How to choose a water tank

The cost of installing a rainwater tank can vary depending on the type of tank, installation and other requirements (like if you need a building permit). Whether you are building a new home or retrofitting a rainwater harvesting system, doing a little research can help you make the right choice for you. Here at Tankful.nz, we’ve done the hard yards for you.

Consider where will you install your rainwater tank In an ideal world, your site would be...

• Stable level ground that requires no excavation
• Next to downpipes that feed from major gutters e.g. at the end of the building (check with your look council for guidelines)
• Within a few metres of the dwelling that you are catching water from
• With a large enough area with at least 30cm additional space around all sides of the tank installation site
• Within easy access for delivery truck to get to from the road or driveway
• In a place that is aesthetically pleasing for you

If you are not sure if your site is suitable then give your local tank merchant a shout and see what they think. They might need to come and check your site.

Before you get too far ahead of yourself you’ll also need to consider the following:

• Any council requirements/permits access for emergency
• It’s likely you’ll need to consult with a structural engineer and your local council if you are looking to install a below ground tank
• Preparing the foundations for the weight of the tank when it is full (water is heavy); options are concrete slab or crusher dust
• Overflow needs to be contained on your property or diverted to the stormwater system (if overflow from your tank damages your property and neighbouring properties, you could be held liable).

What size water tank will you need?

Choosing the right size tank for your needs is often a decision based on what fits on your property versus how much water you need.

If your rainwater tank is your primary or single source of water then the general rule of thumb to get through the driest periods in your region is to get the biggest tank you can. This is especially the case if you live in an area that has unpredictable rainfall. Niwa predicts that the number of drought days we will experience will double in most regions in New Zealand over the next 20 or so years. So if you plan to stay where you are for a while, then the bigger the better might be the best solution to invest in future proofing your supply.

To work out how much water you need to ‘catch’ and store:

Work out the amount your catchment area can harvest by multiplying the roof area by the annual rainfall in your area...

• You can estimate the size of your roof by calculating the length x wide the of your house if it is rectangle or square, otherwise break your house into rectangles and measure each area separately, then add the totals together
• If you already know the footprint of your house eg. 170m2 then you can use this as a guide - if your house is single story and has eaves then your roof catchment area is usually greater than this.
• For average rainfall in your area
  • Go here for New Zealand rainfall data or here for Australia rainfall data (Search monthly rainfall for your area then use the annual total in the far right hand column - you may need to scroll down to get the most recent data)
• For every 1mm of rain, estimate 1 litre of water per square metre

Here’s the equation:

1. Annual rainfall for your area A mm = A litres per square metre of catchment
2. A litres per square metre of catchment multiplied by the catchment area B (roof area) = C litres of harvestable water

Example:

1. Annual rainfall for Auckland (A) = 1212mm = 1212 litres per square metre of catchment
2. 1212 litres (A) x 170m2 (B) roof area = 206,040 litres of harvestable water (C)

Now work out how much water you need e.g. garden use vs sole source of water

• You may be able to estimate garden water use in drier months by checking your water bill (if you are on town supply) and subtracting the wetter average monthly volume/usage from the drier months
• An average NZ family (2 adults, 2 children) use 575 litres per day (minimum of 375 litres - maximum of 800 litres per day which varies considerably due to water efficient appliances)
• An average 2 person household use 397 litres per day (minimum of 275 litres - maximum of 569 litres per day)
• For specific amounts, use a water usage calculator like this one (we've looked at heaps and think this is the best out there)

Example:

Family of 4 = 575 litres per day x 365 days = 295,875 litres per year If your catchment area x average rainfall doesn’t meet this requirement you will need to have a secondary source of water (town supply or purchase bulk water deliveries) or reduce your water usage, which can be done (see case study below).

Case study:

We have 2 adults and 2 kids living in our 188m2 house and one 25,000 tank in Mangawhai. With a total rainfall of 1025mm in the last 12 months we have easily survived without ordering further water (we don’t have town supply here). That would mean our water usage is much less than an average NZ family household however I don’t feel as though we try very hard. Yes we don’t leave the tap on when we brush our teeth like we did when we’re on town supply in Auckland but we still have an average 8 minute shower, an older top-loading washing machine (the most water sucking appliance in a house) and the kids have daily baths etc. We do need to be conscious of our water use in the drier months and had to top up with a 6,000 litre delivery in January 2017 when we had weeks without rain, but for $180 cost of delivery it’s still much less than what we were paying Watercare in Auckland.

What sizes of rainwater tanks are there?

