I have a problem on a job that I am working on. I am installing a rainwater harvesting system and want to estimate the cost of pumping water up from the main 3000l tank up to the holding tank in the loft which is about 6 metres above ground. What I am interested in working out is the pumping cost of filling the wc cistern using free water from the tank compared to just paying the water company to fill it directly instead. I am also aware that the initial jolt in the pump to get the water moving can require several times more power than is required to keep the water flowing. I am guessing that it is a penny or so per loo flush which makes about fifty pounds a year. Does anyone have any ideas how to work this`out?

This was covered by another thread ... somewhere. The gist of it was that it costs peanuts even if you have as I do a pretty inefficient system - 1.5% - giving me a cost of about 17p per 1000litres up 7m. Say 8 litres per generous flush means 125 flushes for 17p.
A more reasonable efficiency would be 5%. Higher is possible but hardly worth the effort.

The main thing is to choose a pump with a bit more max. head than your 6m, say 8 or 9m. You don't need a high a flow - you wouldn't want more than the overflow could cope with if anything went wrong - and it's likely that most of the centrifugal pumps around giving this sort of head will be working at the lower end of their quoted flow rate. 200w - 300w should be plenty, and deliver something like 10 or 15 litres a minute.