Even though larger electric water heaters, 80 or 100 litres, consume more kWh, they can also heat much more water. This is why the monthly cost in € is lower compared to water heaters with less capacity.
Of course, all these calculations depend to a large extent on the electricity tariff you have contracted (€ per kWh). The consumption for an electric water heater also depends on whether you use more or less hot water throughout the day.
As a general rule, a water heater uses electricity 24 hours a day, either because it is heating water or when it is idle. This is why it is always best to choose a tariff that ensures a stable price for electricity throughout the day, such as One Luz, for example.
There are also newer options for which you do not need to worry about daily consumption or about kWh. These are the flat-rate electricity tariffs, where you always pay a fixed quota that is totally customised in accordance with your energy.
Using an electricity tariff recommender or price comparer is a good option to find the tariff that best suits your needs and lifestyle.