A method for transmitting wireless power from a wireless power transmitter, the method can include performing a negotiation process with a wireless power receiver for generating a negotiated power transmission condition, the negotiation process including receiving a request signal from the wireless power receiver to retrieve a capability of the wireless power transmitter, the capability including an available amount of power of the wireless power transmitter, transmitting the available amount of power to the wireless power receiver, in response to the request signal, to allow the wireless power receiver to determine a demanded amount of power based on the available amount of power of the wireless power transmitter, and subsequently, receiving the demanded amount of power as the negotiated power transmission condition from the wireless power receiver; and transmitting wireless power to the wireless power receiver 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.