Rezence (pronounced reh-zense) was an interface standard developed by the WiPower (A4WP) for wireless electrical power transfer based on the principles of magnetic resonance. The Rezence system consisted of a single power transmitter unit (PTU) and one or more power receiver units (PRUs). The interface standard supported power transfer up to 50 watts,[1] at distances up to 5 centimeters.[2] The power transmission frequency is 6.78 MHz, and up to eight devices could be powered from a single PTU depending on transmitter and receiver geometry and power levels. A Bluetooth Low Energy link was defined in the A4WP system intended for control of power levels, identification of valid loads and protection of non-compliant devices.[3][4]