The Surprising Benefits of Smaller Water Tanks in Motorhomes

There are surprising benefits to carrying a modest amount of water in a motorhome. We all know the story of Goldilocks and how she was first attracted to the biggest bowl of porridge in the house of the three bears. After sampling the two largest bowls, she eventually found that the smallest bowl suited her best. You can apply this same analogy to motorhomes and water tank sizes. 

It’s tempting to choose the motorhome with the largest tank capacity without realising how the increased weight of the larger water reservoir negatively affects other aspects of the motorhome. We need to consider the bigger picture before automatically adopting the motorhome with the largest water tank size. So, what is the optimal size for water tank storage? 

The motorhome manufacturers with the most thorough research and development and design expertise are those based in Germany. They generally opt for water tanks of 100L–120L as their experience has shown that tanks of this size range represent ‘the Goldilocks zone’. 

However, with new motorhomes on the market fitted with 200L tanks, we ask the question: Is this extra capacity handy to have, or an oversupply that has detrimental effects on other aspects of the motorhome? 

In this blog, Joe Barton, retail sales consultant at our Auckland sales centre, shares his expert insights to help you make an informed decision on the size of your water tank. We’ll delve into the smart design philosophy behind German brands' preference for smaller water tanks and explore how it enhances performance, safety, and efficiency.

Payload: the reason every kilogram of water storage matters

Let’s start by explaining the term “payload” and why it matters in relation to water tank size. 

Payload is the total weight of what a motorhome can legally carry in terms of load. The manufacturer sets a total weight limit (GVM) for the motorhome, and the payload is the limit minus the weight of the empty motorhome. 

The total weight of everything that you add from that point on must not exceed the maximum number of kilograms the vehicle can carry as specified by the manufacturer. It’s an offence to exceed the GVM.

The payload of a motorhome must therefore provide enough capacity to fully accommodate the weight of all passengers, fuel, luggage, and supplies. It also needs to be high enough to account for any towing capacity required and the extra weight added by any bike, luggage or kayak racks fitted.

How does water storage affect payload?

A litre of water weighs one kilogram, so selecting a motorhome with a 100L freshwater tank instead of a 200L reservoir will free up 100kg of payload. This precious saving can then be used to carry something more essential, like golf clubs or bikes.

For those who intend to tow something with their motorhome, towing a loaded trailer weighing 1000kg consumes 100kg of the payload. If your motorhome has a 100L freshwater tank instead of a 200L reserve, the payload saving can be utilised for towing a trailer without affecting the ability to legally carry other stuff.

Learn more in our guide to motorhome weight and payload in New Zealand.

How the greywater tank capacity also affects the payload

When considering your motorhome's total weight, you have to account for both the fresh water tank and the grey water tank, which collects the used water from your kitchen, shower, and sinks. In many European motorhomes, the grey water tank is actually slightly smaller than the freshwater tank. This is because manufacturers assume not all fresh water ends up in the grey tank (for example, the water you drink or use for cooking).

This leads to an important tip: always empty your grey water tank when you get the chance, even if you don't need to top up with fresh water. It’s just dead weight you're carrying around.

This is especially true if you're freedom camping. You might find a way to add more fresh water (like from a portable container), but have no dump station nearby. In this situation, you would be carrying the new fresh water plus all the old grey water, making your motorhome significantly heavier.

Why do premium German motorhomes have medium-sized water tanks? 

There's a smart reason why many modern motorhomes feature smaller, more practical water tanks. This design choice offers several key benefits, including improved vehicle handling and safety, enhanced fuel efficiency, and for manufacturers to apply smarter interior design.

Let’s look at each in more detail.

Improved vehicle stability

When full, a water tank is one of the heaviest items to be carried inside a motorhome. So how does this lump of liquid mass affect stability? Let’s start with towed recreational vehicles. 

There’s a valid reason why many caravans are fitted with detachable, portable water tanks that are emptied before travel and refilled during camp setup. This removes the pendulum effect that a partially full water tank can have on the stability of the caravan when it’s under tow. The lateral G-force created when turning corners sloshes the water towards the outside of the caravan in a turn. When the cornering phase is over, the water sloshes back again.

Motorhomes are less vulnerable to the effects of this liquid mass transfer because they’re heavier than caravans and are moving under their own power. However, it’s still something that needs to be considered. This is why premium brands within the Erwin Hymer Group, such as Bürstner and Carado, choose to fit tanks sized to supply a useful amount of water while limiting any degradation of vehicle dynamics. 

Cornering transfers considerable weight to the tyres travelling on the outside arc of the bend, and the centre of mass quickly returns to the middle of the vehicle once the road straightens up again. Carrying twice as much water as you need adds significant weight, which increases the lateral (side-to-side) forces your motorhome experiences when cornering and can negatively affect handling.

A 200L water tank could have 180kg of liquid moving from one side of the motorhome in the direction of the other and back again when travelling down a winding road. A 100L water tank filled to a similar level will have 80kg. A heavy or unbalanced motorhome is significantly less stable on a winding road, especially when a strong crosswind gust pushes in the same direction as the cornering force. This combination dangerously increases the risk of losing control.

Improved vehicle safety

Carrying less water gives you a real advantage when it comes to staying safe on the road. Weight directly affects vehicle safety, so it's worth considering whether you really need an extra-large tank over a smaller one, as the weight savings make a real difference when driving.

