Former Vice President Dick Cheney once said that "Conservation may be a sign of personal virtue, but it is not a sufficient basis for a sound, comprehensive energy policy." But in the US, increased energy efficiency has helped drive a drop in total electricity use. That, combined with the rise of renewable power, caused the use of both coal and natural gas to decline last year.

The changes, according to the Energy Information Agency, are relatively small. Total electric generation last year was down 1.5 percent compared to the year before, a drop of 105,000 GigaWatt-hours. But both coal and natural gas saw declines that were even larger. Coal use was down by 2.5 percent, a smaller decline than it has seen in many recent years. But the numbers for its future aren't promising; no new coal plants were opened, and 6.3 Gigawatts of coal capacity were retired in 2017.

Continuing recent trends, 9.3GW of natural gas capacity were brought online, although that was partly offset by the retirement of 4.0GW of older gas plants. Despite the additional capacity, however, natural gas use was also down, dropping by nearly 8 percent.

Overall, those numbers indicate that something must be displacing our use of fossil fuels. That something was clearly not nuclear, which held steady over the last year. This leaves us with renewables.

The return of a snowpack to California helped boost the production of hydropower, which allowed it to stay ahead of wind as the largest source of renewable energy. Look for wind to displace it within the next two years, as it generated about 85 percent of hydro's total and saw more than 6GW of new capacity added. 2017 was a record year for solar as well, both for utility-scale and residential—combined, they saw 8.2GW of capacity added. Solar is still growing from a small base, however, as utility-scale generation only produced about 17 percent of what was generated by hydropower.

Combined, these renewable sources produced well over 15 percent of the US' electricity last year. Throw in biomass, and renewable generation climbs above 18 percent. That means renewable production is likely to surpass the 20-percent share of nuclear energy within the next few years. Coal and natural gas, each of which are around 30 percent of the generating mix, will take quite a bit longer to surpass.