Duke, from what I understand, you would multiply the amps x nominal voltage for a wattage rating. From what I've seen recently, all Mag bulbs can pull somewhere around .8A or.9A across the board. So, in this case, it would be 6V nominal times .9A equals 5.4W. This sounds powerful until you take into consideration that alks can only maintain this level for a brief period before the voltage collapses, dimming and yellowing the output. Thats why Mag bulbs are actually fairly respectable, superbulbs not included, if driven or overdriven with Nimh which can maintain voltage and current to specs.
I have a cheapie 5 buck plastic ROV 4AA lantern that I get good output from by using Nimh and a Mag 3 cell bulb.