I think the idea is the powder has to be put into Hot water to kill the bacteria rather than adding it to previously boiled cooled water. If I was pre making bottles I would imagine it's safer to add the powder then cool & store in fridge however I believe the guidelines are make each bottle up fresh. Perfect prep machine is what the pp is thinking of & although pricey I will be investing.
Oh right! I'll be also spending out on the prep machine, although thats not going to be any good when out and about for the day :/