My "header tank" as you call it is 45000 litres of which by law a large percentage is to be dedicated to fire fighting. The remainder, approx 20000 litres is the required amount to cover us through a dry year, of which we have had numerous over a 30 year period. A "dry" year off grid may not necessarily be low rainfall, but rain at the wrong time of year to cover the drier spells. ie if you plan on getting rain every week to use over the next week, then it aint long before you go thirsty. Our very low rainfall season can be four months. Two of those months can be at or close to zero. We have easily five months coverage in storage, this is at only 133 litres per day. Our normally usage without compromises is approx 250litres per day. In practice we get the majority of our rain around our 4 winter months so we need to have a total approx 15000litres at the harvest points to cover the big rainfall events. We have approx 300square metres of roof area available for harvest. We can get 100mm in one day ie 30000 litres which we need to harvest and we may need to pump that up within the next 24 hrs before it rains again. If we dont harvest it at the time we might well be short several months down the track. My 100 year old pump driven by a petrol engine uses a bit under 500ml of fuel to pump 2500 litres of water to a head of 28 metres in 1 hour. Just to save you some time that is 22 litres of fuel over a 12 month period. Obviously there is no sunshine to run a solar powered pump during those cloudy wet periods, which at times have extended for weeks. Having said all that, if you can build me a solar setup that can reliably pump my water during sunshine and cloud over the yearly conditions supplied above, for less than 21 per year, then I am all ears. If you don't think about water in a yearly cycle when you are off grid, then you are on a hiding to nowhere.
