What Size Water Tank Do You Need? A Guide for Kiwi Homes

Installing a water tank is one of the most effective ways to reduce reliance on mains water, save on water bills, and ensure a steady water supply year-round. Whether you need a tank for garden irrigation, household use, or full off-grid living, choosing the right size is essential.

Many homeowners are unsure what size water tank they need, but with a few simple calculations, it’s easy to find the perfect option. This guide will help you determine the right water tank size based on household needs, rainfall, and property space, so you can make an informed choice.

Why Water Tank Size Matters

Selecting the right size ensures you:

  • Have enough stored water to meet daily household or garden needs.

  • Maximise rainwater collection based on local rainfall levels.

  • Avoid unnecessary overspending on a tank that’s too large for your usage.

  • Comply with space limitations while still getting the best efficiency.

A well-sized water tank ensures efficient storage and reliable access to water, whether you’re supplementing mains water or going fully rain-reliant.

How to Calculate the Right Water Tank Size

The best way to determine what size water tank you need is to consider:

  1. Your water usage requirements

  2. Average rainfall in your region

  3. Available space on your property

1.Estimating Household Water Use for Small-Scale Storage

For those looking to store water for smaller household needs, a compact water tank can be a practical solution. While a typical New Zealand household uses 140-180 litres per person per day, not all water needs to come from the main supply. A 100L water tank is ideal for capturing rainwater for garden irrigation, washing hands, flushing toilets, or providing an emergency backup supply. It’s a great option for apartments, townhouses, and small homes where space is limited but water efficiency is a priority.

2. Factoring in Rainfall Levels

New Zealand has varying rainfall levels depending on location. To maximise water collection, consider how much rain your area receives annually.

  • High rainfall areas (Auckland, Wellington, West Coast): A medium to large tank can store significant rainfall, reducing the need for mains water.

  • Lower rainfall areas (Canterbury, Otago, Hawke’s Bay): A larger tank is ideal to ensure water availability through drier months.

You can calculate potential collection using this formula:

Roof Area (m²) x Annual Rainfall (mm) x 0.9 (runoff factor) = Total Litres Collected

For example, a 100m² roof in an area with 1,200mm annual rainfall can collect:
100 x 1,200 x 0.9 = 108,000 litres per year

This helps determine whether a small, medium, or large water tank is the best fit for your needs.

3. Choosing the Right Tank for Your Property

The size of your tank will also depend on available space. Larger tanks are best for rural properties, while urban homes may need slimline or underground tanks to maximise space.

  • Slimline tanks – Ideal for urban homes with limited outdoor space.

  • Round tanks – Best for larger properties with enough room for installation.

  • Underground tanks – A space-saving option that allows for larger storage without taking up visible space.

Why High-Density Polyethylene (HDPE) Water Tanks Are the Best Choice

When choosing a water tank, durability, safety, and efficiency are key factors. High-density polyethylene (HDPE) tanks provide a strong, lightweight, and long-lasting solution, making them perfect for residential and urban water storage needs.

Benefits of HDPE Water Tanks

1. Strong and Long-Lasting

HDPE tanks are highly durable and impact-resistant, ensuring they can withstand everyday use without cracking or corroding. Unlike metal tanks, they will never rust, making them ideal for long-term water storage.

2. Lightweight and Easy to Install

Unlike concrete or steel tanks, HDPE tanks are lightweight and easy to handle, allowing for quick installation without heavy equipment. Their compact size makes them ideal for small spaces, patios, and balconies.

3. Safe for Water Storage

Made from food-grade polyethylene, these tanks are BPA-free and UV-resistant, ensuring safe and clean water storage for drinking, gardening, or general household use.

4. Low Maintenance and Easy to Clean

HDPE tanks require minimal upkeep. Unlike metal tanks that may corrode or develop leaks, these tanks remain intact with no risk of rust contamination.

Choosing the Right Water Tank Size

Your ideal tank size depends on how you plan to use it. Here’s a quick guide to help you choose:

  • 100L Water Tank – Ideal for small outdoor spaces, watering plants, or as a backup water supply.

  • 200L Water Tank – A good option for home gardens, washing cars, or light household water use.

  • 210L Water Tank – Great for compact urban homes needing extra water storage for everyday tasks.

  • 300L Water Tank – Best for households wanting a larger water reserve while maintaining a space-efficient design.

Whether you need a compact tank for urban water storage or a larger tank for garden irrigation, HDPE water tanks provide a durable, easy-to-maintain solution for long-term use.

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