[IanW]

Dont assume that Economy 7 automatically saves you money. There will be a break even percentage of night use that you need to achieve. Often this needs to be more than 20 percent of your total use.

[theunknowntech]

I'd be tempted to agree with this, even as someone who has never needed Economy 7. I think a more realistic figure would be roughly 34% of your usage falling within E7 Off-Peak hours, but it depends on your specific circumstances. If you use very little energy overnight and/or have gas heating for example, Economy 7 probably isn't appropriate. In actual fact, one of the few use cases where it will help you is for things like Storage Heaters (which was the original purpose), which need several hours to build up enough charge to run until the next cycle.