If you are looking for what mA a particular light bulb draws, you can take an ohmmeter and measure the resistance of the bulb. Then using ohm's law ( Volts = Amps x Ohms) divide your 12 volts by the ohm reading of the bulb. If you are powering them with a DC power pack, that how many amps the bulb will draw. To figure out mA divide your answer (amps) by 1000. AC is a bit different, but close enough for incandescent light bulbs.
Or if you are using a common GE lamp, you could look up the data sheet on line for that number (like 53, 57, 1445, etc.) and that will tell you the mA it draws.
Or you could just err on the side of caution and call each lamp 5 watts like they used to. Then add up all your watts and size your transformer according to that.
So there's three different ways; there will probably be dozens more here by the time all is said and done 
J White
