I am with welsh water so charges may be different to where you are i have a meter for the water running to the house then i have an outside tap with a meter attached which is used for my window cleaning and because this water from this tap doesnt go down any of their drainage systems i dont pay sewage charges from this tap which is the biggest charge on any water bill, from the outside tap they charge me £1.25 a cube which is 1,000 litres. Hope this helps.