Best answer by Tim_OVO
Hope you found a tariff that you like,
Your first Direct Debit payment will tend to be soon after your supply start date. This will be confirmed in your Welcome email, sent 3-4 days after you click on that signup button!
You do have the chance to change this Direct Debit date though: https://forum.ovoenergy.com/billing-payments-35/changing-direct-debit-date-617