A method for transmitting wireless power executed by a power transmitter comprising: performing digital ping and receiving a response from a power receiver; acquiring configuration information of the power receiver and generating a power transfer contract with the power receiver by using the configuration information; transmitting power to the power receiver based on the power transfer contract; transmitting indication information for initiating communication with the power receiver to the power receiver; the method characterized by: receiving, from the power receiver, a response packet for granting the communication; transmitting, to the power receiver, a capability packet including power transmission capability information; receiving, from the power receiver, a renegotiation packet requesting to proceed to a renegotiation phase; transmitting, to the power receiver, an acknowledge, ACK, to enter the renegotiation phase in response to the renegotiation packet; receiving, from the power receiver in the renegotiation phase, a first specific request packet including information for a guaranteed power level requested by the power receiver; transmitting, to the power receiver in the renegotiation phase, an ACK to accept the guaranteed power level in response to the first specific request packet; receiving, from the power receiver in the renegotiation phase, a second specific request packet requesting the end of the renegotiation phase; and transmitting, to the power receiver, an ACK to end the renegotiation phase in response to the second specific request packet. performing digital ping and receiving a response from a power receiver; acquiring configuration information of the power receiver and generating a power transfer contract with the power receiver by using the configuration information; transmitting power to the power receiver based on the power transfer contract; transmitting indication information for initiating communication with the power receiver to the power receiver; the method characterized by: receiving, from the power receiver, a response packet for granting the communication; transmitting, to the power receiver, a capability packet including power transmission capability information; receiving, from the power receiver, a renegotiation packet requesting to proceed to a renegotiation phase; transmitting, to the power receiver, an acknowledge, ACK, to enter the renegotiation phase in response to the renegotiation packet; receiving, from the power receiver in the renegotiation phase, a first specific request packet including information for a guaranteed power level requested by the power receiver; transmitting, to the power receiver in the renegotiation phase, an ACK to accept the guaranteed power level in response to the first specific request packet; receiving, from the power receiver in the renegotiation phase, a second specific request packet requesting the end of the renegotiation phase; and transmitting, to the power receiver, an ACK to end the renegotiation phase in response to the second specific request packet. receiving, from the power receiver, a response packet for granting the communication; transmitting, to the power receiver, a capability packet including power transmission capability information; receiving, from the power receiver, a renegotiation packet requesting to proceed to a renegotiation phase; transmitting, to the power receiver, an acknowledge, ACK, to enter the renegotiation phase in response to the renegotiation packet; receiving, from the power receiver in the renegotiation phase, a first specific request packet including information for a guaranteed power level requested by the power receiver; transmitting, to the power receiver in the renegotiation phase, an ACK to accept the guaranteed power level in response to the first specific request packet; receiving, from the power receiver in the renegotiation phase, a second specific request packet requesting the end of the renegotiation phase; and transmitting, to the power receiver, an ACK to end the renegotiation phase in response to the second specific request packet.