determining tank size and security of supply

these graphs estimate the size of tank you might need assuming a demand of 200L/day.
Step 1: measure the roof area in metres squared

step 2: measure how much water you use, a rough guide is 120 litres per person, per day in the city and 60 litres per person per day in country areas.

step 3: find average rainfall for your area

Step 4: decide how much security you want, do you want to rely on the tank 99 percent of the time or only 90 percent and use carted water if supplies run out. we would be assuming 99 percent of the time for Domus purposes

Step 5: calculate the size of your tank by locating your roof area on the x axis of the graph and follow it up until you find the average rainfall for your area. draw a line horizontally from this point and that point on the y axis will indicate the tank size to use so that you dont loose too much water through having an overflowing tank.

if the vertical line does not intersect with the average rainfall you may not have provided enough roof area to supply the tank with sufficient rainwater.

(taken directly from Sustainable House, by Michael Mobbs, p.50-51)