Devices, systems, and techniques are described to detect when a power transmitting and receiving system is in an inefficient position, which may cause a thermal response that less desirable than a more efficient position. The system may power transmitting device configured to wirelessly transfer electromagnetic energy to a power receiving device. Processing circuitry of the system may compute a target output power deliverable by the power transmitting device for a first duration and control the power transmitting device to output the target output power based in part on a heat limit. The processing circuitry may further calculate an energy transfer efficiency to the power receiving unit, update an adjustment factor based on the calculated energy transfer efficiency, and apply the adjustment factor to the heat limit for a subsequent duration.