Rainwater harvesting.

[bwana]

I think that is the whole point of the OP's calculations! Cost per liter stored is only the starting point. Even if capacity is unlimited, roof space then becomes the limiting factor.

The most important parameter is how many times per year the tank can be filled and drained, in order to make cost/benefit estimates. In CT (outside of winter) the number of good rain events (20mm+) is vanishingly small, compared to other parts of SA.

Our problem is that even though we have plenty of roof area (280m2) the roof is segmented and the down pipes distributed all around the house so having one tank farm isn't going to happen. I've got about 110m2 of my roof servicing 12000l of tanks and it's plenty for us.

EDIT: that 12000l only services our washing machine at the moment but can provide water to the entire house at the flip of a valve. Unless I can find a meter for the tanks I can only guess how much rainwater we actually use but at 50l a load and 15+ loads a week it adds up.