A method of calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter module of a wireless communication unit. The method includes, within at least one signal processing module of the wireless communication unit, determining combinations of the power amplifier supply voltage and power amplifier input power that provide a power amplifier output power equal to a target output power, obtaining battery current indications for the determined combinations of the power amplifier supply voltage and power amplifier input power, selecting a combination of the power amplifier supply voltage and power amplifier input power that provide a power amplifier output power equal to a target output power based at least partly on the obtained battery current indications therefore, and calibrating the envelope tracking system using the selected combination of the power amplifier supply voltage and power amplifier input power.