It's irrelevant what the 'savings' are, that's just marketing rubbish. All that matters is the cost going forward.
I just track all my meter readings on an excel so I can reconcile the billing, I have little trust in energy suppliers to get it right. Far too many horror stories.
I'm with Economy Energy, until they resolve my billing issue, they randomly just added £200 to my charges between bills.
4 months later they still haven't corrected it.