With the Gutter System installed on your water tank, you can harvest almost all of the rainfall from the roof of your tank. Your tank will fill by the amount of rain that your receive. i.e. if you have 20mm of rain, the tank will fill by 20mm.
Using an example of a 100,000 litre tank (7.8m diameter), in a rainfall area of 750mm per year, you will collect up to an extra 35,000 litres each year. This can equate to three tank loads of water per year. The alternative is to buy this water commercially, with a potential cost in excess of approximately $1,500 per year (depending on the area where you live). This would more than cover the one-off cost of purchasing the gutter system itself.