The power transmitter (101) providing power to a power receiver (105) comprises a communicator (309) communicating with the power receiver (105) and a negotiator (305) negotiating a guaranteed power level with the power receiver (105) prior to a power transfer phase. The guaranteed power level is a minimum power level guaranteed by the power transmitter (101) throughout the power transfer phase. During the power transfer phase, a determiner (307) dynamically determines an available power level based on the prevailing operating parameters. The available power level is one that can currently be provided but is not guaranteed. The power controller (309) is arranged to, during the power transfer phase, increase the power level above the guaranteed minimum level in response to power control messages, and to reduce the power level regardless of the power control messages in response to a detection that the power level exceeds the available power level.