A retail electricity provider and power generator has outshone the glamor technology stocks this year. Power utilities aren't always seen as the most exciting way to invest, but investors might need to rethink that opinion, because the top-performing S&P 500 index stock of the year is retail electricity and power generation utility Vistra (VST 3.12%), up a whopping 210% this year. That beats Nvidia's (NASDAQ: NVDA) 155% increase. The two events are not unconnected. Here's why and how Vistra stock has perfor ...