A method for performing wireless communication by a first device and a device for supporting same are provided. The method may include receiving, from a second device, a Physical Sidelink Control Channel (PSCCH) and a Physical Sidelink Shared Channel (PSSCH), based on information on a first transmission power for the PSCCH and the PSSCH, included in the PSCCH or the PSSCH, determining a first Reference Signal Received Power (RSRP) of a DM-RS (Demodulation-Reference Signal) received through the PSSCH, and performing sidelink communication based on the determined first RSRP. For example, the information on the first transmission power is based on a maximum transmission power and an allowable threshold.