The Rain Backup in a Box automatically provides mains water to supplement a rainwater harvesting system when the rainwater runs low. It uses a float switch or level sensor to detect when the tank's water level is critically low, at which point it opens a solenoid valve to allow a measured amount of mains water into the tank, ensuring appliances are never without water. The system is simple to install and use, and replaces multiple components with a single easy-to-use unit.