water will be heated every morning during the lower rate, you won’t have to worry and the costs will be lower.


Although today's water heaters are of better quality, care should be taken when using the water heater in such a way that it is constantly on.