How to size a water storage system

On a separate topic I discuss the concept of capturing and using rainwater for domestic purposes.
benefits-rainfall-collection
This system below is something I would not use or recommend because in my experience a single larger water tank is best.
BUT I appreciate that some folks need to build up storage volume. This company helps those and this presentation explains the concept simply
I hope its clear to you.

https://www.bluebarrelsystems.com/diy/size-your-system/

They only talk about how much you can get off your roof, for many of us with barns the limiting factor isn't how much water we can collect, just using the barn roof and not touching the house I can collect roughly 18,000 gallons per month. or 327 of their barrels. But how much water you can use. in a year we use 27,000 gallons. I would say that we would need a 4000 gallon tank to just run everything off of rainwater, That's allowing for higher usage in the summer than in the winter, and a dryer than average couple of months.

In Australia I would store as much as I can afford, because its so dry in summer.
I store 1/2 million Litres.

Skamdi, some of us just dont have the rainfall you have, but think of those where 10 inches is the total for 12 months.
Its about 16 inches at my place, per annum,

John C Daley wrote:In Australia I would store as much as I can afford, because its so dry in summer.
I store 1/2 million Litres.

Skamdi, some of us just dont have the rainfall you have, but think of those where 10 inches is the total for 12 months.
Its about 16 inches at my place, per annum,

We get about 32inches per year, of course we get very little evaporation and I have a huge barn, but my point was rather than sizing to the roof size to what you can/will use