At the end of the 20th century, electric generation in the U.S. came primarily from five sources; coal, nuclear, natural gas, petroleum, and hydroelectric. In 1999, 45 states relied on three of those five energy sources for their top three sources of electric generation. The exception was five states, which had biomass in their top three. There wasn’t much energy diversity in electric generation back then.

According to the Energy Information Agency (EIA), in 2015, over half of U.S. states had a source of electric generation outside of those five energy sources, and the number of sources of electric generation in the top three in at least one state increased from six sources to nine sources. Utilities and power producers have more options than ever before to supply electricity to their customers.

Fossil fuels are seeing their dominance decline. Petroleum fell out of favor as a source of electric generation following price spikes in 2005. In 1999, 13 states had petroleum in their top three. In 2015 it was only four. Combined, petroleum liquids and petroleum coke still made up 3 percent of electric generation in 2005; but it was less than 1 percent in 2015.

Natural gas has continued to grow; from 15.4 percent of electric generation in 1999 to 32.8 percent in 2015. Natural gas was the number one source of electric generation for only six states in 1999. By 2015 it was 16 states, and in 2016 it could be as high as 18 states. Coal was the number one source of electric generation in 31 states in 1999, even rising to 32 states in 2000. But in 2015 it was number one in only 20 states. In 2016 that number may drop to as low as 18 states.

Nuclear power is generated in 30 states. It was generated in 31 states through the end of 2014 before the Vermont Yankee Nuclear plant shut down at the end of the year. That number is expected to decline to 29 states by 2020 as the Pilgrim Nuclear plant in Massachusetts is expected to shut down in 2019. Since 1999, nuclear power has been the number one fuel for as few as five states and as many as eight states. It was number one in seven states in 2015, which will likely still be the case in 2016.

Now let’s consider wind, solar, and geothermal energy. In 1999, only 10 states generated a measurable amount of electricity from at least one of those three sources (i.e., at least one GWh, or enough to power 1,000 U.S. homes for a year). By 2015, every state generated a measurable amount of electricity from at least one of those three sources.

Being a “top three” source of electric generation means that a fuel source has become mainstream. Most states use a variety of fuels for electric generation, and if a renewable energy source breaks into the top three, there had to be some significant investment to get there.

In 1999, wind, solar, and geothermal were insignificant. No states had wind, solar, or geothermal energy in their top three. However, in 2015, 22 states had at least one of those technologies in their top three. In addition, 2016 appears likely to have 23 states, and by 2020 it could be more than half of U.S. states with one of those sources in their top three.

No state had wind power in their top three until 2003 when Iowa, New Mexico, and Wyoming each had wind as their number three source of electric generation. It wasn’t until 2014 that any state had solar in their top three, when Hawaii reached that feat. Geothermal didn’t make the top three of any state until 2014, when Nevada eclipsed hydroelectric for number three in the Silver State.

Iowa may become the first state to generate a majority of its power from wind. Iowa wind was 31.3 percent of the state’s electric generation in 2015 and 40 percent through the first half of 2016. Meanwhile, coal-fired power in Iowa declined significantly through the first half of 2016 and was 41.6 percent of electric generation during that time. With Mid-American Energy planning another 2 GW of wind by the end of the decade, Iowa could generate over half its energy from wind by 2020. Kansas is also getting closer to catching coal for number one in that state.

Solar is unlikely to be the number one source of electric generation in any state in the near future. But solar was already second place in California in 2015 and Nevada is likely to have solar at number two by 2017. Nevada is likely to have solar and geothermal in second and third place in 2016, making it the first state to have two renewable sources in its top three.

What the data means is that renewable energy sources like wind, solar, and geothermal energy (in some Western states) have become mainstream sources of electric generation. No longer do states have to rely solely on fossil fuels, nuclear power, wood, waste, and trash, or lakes, rivers, and streams. There are other clean energy options, and they are growing every year. With ways to store excess energy from renewable sources becoming more and more economic, the best days for renewable energy lie ahead.