There are a few points to consider when choosing a water tank, not only the colour or the shape but also, for peace of mind, the quality and size. While you can never have too much water stored, you can certainly have not enough, especially to last through a dry season.
Start with two questions
To calculate the capacity you require, or the size and number of tanks, work through two simple questions. First: what will be my daily water consumption? Second: how long is the dry period with little rain for the area I live in? Those two figures together set the storage you need to carry you across the gap. Both are specific to you, which is exactly why there is no single right tank size and why a recommendation borrowed from a neighbour is worth very little. Our rainwater tank size calculator will help you work the household consumption figure through.
A worked example
A four person home with a 50 day dry period needs to have stored at least 32,500 litres of fresh water to last them through when there is no substantial rain to replenish their tanks. This means that with an adequate buffer the owner should be installing at least 2x 25,000 litre water tanks. Note what the buffer is doing there. The storage figure is the minimum needed to get through; the tanks chosen sit above it rather than exactly on it. Sizing to the bare number leaves nothing at all for a dry period that runs longer than expected. Tanks at that scale sit in our large water tanks range.
Add fire storage on top
If you are required to store water for fire purposes this will also need to be added. It is added to the domestic figure rather than absorbed into it, so the sequence runs: work out household storage first, then add the fire volume, then choose tanks that cover the total. Whether a fire requirement applies to your property is not something to assume in either direction. Our article on fire service kits covers the connection side of that requirement, which is a separate question from the volume itself.
How much rain your roof can collect
To calculate the amount of rain water your roof can collect, the formula is – 1mm of rain falling onto 1 sq. metre gives you 1 litre of rain water. That is the whole formula, and its usefulness lies in the fact that both halves of it are things you can actually find out. Roof area is measurable. Rainfall for your area is a matter of record. Put together they tell you what your catchment can deliver, which is the other half of the sizing question: storage tells you what you can hold, catchment tells you how quickly you can refill it. Both figures are worth having before you commit to a size.
Not all polyethylene tanks are the same
After finding the amount of storage you require you will need to choose the right tank. Not all polyethylene tanks are the same, but can be differentiated by design and manufacturing processes used; for example, a tank with vertical parting lines will split open easily. Wall thickness is important, especially in the base side wall area, which must withstand the greatest weight pressure of anywhere on the tank. The best manufacturing process is what is termed as a one-piece construction, which means the lid or dome is not added on separately. None of this shows up on a price list, which is precisely why it is worth asking about. We go further into it in which is the strongest water tank.
Standards, UV and the lid
Ensure the tank complies with New Zealand Potable (Drinking) Water and Food Contact Standards, is fully UV stabilised and has a suitable manhole entry and ventilated lid. Each of those covers a different risk. The standards cover what the material does to the water. UV stabilisation covers what years of sun do to a tank standing outdoors. A suitable manhole entry and ventilated lid are about what happens after installation: access, inspection and cleaning, which matter far more than people expect the first time a tank needs attention. All of them are easier to check before you buy than after.
Buried and hillside installations
If your water tank needs to be situated on a hillside or lowered into the ground due to height restriction and/or for aesthetic reasons, choose a corrugated style tank that remains fully warranted when buried, and especially so with no unsafe cavities left around the tank. The warranty wording is the point to press on. A tank being physically capable of going into the ground and a tank remaining warranted in the ground are two different claims, and the difference between them only surfaces when something has already gone wrong. Get it confirmed before the tank is delivered, not after the hole is dug.
Fire couplings and your warranty
When you are required to install a Fire Service Outlet Coupling Kit, ensure the tank wall is strong enough and also that the tank warranty will not be made void with a larger penetration made into the wall. This is the same warranty question wearing a different hat. A larger penetration asks more of the wall than a standard fitting does, so both points, strength and warranty cover, are worth confirming with the manufacturer before the kit is specified rather than after. It is not a modification you can undo, and discovering afterwards that it voided your cover is an expensive way to learn it.
Colour and wall style
Some manufacturers offer up to 14 different colours, smooth wall and corrugated wall styles, all made from polyethylene, which allows you to select the best colour to suit your surroundings. Surprisingly, the visual variation of the corrugation design really helps your water tank to fit into natural surroundings. This is the last decision rather than the first, which is the whole reason for working through the sums before the swatches. Once capacity, construction and installation are settled, colour and wall style are where you get to make the tank disappear into the site instead of standing out on it.