Circuit's power budget and power dissaption
There are a few ways to do this:
1) Predict and model the design. This is done a few ways. If it's a circuit, simulating each of the pieces gives a nice idea of how much it will use. A spice package such as LT Spice can be great for this (if you hover over a part in LT spice an hold down alt it can graph out a power for that component). Estimation and bounding goes a long ways, usually pouring through datasheets and finding quiescent currents (min current) and max currents. Then estimate something in between. With microprocessors it is much more difficult to determine how much power they will draw, so a max case is usually best. 

Estimate power supply efficiencies and DC to DC converters, get a current draw for the total design. It's better to give yourself some margin (like add 30%) to make sure you size a supply right and don't overdraw current.

2) The next way is to actually get some parts with prototyping and measure the currents to give you an idea of how much current the total design will draw.