How much electricity does the Hoover Dam supply?

It's very difficult to answer questions like this because of the way the US power grid works. Power producing sites put power on the grid, and power consuming sites take power off the grid, and it's not really possible to track exactly what goes where. Think of it this way: the power grid is a big barrel full of water. Las Vegas has a pipe coming out of the barrel, and Hoover Dam has a pipe going into the barrel, but there are many other pipes coming out and going in. Finding out if a particular drop of water flowing through the Las Vegas pipe came from the Hoover Dam pipe would be difficult. The power grid is actually worse than that, because we could theoretically put some dye in the water, but we can't do that with power; an electron is an electron is an electron. That said: Las Vegas actually does not get much of its power from Hoover Dam, since the electric company serving Las Vegas is NV Energy, which owns its own power plants and does not operate the generators at Hoover Dam. NVE's own website states that 70% of their power generation for southern Nevada comes from natural gas fired plants, so at most 30% of Las Vegas' power would come from Hoover Dam (and most likely much less than that). Also, the US Bureau of Reclamation (the agency that operates the generators at the dam) does not list Las Vegas as a recipient of the power generated. They do list the State of Nevada in general (which gets about 23% of the power generated by the dam), but the only specific city in Nevada that's listed is Boulder City, NV.