Economy 7, 8, 9, 10 and 18 are all tarriffs for supported meters that provide additional off-peak electrcity. Economy 7 offering 7 hours, economy 10 offering 10 hours etc.

Economy 7 (and to the best of my knowledge 8 and 9) all can be run from a single meter where the time switch is configured to swtich from off-peak to peak after , 8 or 9 hours duration. There may be higher unit charges for the peak rate electrcity used and even a standing charge for the meter on the non-standard tarriffs. (anything that's not single rate or economy 7), so a clear understanding of your consumption and analysis of yearly costs based on this consumption versus these tarriffs may be required.

Economy 10 and 18 will always have an additional meter used to record all energy used to charge the stored heat, whereas Economy 7-9 is equally supported using one meter or two.