Electric generator capacity is the maximum output, commonly expressed in megawatts (MW), that generating equipment can supply to system load, adjusted for ambient conditions. Generating units vary by intended usage; that is, by the three major types of load (generally categorized as base, intermediate, and peak) requirements the electricity producer must meet.

• A baseload generating unit is normally used to satisfy all or part of the minimum or base load of the system and, as a consequence, produces electricity at an essentially constant rate and runs continuously. Baseload units are generally the newest, largest, and most efficient of the three types of units.
• A peakload generating unit, normally the least efficient of the three unit types, is used to meet requirements during the periods of greatest or peak load on the system.
• Intermediate-load generating units meet system requirements that are greater than base load but less than peak load. Intermediate-load units are used during the transition between baseload and peak load requirements.

The United States has over twice the generating capacity of any other country. By the end of 2007, 994,888 megawatts of capacity supply the nation’s demand for electricity. Capacity measures the steady hourly output that a generating system can supply.

Capacity additions in 2007 increased only 1 percent from the previous year. The industry added only 8,673 megawatts of new capacity (new generators) in 2007. About 53 percent of the new capacity was fueled by natural gas.

Since the late 1990s, natural gas has been the fuel of choice for the majority of new generating units. In 2007, natural gas capacity accounted for 40 percent of the total generating capacity. Coal-fired capacity remained unchanged at 31 percent of total generating capacity. Petroleum-fired capacity decreased 2,029 megawatts from 2007 and accounted for 6 percent of total generating capacity.

What is the outlook through 2030?

• Growth in coal-fired generating capacity is limited by concerns about greenhouse gas (GHG) emissions and the potential for mandated limits, but existing plants continue to be used intensively.
• Natural-gas-fired plants account for 53 percent of capacity additions, as compared with 22 percent for renewables, 18 percent for coal-fired plants, and 5 percentfor nuclear.
• Escalating construction costs have the largest impact on capital-intensive technologies, including renewables, coal, and nuclear; but Federal tax incentives, State energy programs, and rising prices for fossil fuels increase the cost-competitiveness of renewable and nuclear capacity.
• New nuclear capacity additions fall after 2020 because a production tax credit expires.  However, rising gas prices (and falling nuclear costs due to learning and other factors) eventually result in nuclear becoming economic even without the tax credit in the later years.

Least Expensive Technology Options Are Likely Choices for New Capacity

• Technology choices for new generating capacity are made to minimize costs while meeting local and Federal emissions constraints. Capacity expansion decisions consider capital, operating, and transmission costs. Typically, coal-fired, nuclear, and renewable plants are capital-intensive, whereas operating (fuel) expenditures account for most of the costs associated with natural-gas-fired capacity. Capital costs depend on such factors as interest rates and cost-recovery periods. Fuel costs can vary according to plant operating efficiency, resource availability, and transportation costs. 

• Regulatory uncertainty affects capacity planning decisions. Unless they are equipped with carbon capture and storage (CCS) equipment, new coal-fired plants could incur higher costs as a result of higher expenses for siting and permitting. Because nuclear and renewable power plants (including wind plants) do not emit GHGs, however, their costs are not directly affected by regulatory uncertainty. 

More information on this subject can be found in the following EIA publications:
   Electricity projections from the Annual Energy Outlook
   Electricity capacity from the Electric Power Annual
   Additional basic electricity information 
   Electric Power Industry Overview 2007: Generating Capability/Capacity