You need to distinguish worst case and average power consumption. It occurs to me you are more interested in the latter.
Average power consumption
Here you have to understand how RFID and specifically
14443 A work. In a nutshell, the reader keeps its antenna resonating at 13.56 MHz, and when a token is in the field, the latter gets power. Token has an antenna tuned to the same frequency ±manufacturing error. Thus both antennas resonate together.
Furthermore reader sends out commends to the token by modulating the field, while token responds by by modulating losses in the established resonating circuit.
In the simple case, protocol requires only a few messages back and forth.
This should give you the baseline, that is the minimum work required to identify a token.
Real energy use will be much higher than baseline:
- most of the time there is no token and energy is lost in reader antenna and surrounding air. although only a little at a time, it will amount to a lot over hours of operation.
- some tags are worse than others, losing a lot more energy per read
- if several tags are present in the field, reader goes through anticollision sequence, totally oversimplifying the subject, think of reader trying to talk to every card it may see in sequence over and over
- if your environment contains any conductive parts, energy is lost as Eddy current
- your reader and microcontroller need energy to run too
In practice you need to build a prototype, go out in the real world and test your system. You will be surprised how much your runtime will depend on simple factors most notably temperature (cold kiss batteries) and nearby objects (your body or metal of your car will eat up reader field). Smaller factors are humidity, how slow tokens are brought in and removed, and many others.
However not all is lost, there are energy efficient readers out there, here are some tricks I've seen in the field:
- reader is not constantly on, the antenna is switched on long enough to detect and card, then switched off for a "short" period of time
- read every second -- requires that card is brought next to reader and kept there for a while
- read 4~10 times a second -- almost imperceivable to average user, yet massive power saving
- extra input to detect that a token is likely nearby
- light sensor, when there's a sudden ambient light change, typically it got darker, start scanning for tokens and give up after several seconds
- antenna integrated into or around a button, user has to push the button with their token to have it read. microprocessor and reader ic can be put to deep sleep until button is pressed. IIRC manufacturer claimed a year or several years of operation on a 9V lithium battery, several reads per day and some electromechanical actuation.
Worst case power consumption
While already covered by Oli's answer, perhaps you want to distinguish absolute worst power consumption (antenna internally shorted) from reasonable, external worst case, such as operation in rain, next to metal or sources of interference.