When is the Power Move credit added to the bill?

If successful when is the power move credit added to  (subtracted from)  the bill? I was invited to sign up part way through August and got notification that I had achieved the target but should my (pro rata) power move credit come off August's bill or is it off September’s bill? (I did phone Ovo to clarify but sadly the very nice man I spoke to was not aware of power move at all ….  @Tim_OVO as power move has been around for nearly a year it did surprise me that he was unaware.

My apologies if this has been asked and answered elsewhere - just point me in the right direction - thank you.