Monthly Payments re Credit Balance

My monthly payments are being paid via Direct Debit, however I have a credit balance on my account. Why is it not deducting from the credit balance and is being taken from my bank account?

It’s doing both. Your monthly usage is deducted from your Bulb account. Your monthly payments are credited to your account.

You still pay the same amount monthly, even if your account is in credit, because your estimated annual usage is calculated and then spread out over 12 equal monthly payments. This prevents you having huge bills in the winter and small bills in the summer.