Calculating how much an electric water heater consumes can sometimes not be so intuitive. The consumption of a water heater essentially depends on its capacity in litres and its energy efficiency qualification (A-F).
Here are the factors you need to take into account to calculate how much your electric water heater consumes and to be able to calculate the price for heating the water with it. With these calculations you can also always compare between two water heaters and choose the one that best suits your needs, because of how much it consumes or its size.
How much does an electric water heater consume?
The consumption of electric water heaters varies between 400 and 3000 kWh/year. You can usually find the consumption value on the heater's energy efficiency label.
This is the approximate consumption depending on the average time of use. However, with the power and heating time data for the specifications you will know how much an electric water heater consumes for each hour of use.
To do this, it is best to take as an example a number of different recommended electric water heaters depending on the number of people living in your home.
Consumption table depending on the electric water heaters' capacity
| Water heater model | Number of people | Power | Heating time | Water heater electricity consumption (kWh) |
|---|---|---|---|---|
| 30 litres | 1 | 1500 W | 55′ | 1,375 kWh |
| 50 litres | 2 | 1500 W | 1h30′ | 2,250 kWh |
| 80 litres | 3-4 | 1500 W | 2h15′ | 3,375 kWh |
| 100 litres | 5-6 | 1500 W | 2h50′ | 4,245 kWh |
How is the consumption of an electric water heater calculated in kWh?
When you know the power in watts (W) and the heating time (hours and minutes) you can calculate the water heater's electricity consumption in kWh by taking the following three steps:
- To change the power to kW: by dividing by 1000 you change the power units from watts (W) to kilowatts (kW).
- To calculate the heating time in hours: divide the minutes by 60 and add them to the hours. This gives you the value of the water heater's heating time in units of hours, exclusively.
- Multiply the power by the time: as you already have the power in kW and the time in hours, when you multiply them you get the water heater's consumption in units of kWh.
Examples using the same water heaters as in the previous section:
| Electric water heater | Power (kW) | Heating time (hours) | Water heater electricity consumption (kWh) |
|---|---|---|---|
| 30 litres | 1,5 | 0,917 h. | 1,375 kWh |
| 50 litres | 1,5 | 1,5 h. | 2,250 kWh |
| 80 litres | 1,5 | 2,25 h. | 3,375 kWh |
| 100 litres | 1,5 | 2,83 h. | 4,245 kWh |
Another important factor: the water heater's consumption when it is idle
All water heaters, whether they are electric or gas, have an energy consumption when they are idle that is measured by the thermal dispersion at 65ºC and is shown in kWh/24 hr. This means that, as they are hotter than the place where they are installed, they cool down and need to consume a little energy to keep the water hot.
The average thermal dispersion for water heaters is between 1 and 2 kWh/24 hr. This means that an electric water heater that is idle consumes between 1 and 2 kWh every hour.
How much does it cost to heat water with an electric water heater?
Heating water with an electric water heater costs between €0.2 and €0.8 per litre. If you take into account that a person uses on average 30 litres of hot water a day, this means that we would be talking about a cost of between €3.5 and €8 per month.
| Electric water heater | Water heater electricity consumption (kWh) | Cost per litre of water(1) | Monthly cost(2) |
|---|---|---|---|
| 30 litres | 1,375 kWh | 0,70 cents | 7.3 € |
| 50 litres | 2,250 kWh | 0,42 cents | 4.9 € |
| 80 litres | 3,375 kWh | 0,26 cents | 3.8 € |
| 100 litres | 4,245 kWh | 0,21 cents | 3.5 € |
(1) Calculated by applying an average price of 0.14 kWh with a stable price electricity tariff like One Luz.
(2) The monthly cost is obtained by multiplying the price per litre of water by 30 litres per day of average consumption and by an average 30 days per month. To calculate the monthly cost we have also added the cost when the electric water heater is idle, which is usually between €1 and €2. The greater the capacity, the greater the consumption when the heater is idle.
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.
Comparison of Electricity and Gas Tariffs
You don't need to adapt to Endesa's tariffs because they adapt to you. If you go to our catalogue you can compare the different tariffs for yourself. Or if you prefer, you can answer a few questions and we will take care of comparing all the different electricity and gas tariffs and then make a customised recommendation.
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