benefits of rainfall collection

Another set of calculations
Rainfall is about 25mm per week across 12 months
So you have about 80 sq m of roof [ 780 sq ft ] x 10 weeks x 25mm
= 80 x 250mm
= 80 x .25M
Total volume captured over 10 weeks is
= 20 cubic M which is 20,000 Litres or about 5000 gals.


My source of data for water consumption is water consumption
Essentially from that data an efficient house would use 1000L per person in a very water efficient house per week.

GOING BACK TO THE QUESTION OF WHAT VOLUME OF WATER NEEDS TO BE STORED FOR A FAMILY OF 4 OVER 3 DRY MONTHS
From my data of Melbourne a water  efficient household uses 750- 1200L of water per person per week
water usage data

Lets allow 1000L per person per week over 13 dry weeks.
1000L x 4 people x 13 weeks gives a required stored water volume of 52,000 L

MY EXAMPLE EARLIER IS INCORRECT.
HAVING BEEN ASKED TO EXPLAIN, I WENT THROUGH MY NOTES AND REALISED I HAD EXCLUDED SHOWERING AND KITCHEN USE.
I AM WORKING AT NIGHT HERE, PERHAPS I RUSHED I AM SORRY.
BUT I CANNOT YET EDIT THE OLD POST.
SO 52,000L needs to be stored not 26,000L