Rainwater tank sizes

The largest manufactures of plastic rainwater tanks in New Zealand generally supply them in the following sizes: 1,000L, 2,000L, 3,000L/3,500L, 4,000L, 5,000L 9,000L, 10,000L, 13,500L, 15,000L 25,000L, 30,000L

The largest manufacturers of concrete rainwater tanks generally supply them in the following sizes: 1,100L, 1,800L, 5,000L, 6,300L 9,000L, 12,500L 22,500, 25,000L

Slimline water tanks: 1,000L, 2,000L, 3,000L, 4,000L, 5,000L

Choose the type of water tank that fits your needs

Each tank manufacturer will be working hard to convince you the material they use is the best. Whether its a concrete water tanks versus plastic tanks, each have their pro’s and cons. Plastic water tanks have become more popular over recent years however Kiwi’s and Aussies still love a good concrete tank. It often comes down to personal preference, price vs position - where will it go, does it need to be recessed or below ground or does it need to be easy to delivery and install.

Here’s a brief list of the good stuff and not so good stuff with the most popular materials in rainwater harvesting.

Concrete rainwater tanks

Good stuff

• Concrete water tanks are strong, resilient and long lasting
• Can be installed underground or partly submerged
• Are designed to keep your water cooler
• Keep the water darker which reduces algae growth
• More reliable and convenient water storage tank in instances of fire (although we hope you never have to count on this benefit)
• Good for high wind areas as they don’t require additional restraints
• No metal = no rust

Not so good stuff

• Concrete tanks are heavy which means they are harder to transport (they need to be delivered by a crane truck) and install
• Access to install has more requirements like no overhead wires for crane to navigate and access ground from road to install site must be level
• More expensive to build
• New tanks have been reported to leak lime into water although some say they can be washed out before use and others say this actually helps to reduce theacidity of rainwater
• Can be prone to cracks and leakage although they are relatively easy to repair

Polyethylene or Plastic rainwater tanks

Good stuff

• Plastic tanks are the cheapest water tank available (except maybe metal)
• Most popular tank in USA, Australia and New Zealand
• Lighter which means they are easier to transport to install
• A variety of different shapes, sizes and colours
• Manufactured with no seams that can crack, erode or rust over time
• Some corrugated plastic tanks can be up to 1 metre in the ground (check with the manufacturer)

Not so good stuff

• Most plastic tanks can not be buried, even partially
• Whilst plastic tanks need to be made food grade and fit for human consumption some query the safety of using plastic softeners such as Bisphenol A which is a well known endocrine disrupter that may lead to health hazards. Whilst this is not always used in the manufacturing of plastic rainwater tanks, some manufacturers are unclear whilst others clearly state their products are BPA-free. If you are concerned if BPA is in a plastic rainwater tank you are looking to purchase, contact the manufacturer to gain further clarity.

Metal or Steel rainwater tanks

Good stuff

• Steel tanks can be made to hold large capacities of water making them an option when an extremely large storage solution is required
• Strong and lightweight
• Attractive shapes and colour selection from slimline, round and square

Not so good stuff*

• Whilst most steel tanks are made with a zinc coating that protects the steel from corrosion, it will still rust and corrode over time
• Zinc can leak into the water causing a metallic taste, a food grade coating can be applied however this can be costly.

*Stainless steel tanks mitigate these issues, however they are more costly

Fibreglass rainwater tanks

Good stuff

• Fibreglass tanks are robust
• Lighter to transport
• Available in a variety of different shapes and sizes (although these are not always easy to find in some regions)
• Can be used above or below ground
• They won’t rust

Not so good stuff

• They are rigid and lightweight but can be more prone to cracking
• If not installed below ground they can allow more light to enter the tank which you want to avoid to reduce the chances of algae growing but they alsoneed to be made with a food-grade coating on the inside for safety standards and this can reduce the light entering
• Quality fibreglass can be rather expensive
• There’s not a lot of availability in NZ

Treating your rainwater

Some governments and councils will only allow you to use rainwater for drinking, cooking and bathing purposes if it’s been filtered or treated. Check with your governing body for regulations.

Legal bits

In New Zealand you will need a building consent if you are connecting a rainwater system to the plumbing of a house that also has a mains supply. The rainwater system needs to be separate from the mains water by law which can be done by using a back flow prevention device installed by a qualified plumber. Even if you want to go completely off the grid, most councils will still require you to connect to the mains supply if it is available.

Click here for a full list of New Zealand councils website links and contact details. It’s a bit of a mine field navigating their sites to understand what legal requirements you need to adhere to so it might pay to pick up the phone and let them guide you in the right direction.

Article fine print

This is an independent article which means no manufacturer has paid to be featured or has influenced us writing about them in any way. Yay!