I was talking to a dear friend the other day about my hopes and dare I use the term, plans, for getting our house running entirely off rainwater and my dreams for 40,000L-50,000L of water tank capacity. She brought up a very valid point, one I had not yet considered.
Do you have enough roof to collect that much water and do you get enough rain?
Good point. 😉
She put her husband on the phone, a man who never ceases to astound me with his ability to take a glance at an idea and then see the bigger picture. I see rain water tanks connected to down pipes and he sees roof space, location of down pipes, gravity feed, pumps, capacity and local rainfall. He’s pretty amazing really. 🙂 He gave me the mathematical formula to figure out whether or not we can fill 50,000L and explained what it really means too. Thank you! 😀
Now before you flinch or run away from the word “mathematics” do not worry. This is a very easy formula, particularly for those with lovely square (or rectangular) houses. 🙂 To work out if you have sufficient roof space to collect enough water to fill a tank you need to work out the square meterage of your roof space and then multiply it by your annual average rainfall. The thing is you’re not measuring how much roof you have but how much horizontal space it takes up. 🙂 The easiest way to do this is to measure your floor space of your house and any eaves too.
The formula is this:
Length of the house x Width of the house x annual rainfall = potential Capacity in Litres
L x W x AR = C
Jasper and I grabbed the biggest tape measure we own (50m thank goodness) and headed around the house to measure up. Our house is 9.6m wide excluding the small bay window area which was too tricky to work the maths for (which would be a few square metres at most) and 15m long. Our annual rainfall was a little harder to find for accuracy. The Ballan (Fiskville) figures run from 1928 until 1969 which isn’t all that recent these days (44 years of data missing) and otherwise there is Bacchus Marsh which is a lot closer to sea level than we are, or Ballarat which is quite a way away. I decided to base my figures on the old data of Ballan. 🙂
So, our formula works out to be:
9.6 x 15 x 571.2 = 82252.8L
Yes, we have enough roof to collect 50,000L. 😀