Energy source mix varies among the three U.S. electricity grids

The bulk electric system of the Lower 48 states consists of three independently synchronized electric interconnections: Eastern, Western, and the ERCOT part of Texas. Because of minimal transfers of electricity between the Interconnections, each interconnection essentially meets its demand with its own generating resources along with modest contributions from international imports.

The three U.S. electric Interconnections differ widely in their amount of generating capacity, energy source mix, and type of capacity actually used to produce electricity. The Eastern Interconnection had 748 gigawatts (GW) of total generating capacity in 2015. This is significantly more than the Western Interconnection at 212 GW and the ERCOT part of Texas at 98 GW.

The charts below present generation capacity in each Interconnection by energy source. The total bar indicates total capacity by energy source. The shorter darker bar represents how much of the total capacity was used during the year on average. The ratio of average use to total capacity is called the capacity factor (CF). These dark bars represent the relative output of capacity by fuel source.

The amount that each energy source contributes to meeting the demand depends on how much of each type of capacity is used. Certain plants operate closer to their full capacity more of the time than others, depending on economic factors such as fuel cost and resource factors such as the availability of wind, sun, or water. In general, nuclear plants, coal plants, and natural gas combined-cycle plants operate closer to their full capacity more of the time than other generating technologies.

In the east, coal represents the largest capacity resource. Although the total capacity of all natural gas technologies exceeds coal, due to distinct performance and operating attributes, combined-cycle natural gas-fired applications are considered separately from simple-cycle natural gas-fired applications. However, based on output, coal exceeds all natural gas technologies, either together or separately. Renewable capacity and generation are significantly smaller than the other energy sources.

In the west, hydroelectric capacity largely on the Columbia, Snake, and Colorado rivers surpasses the capacities of all other energy sources. Natural gas combined-cycle had the highest output in 2015, slightly higher than coal. The individual capacities of wind and solar PV are greater than nuclear capacity. But nuclear's higher capacity factor means its output is higher than either of those renewable sources.

In Texas, natural gas combined-cycle generators are the largest in both capacity and output. Wind capacity rivals that of fossil fuel generators and, even with its relatively low 20% capacity factor, wind equals the output of nuclear units.

An interesting feature of the Western Interconnection is that coal accounts for a comparatively large share of generation (26%) relative to its capacity (15%). This relationship is explained by the fact that coal plants in the Western Interconnection operate on average near a 70% capacity factor, which is significantly higher than the 53% capacity factor in the Eastern Interconnection (36% generation, 31% capacity) and the 57% capacity factor in Texas (25% generation, 19% capacity).

One aspect of consistency across the interconnections is the relatively high capacity factor of nuclear plants, which averages between 91% and 92%. This high capacity factor is attributable to the facts that nuclear plants perform better when operating under relatively steady conditions and tend to have comparatively low variable costs.

The overall capacity factor in the west (39%) is significantly lower than in the east (45%) or in Texas (43%). These differences are consistent with their capacity mixes. The lower overall capacity factor of the Western Interconnection reflects a lower number of nuclear and coal plants, both of which tend to operate at high capacity factors. In contrast, the Eastern Interconnection, with relatively large amounts of nuclear and coal capacity, has the highest overall capacity factor.

Principal Contributor:

Glenn McGrath
(Glenn.McGrath@eia.gov)