I'm about to take delivery of 2x 25,000L tanks which will be installed on a hill with about 30m of vertical head between them, about 60-80m away. I will have a shed about 5m above the lower tank, collecting rainwater to the lower tank, and will pump up to the top tank to supply the property with decent pressure via gravity.

The property is off grid, so the transfer to the top tank will happen by use of a petrol driven pump until there is a house feeding the top tank in a few years.

This post is mostly about sizing the polythene pipe. I'll buy about 100m of it and the price of the pipe doesn't vary a great deal between 32,40 or 50mm so my initial thought was to go for 50mm, but when I price out the various valves and fittings, there is an exponential impact on the price for fittings depending on the size so I don't want to go for significantly larger than I need, and I'm thinking a 50mm system is overkill for the price.

A good honda powered pump will handle the head at either 2" or 1.5" outlets, so I'll set up the system with some Tees, Ballcocks and camlocks (to connect the transfer pump when necessary). 

A couple of retailers are happy to sell me whatever I want, without much advice as such, so, what would a 'typical' pipe size be for a system like this, without greatly affecting transfer flow up much or supply flow & pressure down?  I'll buy 100m roll to do as much as I can, and reduce it down for supply at the shed. 

Thanks.