The fact is that you can use rainwater for anything if the right system is installed. It is recommended to use water from the main supply for drinking and cooking for health reasons. The absence of fluoride in rainwater should also be taken into account in the case of children in the home. Rainwater may be able to withstand 100% of domestic use if the right system is installed.
While a rainwater collection tank can help you weather droughts, the amount of water they store is unlikely to last forever. An example of the amount of water that could be collected in Sydney in a year with a 100 m² roof would be more than 38,000 L of water. If you only use it for washing, toilets and watering the garden, it's obviously not important to maintain good water quality. Keep in mind that the amount of water used for drinking and cooking is relatively small, and that state and territory health departments recommend using the public supply of cross-linked water (if available) for drinking and cooking because it is filtered, disinfected, and generally fluoridated.
And for some, the cost of water is relatively cheap, so they may prefer to have more additional space for other purposes (not having a rainwater tank) and spend less time maintaining their home. The lifespan of a water tank that is properly and regularly maintained can be up to 20 years, which is a long time to accumulate cost savings. The best way to calculate this is that one square meter of roof catchment area multiplied by 1 mm of annual rainfall is one liter of water that you can trap in your tank. For some people, one of the most daunting aspects of buying a water tank in the short term can be the price.
I don't use it much just once in a while to water plants and it's not currently connected to my main water. Therefore, having a plumber to connect a rainwater tank is cost-effective if you have many more kiloliters of water available. This water can become contaminated as it moves above the ground and affects the ecosystem of water bodies. When it comes to what size of water tank to install, there's no one-size-fits-all answer, but realistically, you should install the largest water tank you can afford, that fits on your property and you expect to be able to fill it with your annual rainfall and roof catchment area.
When it doesn't rain for weeks, it's expensive to use tap water to keep vegetables and plants in the garden alive. If you're a business owner and water comes at a significant cost, especially for industrial companies, installing a rainwater tank could help protect you from these increases.