How much water can rain collectors harvest?

Rain collectors can harvest a lot of water. This is determined by your annual rainfall, the size of your roof and the amount of rain that can be collected.

rain collectors

Before deciding to install a rainwater harvesting system, you might be interested to know how much rainwater you will be able to collect.

Rain collectors are becoming increasingly popular. They can harvest a lot of water.  Once captured and stored this water can then be used to provide your garden with water to flush your toilets.

Rain collectors formula for harvesting:

It is possible to calculate the amount of water rain collectors can harvest by using the following formula.

1” of rain on one square foot of a roof area produces 0.52imperial gallons of water


1mm of rain on 1 square metre of the roof area produces 1litre of water.

Factors that affect how much rain collectors can harvest:

Annual rainfall:

The amount of rainwater that you are able to collect would firstly depend on the annual rainfall within your local area.

As there will be different amounts of rainfall throughout the year you should check whether your area has sufficient rain in order for your system to operate effectively.

The adequate amount of water would be essential. This information can be found at www.climate.weatheroffice.  In cases when there is not sufficient rainwater collected, the systems would fail and the building would have to use an alternative source.


Size of roof:

Another factor to consider would be the size of your roof. The area that should be measured is the horizontal plane under the roof and you should also include any overhangs.

It has been proven that larger scale commercial buildings are more suited to rainwater harvesting systems as they have a larger catchment area.



Amount of rainfall that is collectable:

The third issue to consider in order to gain a better idea of how much rainwater you might be able to collect, would be the portion of rainfall that is actual collectable.

In order to work out the collecting efficiency of your roof, factors such as prevailing winds, tree coverage and type of roofing would need to be taken into consideration. Well-designed rain collectors would be able to capture roughly 75-85% of the total rainfall.


How much water would I need?

It is important to consider what is it that you want your system to do. Which parts of your domestic routine would be making use of the rainwater?

If your system would be providing water for not one but several systems such as flushing toilets, watering the garden and washing clothes, the property would require a sufficient amount of rainwater. This should be assessed when calculating the amount of water that you need.


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