To me it doesn't work on a monthly basis though. If a supplier wants to alter my DD based on usage then fine set it up so I pay for what I use and actually promote that's how the DD will work, I've no issue with that and have available funds to do so. Hell I've the funds to pay for my years usage up front if they wanted it. The whole point of monthly payments to me is to take a years worth of usage, divide by 12 and pay that each month. I've provided evidence of what I use in a year and offered to pay a slightly higher rate than that even. Why do you need to be in debit? that just seems crazy to me. I'd much prefer my money to be in my account making me interest rather than my suppliers account.
By regulation the supplier cannot hold on to more money than is required to clear your account after 6 months. Any more than that and they have to give it back to you.
This was brought in back in 2016 to stop the public loosing out if a company.went under (this happened several times in a couple of years before 2016).
Energy firms were floating the available cash on th stock market and making huge profits on money that did not belong to them. Eon, Npower and SSE were all fined very heavily for this irrisponsible practice.