Water Saver

Rainwater harvesting means capturing and storing rain that falls on-site (usually on roofs). It is generally used for irrigation and toilet flushing or other greywater uses, though it can also be used for drinking water if it is adequately treated.
Capturing rainwater can be a valuable way to reduce or even eliminate a building's use of municipal potable water, without requiring reductions in water use by occupants. However, it is of course more effective in rainy climates than dry ones.

Rainwater harvesting systems are measured by their area for collecting water (in m2 or ft2) and the volume of water they store (in liters or gallons).

Simple rainwater collection systems have three main elements: the roof or other catchment area, the storage tank(s), and the gutter and other piping that directs the water from the catchment area to the tank.
Rainfall
Weather data from TMY files can be used to determine rainfall patterns. These will be in mm or inches of rain.
Be sure to calculate average monthly rainfall for the different months of the year, not simply an annual total. Most sites have much more rainfall in some seasons than others, and excess water can always be drained, but a lack of water requires municipal water use to compensate.
Efficiency
Different gutter systems, different roof pitches, and different materials can affect system efficiency. For example, lower-pitch roofs cause less loss than steeply-pitched roofs.!