A power transmitter for wirelessly providing power to a power receiver via an inductive power transfer signal; the power transmitter comprising: a communicator configured to communicate with the power receiver; a negotiator configured to negociate a guaranteed power level with the power receiver prior to a power transfer phase, the guaranteed power level being a minimum power level guaranteed by the power transmitter throughout the power transfer phase; the power transmitter characterized in further comprising: a determiner configured to, during the power transfer phase, dynamically determine a maximum acceptable available power level for the power transfer signal in response to an operating parameter of the power transmitter, the maximum acceptable available power level being a power level that can currently be provided by the power transmitter but which is not guaranteed for the power transfer phase; a power controller configured to control a power level of the power transfer signal during the power transfer phase in response to power control messages received from the power receiver; wherein the power controller is arranged to, during the power transfer phase, increase the power level above the guaranteed minimum level in response to the 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 maximum acceptable available power level. a communicator configured to communicate with the power receiver; a negotiator configured to negociate a guaranteed power level with the power receiver prior to a power transfer phase, the guaranteed power level being a minimum power level guaranteed by the power transmitter throughout the power transfer phase; the power transmitter characterized in further comprising: a determiner configured to, during the power transfer phase, dynamically determine a maximum acceptable available power level for the power transfer signal in response to an operating parameter of the power transmitter, the maximum acceptable available power level being a power level that can currently be provided by the power transmitter but which is not guaranteed for the power transfer phase; a power controller configured to control a power level of the power transfer signal during the power transfer phase in response to power control messages received from the power receiver; a determiner configured to, during the power transfer phase, dynamically determine a maximum acceptable available power level for the power transfer signal in response to an operating parameter of the power transmitter, the maximum acceptable available power level being a power level that can currently be provided by the power transmitter but which is not guaranteed for the power transfer phase; a power controller configured to control a power level of the power transfer signal during the power transfer phase in response to power control messages received from the power receiver; wherein the power controller is arranged to, during the power transfer phase, increase the power level above the guaranteed minimum level in response to the 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 maximum acceptable available power level.