A method of controlling an electrical instrument to which a user inputs a reservation time using a rotary dial includes, upon reception of power rates by time periods from a power provider, confirming power rate levels by time periods based on the received power rates, displaying a color window including the power rate levels by time periods around the rotary dial, and displaying the reservation time input by the user through the rotary dial on the clock window. Accordingly, it is possible to provide a user interface that displays power rate information by time periods such that the user intuitively recognizes the power rate information and sets a reservation time on the basis of the power rate information.