If there is a ban then you might not want to use any stored water for cleaning windows you might get lynched. Let assume the worst and water is rationed, not enough to have a bath or shower, bottled water for your kettle and obviously none for the gardens etc and then the window cleaner turns up and chucks out 50 litres or so to clean the windows...I dont think that would go down to well....regardless of whether the water is stored or not, besides if you did have a large amount stored and if there was a serious shortage it would be worth more money to someone than the price of cleaning a 3 bed semi!