I wonder if they would have a grump on if we all averaged our DD out, so we owed then in winter, they owed us in summer? By rights that's exactly what should happen.
That is what should happen (and in my experience often does), you build up a credit over the summer and end up a bit in debt at the end of the winter, but it is difficult to always get the balance right as a mild winter gives lower bills and a harsh winter higher ones.