A wireless charging system has a transmitter and a receiver. The transmitter has (i) a TX coil that wirelessly transfers power to the receiver and (ii) TX circuitry that powers the TX coil and detects receiver removal by comparing TX input power and TX power loss. The TX circuitry can determine (1) TX input power as the product of sampled TX input current and voltage and (2) TX power loss using a mapping based on sampled TX coil current. When the receiver is present, the difference between TX input power and TX power loss has a first value and when the receiver is removed, the difference has a second, lesser value. The transmitter detects removal of the receiver by determining when the difference decreases below a specified threshold level. By frequently generating and analyzing the difference, the transmitter can quickly detect receiver removal and safely power down the TX coil.