This is where vehicle safety comes into play, which is categorised into two types. 

Active safety helps you avoid an accident in the first place, while Passive safety is about protecting you during one.

When it comes to active safety, the weight of your motorhome is critical. A lighter vehicle can simply react better; it can change direction faster, accelerate out of trouble, and stop in a shorter distance.

This is a key reason many German motorhome manufacturers install more compact, medium-sized water tanks. Their designs are rigorously checked by the Technischer Überwachungsverein, or Technical Inspection Association (TÜV), Germany’s transport authority. To gain design approval and be authorised for the road, new models must prove their engineering is sound, including their ability to avoid crashes. That extra margin of active safety provided by saving weight is a crucial part of this process.

Recently, the European insurance company Baloise commissioned the Dynamic Test Centre AG in Basel, Switzerland, to conduct an emergency double lane change test with three motorhomes loaded differently. The aim of the exercise was to inform motorhome owners that care needs to be taken to ensure minimal vehicle loading and balanced added mass.

The test found that the motorhomes carrying the heaviest and most unbalanced loads were less likely to stay within their lanes during the emergency manoeuvre. “In the worst case, they might even tip over,” says Daniel Junker, head of vehicle expertise at Baloise.

Freedom of design

Motorhome designers like simple, uncluttered layouts. Using more compact and practical components gives them greater design freedom. For example, a smaller water tank is much easier to fit safely into the layout. This improves the motorhome's balance on the road and frees up valuable storage space for other necessities.

Enhanced fuel efficiency

As any truck driver or electric vehicle owner already knows, the heavier the load, the more energy is required to shift it. This indisputable law also applies to water storage.

Choosing a motorhome with a water tank half the size of a rival RV’s will not halve fuel use, but it does create a small improvement that will add up to quite a few fewer litres used over time. There’s an old campfire joke amongst motorhomers that Scottish folk empty all their water storage before travel. 

How much water tank storage do you really need?

This is a common question, and the answer is that you often need less water tank storage than you think. For most motorhome trips, especially when staying at holiday parks, a standard 100-120L tank is more than enough. You'll have easy access to fresh water, allowing you to refill your fresh water as often as needed.

But what about freedom camping? This is where many people assume a massive 200L tank is essential, but in reality, your water tank is rarely the limiting factor. The real limit on how long you can stay "off-grid" is almost always your toilet cassette. 

Here’s a simple breakdown:

1. Toilet fills first: A standard motorhome toilet cassette holds about 20L of waste. For a couple, this will typically fill up in just 2-3 days.
   
   

2. You must move: Once the cassette is full, you are required to drive to a designated dump station to empty it. 
   
   

3. Water is still left: In those same 2-3 days, a couple will use about 80-100L of fresh water. This means your 100-120L tank still has water in it when you're forced to move anyway.

The key to successful freedom camping isn't just about having the biggest water tank; it's about managing all your resources. This is especially true for remote spots where you'll be relying mainly on your onboard toilet. Since it's usually the toilet cassette that fills up long before your water runs out, arriving at your site with a full 100-120L tank provides the perfect balance of capacity. 

Even if you carried a second toilet cassette, you would still need to find a dump station to empty both long before that 120L tank runs dry. This shows that managing your resources efficiently is just as important as your tank size for a great adventure.

Learn more about compliant freedom camping in our RV buyers guide to self-contained vehicles. 

What’s the minimum water tank size required for full self-containment certification? 

The number of litres is determined by the number of berths inside the motorhome. The greater the sleeping accommodation, the larger the water tank capacity needs to be.

To qualify for Self-Containment Certification (SCC), a motorhome must carry enough freshwater for each occupant to use 4L per day for four days. The minimum tank size for a four-berth motorhome is therefore 48L, for a six-berth it will be 60L, and for a two-berth it’ll be 24L. Greywater catchment must also have the capacity to store 4L per occupant per day for four days. 

Learn more about self-contained motorhomes in this guide.

If you’re buying a motorhome to travel as a couple, a 120L freshwater tank will allow 15 nights of freedom camping at one spot while staying within SCC boundaries. However, 4L per day per occupant is quite high usage, especially if you’re aware of the many ways to conserve water consumption.    

Here are some tips on conserving your water supply:

• Shower to just get clean as soon as possible.
  
  

• Keep the flow from taps and the shower rose low. Most premium German motorhomes come with low-flow taps and showers as standard.
  
  

• See any public water taps as golden opportunities to do low-impact things, such as rinsing swimwear and tramping boots.
  
  

• Only fill the kettle as needed.
  
  

• Stay at a commercial campground when you need to do the laundry

Make the smartest choice with a smaller water tank size

When you're looking at motorhomes, it's easy to get caught up in the numbers and assume bigger is always better. However, when it comes to water tank sizes, that's not necessarily the case.

A water tank less than 200L and at least 100L in size is the smartest design choice. It offers a useful and convenient supply while minimising the impact that larger tanks have on vehicle dynamics, payload, design, and fuel efficiency. 

If you wish to know more about the range of intelligently designed German motorhomes sold by Wilderness Motorhome Sales, speak to our sales team to learn more about finding the perfect vehicle balance for your needs.