When you pay by by monthly direct debit, it normally means that your monthly payments are fixed across winter and summer, so you pay the same amount based on your estimated annual consumption.
Less commonly some providers also allow you to pay by a monthly variable direct debit. This means you pay for the energy you use each month. So in winter your bills will be higher, with reduced payments over the summer period.
British Gas' All Online January 2019 is an example of a tariff where the only payment method is through a monthly variable direct debit.