In the US, electricity demand is growing very slowly, which means that capacity additions don't have to exceed retirements by much in order to keep the grid functioning. Tracking the comings and goings from the electric grid can help provide a picture of the country's changing energy mix.

We recently discussed the planned generating additions for 2016, which were dominated by renewables and natural gas; no new coal plants were expected. It appears that 2015's retirements were the mirror image.

The Energy Information Administration, which provides data on the US' electric grid, says 18GW of capacity were retired this past year, more than 80 percent of it coal-fired. More than 27GW of utility-scale projects will replace that this year. Note that much of the new generating hardware is wind and solar, which typically have a capacity factor in the area of 30 percent, while the plants they are replacing could have a capacity factor that's much higher, so it's not easy to have a direct comparison between the two.

Part of the challenge is that most of the plants wouldn't run anywhere near their potential capacity because they are old and small. This sort of hardware tends to be very inefficient and is only used during times of peak demand. The most recent of these plants was built before 1970, and it had an average age of 54 years. Most of the US' coal plants were built between 1970 and 1990 (when electricity demand was rising rapidly); these plants are much younger and larger, on average.

While competition with natural gas prices is the major challenge facing coal, environmental rules are likely to have played a role this year. The EIA notes that about 30 percent of the coal plants that were closed shuttered their doors in June, when new EPA limits on mercury emissions went into effect.

The remaining plants that were shut included a handful of oil-fired generators and a number of natural gas plants, which included both steam and combustion-based turbines. These, especially the petroleum plants, are also likely to use older hardware.