It's a much lower number than I expected, probably because in sci fi it's never a signalling laser, it's always a launch laser and that's a whole 'nuther animal. FUN FACT - disturbing portions of the historical record thinks Sirius was red. There are two real ways to resolve this: (1) presume that the historical record, as evidenced time and time again, is faulty (2) presume that the Sirians were pointing a launch/signalling laser at us for a few hundred years back in antiquity. (2) is a lot more fun and could be the impetus for a pretty fun sci fi conspiracy tale, or so I've heard. Or, if you're Larry Niven and Jerry Pournelle, you take the idea and throw it in the distant future at a completely different part of the galaxy so that your social commentary doesn't have to include people. 100MW is chump change. I've worked with powerplants that big. You can buy them on Alibaba. Which leads me to believe that divergence is more important than we're accounting for but I'm too lazy to do more than throw some numbers at an online calculator and watch it choke on the light years.