In January 1999, the Colorado Public Utility Commission (PUC) adopted regulations requiring the state's utilities to disclose information regarding their fuel mix to retail customers. Utilities are required to provide this information as a bill insert or as a separate mailing twice annually, in April and October of every year. The PUC provides a suggested format for the disclosure. Fuel mix percentages are to be based on the power supply mix for the previous calendar year. Supporting documentation concerning the calculations used to determine the power supply mix percentages must be submitted to the PUC for approval.