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In order to optimise DHW consumption, you first need to understand it. DHW consumption per person accounts for 25 to 40 percent of the total water consumption in this country. Given an average total water consumption of 121 litres per day (BDEW for 2018), this amounts to 30 to 48 litres. This wide range is due to the fact that each person uses DHW very differently.
DHW consumption in figures
Daily personal hygiene accounts for the largest share of DHW consumption per person in a household. This includes showering, bathing, face washing, etc. The average value of 30 to 48 litres mentioned above does not refer to the total amount of water used for showering or bathing. It merely refers to the amount provided by a DHW cylinder or a solar thermal system. Someone who bathes more frequently has a higher DHW consumption and thus a higher total water consumption than someone who only takes a quick shower every day. This is because cold water is added to the heated DHW.
The following figures show how closely DHW consumption (hot water) in particular and water consumption (hot and cold water) in general are related:
- Toilet flushing accounts for almost 30 percent of total water consumption. As expected, this is cold, unheated water from the pipe.
- Laundry accounts for slightly more than 10 percent of total water consumption. Unlike toilet flushing, some of this water is heated – usually directly in the washing machine by using electricity.
- As with washing laundry, some of the water used for washing dishes is heated. This task accounts for around 6 percent of total water consumption.
DHW consumption, drinking water consumption, process water and virtual water consumption
If you search for the average DHW consumption per person, you will come across related terms such as drinking water, process water, virtual water or water footprint at some point. But what do they actually mean? The figure of 121 litres per day mentioned at the beginning refers only to the drinkable water used directly from the tap. Process water, on the other hand, also takes into account the amount used in the manufacture of food or other goods – and that transcends national borders. Experts also refer to this indirectly used water as virtual water. Its quantity exceeds that of drinking water many times over. For Germany, the Federal Environment Agency has calculated a daily consumption of 117.2 billion cubic metres. That equates to 3.9 cubic metres or 3900 litres per person. This means that the amount of both direct and indirect water must be taken into account when calculating the actual water consumption per person. The term water footprint is used for this.
Calculating and reducing DHW consumption costs
Technically speaking, DHW can be heated in different ways: at a central point or in a decentralised system. In the case of centralised DHW heating, the task is undertaken by the existing heating system. The water is heated either in the conventional way by the heat generated during combustion, or with the help of sunlight. For the latter, a solar thermal system and a suitable solar cylinder are essential.
Decentralised DHW heating, on the other hand, is separate from the existing heating system. Here electrically operated systems are used to provide the DHW. Which type of DHW heating is used has a major influence on the cost of DHW consumption.
In practice, experts recommend the centralised solution, as it is cheaper due to the fuel used. However, decentralised provision also has its advantages, for example when DHW demand is too low for a connection to the existing heating system to be economically viable. Ultimately, it's up to the individual to decide which version is best. The first important step should always be to get advice from a heating contractor on site.
Calculating the average costs of DHW consumption
If you are a homeowner and also own a solar thermal system, calculating the average costs of DHW consumption may only be of secondary importance to you. Those who live in rented accommodation, on the other hand, should not shy away from doing so. Because it provides both clarity and an incentive to save energy. If you wish to calculate the cost of your average DHW consumption, you need to bear in mind several factors. These include:
- DHW consumption on average
- The temperature to which the water is heated
- The price of the fuel used
Number of occupants influences DHW consumption
The annual DHW consumption for a household depends very much on how many people live in it. Given a DHW consumption of about 40 litres per day/person (see above), this amounts to 43,800 litres or 43.80 cubic metres per year for a family of three (40 litres x 3 persons x 365 days).
The water temperature is also an important factor
The hotter the DHW, the more energy the water heater consumes. On average, the DHW in most systems is heated to between 50 and 60 degrees Celsius. Heating to a temperature below the range may impact on comfort.
It all comes down to the fuel
A significant factor in DHW water costs is the fuel used. While the price per kilowatt hour for conventionally procured electricity is very high, it has been consistently low for logs and pellets for a number of years, as this table shows:
* The figures are indicative only and may fluctuate significantly at any time
If you now wish to calculate the total cost of your annual DHW consumption, all you have to do is multiply the relevant annual consumption in kilowatt hours by the fuel price. The easiest way to find out your annual consumption is to look at your annual heating bill. If you do not have this handy, you can calculate it with this formula:
DHW consumption x factor 2.5 x difference between cold and hot water = annual consumption, this being based on a cold water temperature of 10 degrees Celsius.
In our concrete example, this means: 43.80 cubic metres x 2.5 x 40 degrees Celsius (50 °C set temperature - 10 ° cold water temperature) = 4380 kilowatt hours. If, on the other hand, the DHW is heated to 60 degrees Celsius, the annual consumption is 5475 kilowatt hours = (43.80 cubic metres x 2.5 x 50 degrees Celsius).
Optimising DHW consumption and saving costs
Optimising your own DHW consumption does not mean using less DHW or reducing costs at any price. For a start, the basic need for water must continue to be met. And secondly, aside from the purchasing costs for the solar thermal system, it is possible to generate DHW free of charge. Rather it's more a question of making optimum use of the DHW by implementing technical and manual measures.
How to optimise your DHW or water consumption
- Optimising DHW consumption starts with how it's produced. For DHW heating, opt for advanced, efficient heating technology that suits your needs.
- If possible, invest in a solar thermal system and let it cover most of your DHW needs.
- Use a water saving cistern for your toilet. This saves up to three times more water than a conventional cistern.
- Flow limiters and thermostatically controlled mixer taps can reduce water consumption markedly.
- Also be mindful of how long you spend showering, or consider turning off the tap while applying soap.