I'm a sparky and ive been told that the Economy 7 tarriff times vary from 12-7, 12.30-7.30 and 1-8 but you will deffo get 7 hours a night, the best bet is to make sure your water heating or whatever you are using the Economy 7 for, switches on at say 2am til 7am then you know you will deffo be getting the lower rate, but your energy supplier should be able to tell you when the 7 hours are if not ask the local electricity supplier as the meter is owned by them and not the people you pay your money too....