Just thought this may be of use to someone. I have just received the six monthly water bill and it shows a useage of 65 cubic metres of water. This includes the water used by the household and the business (2 adults, 3 children). I am DI only and have a 400 ltr tank. I also top up the tank wherever I can on customer properties (with permission of course ) So that is 10.8 cubic metres a month on average and 2.7 cubic metres per week. The cost for six months is £360. That is £60 a month. So in short I am happy with this. Thats the household and providing water to my business. The point I'm making is that if you are DI only and you purify on demand you can top up the tank on the move and cut down your costs.