A method for receiving wireless power by a wireless power receiver , the method comprising: performing a negotiation process with a wireless power transmitter for establishing a negotiated power transmission condition, the negotiation process comprising: transmitting a request signal to the wireless power transmitter to retrieve a capability of the wireless power transmitter , the capability including an available amount of power of the wireless power transmitter , characterized by further comprising: receiving the available amount of power from the wireless power transmitter in response to the request signal, determining a demanded amount of power based on the available amount of power of the wireless power transmitter , subsequently, transmitting the demanded amount of power as the negotiated power transmission condition to the wireless power transmitter ; receiving wireless power from the wireless power transmitter based on the negotiated power transmission condition when the demanded amount of power is allowable by the wireless power transmitter based on the capability of the wireless power transmitter ; in response to a change of a state of the wireless power transmitter while conducting power transmission, again entering the negotiation process with the wireless power transmitter to adjust the negotiated power transmission condition, and wherein the change of the state of the wireless power transmitter refers to information about a change of the amount of power to be supplied from the wireless power transmitter . transmitting a request signal to the wireless power transmitter to retrieve a capability of the wireless power transmitter , the capability including an available amount of power of the wireless power transmitter , characterized by further comprising: receiving the available amount of power from the wireless power transmitter in response to the request signal, determining a demanded amount of power based on the available amount of power of the wireless power transmitter , subsequently, transmitting the demanded amount of power as the negotiated power transmission condition to the wireless power transmitter ; receiving wireless power from the wireless power transmitter based on the negotiated power transmission condition when the demanded amount of power is allowable by the wireless power transmitter based on the capability of the wireless power transmitter ; in response to a change of a state of the wireless power transmitter while conducting power transmission, again entering the negotiation process with the wireless power transmitter to adjust the negotiated power transmission condition, and wherein the change of the state of the wireless power transmitter refers to information about a change of the amount of power to be supplied from the wireless power transmitter . receiving the available amount of power from the wireless power transmitter in response to the request signal, determining a demanded amount of power based on the available amount of power of the wireless power transmitter , subsequently, transmitting the demanded amount of power as the negotiated power transmission condition to the wireless power transmitter ; receiving wireless power from the wireless power transmitter based on the negotiated power transmission condition when the demanded amount of power is allowable by the wireless power transmitter based on the capability of the wireless power transmitter ; in response to a change of a state of the wireless power transmitter while conducting power transmission, again entering the negotiation process with the wireless power transmitter to adjust the negotiated power transmission condition, and wherein the change of the state of the wireless power transmitter refers to information about a change of the amount of power to be supplied from the wireless power transmitter . <EOS>