Saving water in a motorhome: how many litres each use costs

Three minutes of showering at 8 litres per minute adds up to 24 litres. At 3.5 litres per minute, it is 10.5 litres. The difference between the two values is 13.5 litres, and it is the tap that decides it, not the length of the shower.

Two items together account for 63%: personal hygiene and toilet flushing. Both are the easiest to change in a motorhome, because showers are shorter and the toilet can manage without flush water. By contrast, only 4% of household water goes on drinking and cooking, around 5 litres a day. How much of this is for drinking alone is covered in the article on the recommended fluid intake.

A full 120-litre tank weighs 120 kilograms. The water alone therefore takes up more than the minimum payload that the regulation requires of the manufacturer of a 7-metre vehicle with four seats. The regulation leaves open whether fresh water counts towards the mass in running order: Article 2(4)(a) mentions only fuel and liquids. The calculation still works as a rule of thumb for trip planning, because every litre in the tank is a litre less for luggage.

Grey water from the shower and sink is waste water within the meaning of the law. § 54(1) no. 1 of the Wasserhaushaltsgesetz (WHG, German Federal Water Act) defines waste water as water whose properties have been altered by domestic use. § 55(1) requires waste water to be disposed of in such a way that the public good is not impaired. § 56 sentence 1 assigns responsibility for disposal to the public bodies obliged to carry it out under state law.

A practical consequence follows from the definition, the disposal principle and the allocation of responsibility. Grey water belongs at a disposal station, i.e. in a discharge point intended for this purpose. If you use 60 litres of fresh water, you have to get rid of most of it again. Drinking water, cooking water and the share that runs into the black water tank have to be deducted.

If the waste water tank holds less than the fresh water tank, it determines the range. Every litre saved then counts twice: once at the inlet and once at the outlet. A look at both tank sizes in the vehicle registration document shows which of the two will be full or empty first.

According to the Verbraucherzentrale, 15 to 20 litres run unused down the drain when you brush your teeth or wash your hands with the tap left running. A jet regulator reduces the water demand at the tap by mixing air into the water jet and reducing the flow rate while keeping the same spray pattern. The EU Ecolabel for sanitary tapware sets clear limits in Decision 2013/250/EU. Kitchen and basin taps may deliver 6.0 litres per minute, or 8.0 litres with a flow limiter. It must be possible to set the limiter's saving setting to no more than 6 litres. For shower heads and shower systems, the figure is 8.0 litres.

Under the Decision, testing is carried out at 1.5, 3.0 and 4.5 bar, with the average of the three measurements being what counts. Under Article 4, the criteria were valid for four years from 21 May 2013. The values remain reliable as an order of magnitude, because the Verbraucherzentrale's figure of 8 to 10 litres per minute falls within the same range.

In the household, toilet flushing uses 27% of drinking water, making it the second-largest item. The Verbraucherzentrale gives 9 litres per flush for old cisterns. Dual-flush systems or a flush-stop button manage with 3 to 6 litres.

On board, the calculation shifts because, in addition to the fresh water tank, the black water tank is also limited. Thetford states a capacity of 17.5 to 19.3 litres for its cassette toilets, which holds flush water and faeces together. Models without a connection to the central water tank have their own flush water tank holding 8 to 15 litres. Every flush therefore shortens two ranges at the same time.

A dry separating toilet keeps urine and solids apart and needs no flush water. Water consumption for the toilet thus drops to zero. How much difference this makes depends on the flush volume of the appliance being replaced. With four people, three flushes a day and 3 litres per flush, it comes to 36 litres a day. The 3 litres are the lower value that the Verbraucherzentrale gives for dual-flush systems in the household; no comparable manufacturer data on flush volume is available for motorhome toilets.

According to the Verbraucherzentrale, washing up by hand under running water uses up to 50 litres for twelve place settings. A dishwasher needs 19 litres on average for the same amount. On board there is usually no dishwasher, which is why hand-washing technique makes the difference.

Two bowls instead of running water are the most effective measure: one for the washing water, one for rinsing. Food residues should be removed dry beforehand, because otherwise pre-rinsing uses more water than the actual washing-up. A brush absorbs less water and washing-up liquid than a sponge, which reduces the amount of rinsing water.

What comes out of the washing-up bowl is grey water and belongs in the waste water tank. Tipping it out under the vehicle is not a permitted way of disposing of waste water.

Saving has a lower limit, and it is based on hygiene. The Deutscher Verein des Gas- und Wasserfaches (DVGW, German Technical and Scientific Association for Gas and Water) gives a clear value in its compact information sheet on stagnation from March 2024. The drinking water in a drinking water installation must be replaced every seven days, or better every three days. From 72 hours up to 7 days, the growth of microorganisms adds to the heavy metal load.

A motorhome tank is a drinking water installation in the same sense. Anyone who reduces consumption so far that 100 litres last two weeks has not achieved a saving goal but created a stagnation problem. Standing water promotes biofilm on the tank wall, and legionella multiply in warm water. Disinfecting the tank fixes this after the event; a quick change of water prevents it in the first place. The legislator itself recognises the conflict between saving and hygiene. § 50(3) WHG obliges water suppliers to provide information on water-saving measures. This explicitly applies with due regard to hygiene requirements.

In practice, this results in a rule that no saving tip can replace. It is better to take on a little water and top up more often than to leave a lot of water standing for a long time. How standing times affect the water in the tank is explained in the guide to water treatment in a motorhome. Why silver ions do not replace this rule is covered in a separate article.

§ 5(1) no. 2 WHG obliges every person to exercise due care in activities that may have an impact on a body of water. The aim is the economical use of water that is required out of consideration for the water balance. The provision is tied to the effect on bodies of water, not to every single shower. However, it frames the economical use of water as a general duty of care.

On the supplier side, the obligation is set out in § 50(3) WHG. Public water supply bodies work towards the careful use of water, keep water losses low and inform end consumers about water-saving measures. Paragraph 1 of the same provision declares public water supply to be a service of general interest. For every cubic metre drawn, waste water charges are also payable, as the Verbraucherzentrale points out.

When it comes to water quality, however, the supplier's responsibility ends early. § 10 no. 1 of the Trinkwasserverordnung (TrinkwV, German Drinking Water Ordinance) sets the point of compliance where the water leaves the point of use. The provision explicitly includes water on board land vehicles. What happens beyond the tap is the camper's responsibility. The guide to filling up with water on the road explains this in detail, and the article on the German Drinking Water Ordinance explains the legal framework.

Searches for a motorhome's consumption are usually about diesel, not water. The two quantities meet at exactly one point: weight. 120 litres of fresh water means 120 kilograms of load that travel with you every kilometre.

If you only fill the tank a third full, you save weight and gain payload for luggage. Reliable figures on the additional fuel consumption per 100 kilograms are not available in robust form for motorhomes, which is why this guide does not give any. The direction is clear in physical terms; the order of magnitude depends on the vehicle, route and driving style.

A simple rule is enough for trip planning: take on water where the next leg of the journey requires it, instead of filling up completely as a precaution. This also reduces the time the water stands in the tank.

The comparison shows where the leverage lies. What separates the first and second scenarios is not a different attitude to saving water, but a different tap and a different showering rhythm. A water canister carried on board extends self-sufficiency further, but again costs payload.

At the same time, the second scenario runs up against the limit described in the section on stagnation. With a range of just under 5 days, the last of the water stands in the tank for longer than the three days that the DVGW recommends as the better value.