Also as far as semiconductors are conserned (that is a PCB with MCUs or other ICs), you cannot use theorems to calculate consumption. It is not fixed. For a microcontroler for example this varies dramatically depending on the state and function performed by it. If yoou drive some external loads like LEDs the consumption is high, if it is in slip mode then only few microamps are drawn. Similarly for an Op-Amp, with high inputsignals, or output, the consumption is higher (always depends output load). Logic IC consume more with higher switching frequencies.
The only way to know the consumption is to measure it with a series ammeter. If you create a profile by taking various measurments you might have a better view of the average current needs of your board.