How do i measure the basic default power consumption ?

How do i measure the basic default power consumption ?

The basic default power consumption is an input by the designer.  If this is an existing sub-system or IP or SoC, you can get this from previous testing.  If it is a commercial product such as a Processor, you can get it from the datasheet.  If it is a completely new product, the designer/user will have to start with assumption.  One of the purposes of system design is determine at what number will the system meet the requirements.  For example, if they estimate 20 mW and it does not meet their requirements, the designer knows that the power must be less than 20 mW.