A method of controlling a printer includes measuring a first voltage value of a power supply of the printer before energizing heat generating elements of a print head of the printer. The method also includes calculating, by a controller of the printer, the number of heat generating elements that are simultaneously energizable based on the measured first voltage value of the power supply, an end voltage value of the power supply, an internal resistance value of the power supply, a resistance value of each of the heat generating elements, and a current value of an electric current flowing through a motor of the printer for conveying recording paper. The method further includes performing printing on the recording paper by the print head using the heat generating elements of the calculated number.