A top-up device is a mechanism used in rainwater harvesting systems to maintain a minimum water level in a storage tank. It works by automatically refilling the tank with water from an external source, such as a municipal water supply, when the water level falls below a certain point. This ensures that there is always a sufficient amount of water available for use, even during periods of low rainfall.

A top-up device can be especially useful in areas where water shortages are common, or for systems that are used to meet a large portion of a building’s water needs. By maintaining a consistent water level, a top-up device helps to ensure that a rainwater harvesting system is able to provide a reliable source of water for various applications.