Electric grid operators
plan that power sources
can be shut down
for maintenance,
usually in the spring
and the fall.
They need maximum
operating capacity
in the summer (cooling)
and in the winter (heating).
According to the report:
“Wind energy's share of total utility-scale electricity generating capacity in the United States grew from 0.2% in 1990 to about 9% in 2019, and its share of total annual utility-scale electricity generation grew from less than 1% in 1990 to about 7% in 2019.”
“Solar energy's share of total U.S. utility-scale electricity generation in 2019 was about 1.8%, up from less than 0.1% in 1990.”
"To ensure a steady supply of electricity to consumers, operators of the electric power system, or grid, call on electric power plants to produce and place the right amount of electricity on the grid at every moment to instantaneously meet and balance electricity demand.
According to the EIA’s description of U.S. electricity generating capacity:
"In general, power plants do not generate electricity at their full capacities at every hour of the day.
Three major types
of generating units
vary by intended usage:
"Base load generating units normally supply all or part of the minimum, or base, demand (load) on the electric power grid. A base load generating unit runs continuously, producing electricity at a nearly constant rate throughout most of the day. Nuclear power plants generally operate as base load service, because of their low fuel costs and the technical limitations on load responsive operation. Geothermal and biomass units are also often operated in base load because of their low fuel costs. Many of the large hydro facilities, several coal plants, and an increasing number of natural gas-fired generators, particularly those in combined power applications, also supply base load power.
Peak load generating units help to meet electricity demand when demand is at its highest, or peak, such as in late afternoon when electricity use for air conditioning and heating increases during hot weather and cold weather respectively. These so-called peaking units are generally natural gas [turbine only] or petroleum fueled generators. In general, these generators are relatively inefficient and are costly to operate, but provide high- value service during peak demand periods. In some cases, pumped storage hydropower and conventional hydropower units also support grid operations by providing power during periods of peak demand.
Intermediate load generating units comprise the largest generating sector and provide load responsive operation between base load and peaking service. The demand profile varies over time and intermediate sources are in general technically and economically suited for following changes in load. Many energy sources and technologies are used intermediate operation. Natural gas-fired combined cycle units, which currently provide more generation than any other technology, generally operate as intermediate sources.
Additional categories
of electricity generators include:
of electricity generators include:
Intermittent renewable resource generators powered by wind and solar energy that generate electricity only when these resources are available (i.e., when it's windy or sunny). When these generators are operating, they tend to reduce the amount of electricity required from other generators to supply the electric power grid.
Electricity storage systems/facilities, including hydroelectric pumped storage, solar- thermal storage, batteries, flywheels, and compressed air systems. These systems typically use (or purchase) and store electricity that is generated during off-peak electricity demand periods (when electricity prices are relatively low), and they provide (or sell) the stored electricity during periods of high or peak electricity demand (when electricity prices are relatively high). Some facilities use electricity produced with intermittent renewable energy sources (wind and solar) when the renewable resource availability is high and provide the stored electricity when the renewable energy resource is low or unavailable. Non-hydro storage systems can also provide ancillary services to the electric power grid. Energy storage applications inherently use more electricity than they provide. Pumped- storage hydro systems use more electricity to pump water to water storage reservoirs than they produce with the stored water, and non-hydro storage systems have energy conversion and storage losses. Therefore, electricity storage facilities have net negative electricity generation balances. Gross generation provides a better indicator about the activity level of storage technologies and is provided in the data releases of the EIA-923 Power Plant Operations Report.
Distributed generators are connected to the electricity grid, but they primarily supply some or all of the electricity demand of individual buildings or facilities. Sometimes, these systems may generate more electricity than the facility consumes, in which case the surplus electricity is sent to the grid. Most small-scale solar photovoltaic systems are distributed generators.
At the end of 2019, the United States had about 1,100,546 MW—or 1.1 billion kilowatts (kW)—of total utility-scale electricity generating capacity and about 23 million kW of small-scale solar photovoltaic electricity generating capacity.
Generating units fueled primarily with natural gas account for the largest share of utility-scale electricity generating capacity in the United States."