What defines a Critical Care customer? A Critical Care customer is residential customer who has a person permanently residing in his or her home who has been diagnosed by a physician as being dependent upon an electric-powered medical device to sustain life. In order to be designated as a Critical Care residential customer, the commission approved application must be submitted to the TDU by a physician. Approval/denial of the customer's application is determined by the TDU and will notify the customer within 2 business days. A customer can be designated as Critical Care for up to two years.