Hi @Daisiegirl , supply charges are to cover maintenance of the infrastructure that connects energy to your home, and they are flat rate separate from usage charges, which are based on the energy you actually use.
Estimated bills are only used when we haven't received an actual reading from your local energy distribution company. They are only temporary and the estimated charge is corrected when we next receive an actual bill. In other words, if the estimate was too low, you'll need to pay extra to cover your usage, and if the estimate was too high, you'll get money back as a credit.
For more info check out Estimated Energy Bills