Installing a rain water tank serves two purposes. Firstly, you get a steady supply if reasonably pure water (unless you live in a heavily polluted locality) which you can use for cleaning, cooking, gardening and even drinking. Secondly, by installing a water tank, you are preserving a precious natural resource and contributing to the ecological cause. However you need to plan beforehand for purchasing a tank so that your tank size and type will suit your unique requirements. See Rainwater tanks.
Things to consider before buying a tank
Do you need the tank for gardening and toilet flushing apart from cleaning and cooking? If your answer is yes, you will need to buy a tank with a capacity larger than 4,500 liters. Find out how much space you have on your property to accommodate a tank and the roof drainage system of your home before the purchase. The kind of tank you get will depend upon the quality and type of roofing material used in your home/ apartment, your budget and the safety regulations of your local council.
Purchasing a water tank is not a standalone expense as you will need a pressure pump, a first flushing device and back flow prevention device. You will also need screens and gutter guards, a pump and a top up pipe. See Plumbing
The total volume of water collected will depend on the surface area of the catchment, tank size and rate of water consumption.
Though tank sizes are standardized, you will get a lot of options in the choice of material.