First, a significant amount of electricity that is generated in the U.S. comes from the burning of natural gas - historically it’s been about 20%. The rest comes from coal (about 50%), nuclear (about 20%), hydro-electric (about 8%) and renewable (about 1%). If 50% of all electricity generated in the U.S. comes from coal, why is the correlation between natural gas more important?  To answer that question, you’ll need to understand the order in which electricity generators are dispatched and how prices for electricity are set.