Roadmap for Incorporating Energy 

Efficiency/Renewable Energy

Policies and Programs into State and Tribal 

Implementation Plans

Appendix B: Overview of the U.S. Electric System
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EPA-456/D-12-001c

July 2012

Roadmap for Incorporating Energy Efficiency/Renewable Energy

Policies and Programs into State and Tribal Implementation Plans

Appendix B: Overview of the U.S. Electric System

By:

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Outreach and Information Division

Research Triangle Park, North Carolina

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Outreach and Information Division

Research Triangle Park, North Carolina
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ACKNOWLEDGMENTS

We would like to acknowledge substantial contributions from members of an inter-office EPA 

team that included the Office of Atmospheric Programs, the Office of Policy Analysis and 

Review, the Office of General Counsel and Regions 1 and 6. This document also reflects 

comments received from a number of stakeholders, including state and local air quality 

agencies.
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Contents

FIGURES ..................................................................................................................................................................... B-4

SECTION B.1: INTRODUCTION................................................................................................................................... B-5

SECTION B.2: ABOUT THE U.S. ELECTRIC SYSTEM .................................................................................................... B-5

SECTION B.3: HOW THE ELECTRIC SYSTEM WORKS.................................................................................................. B-7

SECTION B.4: ROLES AND RESPONSIBILITIES ............................................................................................................ B-9

SECTION B.5: LOCATION OF EMISSION REDUCTIONS RELATIVE TO THE SITING OF CLEAN ENERGY RESOURCES . B-10

REFERENCES............................................................................................................................................................. B-12
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FIGURES

Figure 1: NERC Interconnections............................................................................................................................... B-6

Figure 2: Flow of Electric Power ................................................................................................................................ B-7

Figure 3: Unit Dispatch in a Power System................................................................................................................ B-8
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SECTION B.1: INTRODUCTION

Generating electricity from fossil fuels is the single largest source of anthropogenic carbon 

dioxide (CO2) emissions in the United States, representing 40 percent of CO2 emissions in 2009.

1 

It is also the largest source of criteria air pollutants that affect air quality and human health. 

For these and other reasons there has been growing interest in understanding the impacts of 

state-level energy efficiency and renewable energy (EE/RE) policies on emissions from power 

generation. Much of this interest has come from state environmental regulators interested in 

including emission reductions from EE/RE policies in their plans for improving and maintaining 

air quality. 

Many air agencies are already familiar with the electric system, and the roles and 

responsibilities of energy agencies in their state. For those who want more information on the 

topic, it is provided here in this appendix as a convenience. For these stakeholders and others 

working to analyze the effects of clean energy on air pollution emissions, there is a need to:

Understand the electric system

Understand how the system is likely to respond to the introduction of clean energy 

resources

Conduct analyses that credibly and accurately represent this interaction and estimates 

reductions in air pollution

Appendix B is intended to address these needs, in addition to other resources.

2

 It highlights the 

basic workings of the electric system and addresses important issues that arise in energy and 

emissions planning; most notably quantification of emission benefits for incorporation in State 

Implementation Plans (SIP)/Tribal Implementation Plan (TIP) (see Appendix I for details on 

quantification methods). A key take-away from this Appendix is that the operation of regional 

power systems is complex and dynamic, so predicting how these systems will react to new 

resources – including EE/RE – is likewise a complex undertaking. 

SECTION B.2: ABOUT THE U.S. ELECTRIC SYSTEM

The most common way to generate electricity is to burn fossil fuels to convert water into 

steam, and to use the steam to spin a turbine that is connected to an electric generator. 

Generators can also be turned by water – as is the case with hydroelectric power plants – or by 

wind turbines. In all cases, the electricity generated at these facilities flows across the 

transmission and distribution system to where it is needed to meet customer demand in cities 

and rural areas.

 

1

EPA (2011).

2

EPA (2010).
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The North American electric system is an interconnected network for generating, transmitting, 

and delivering electricity to consumers. Over the past 100 years, the system developed around 

a "central station" model that distributes power from large generating stations (often located 

near a fuel source) to customers located in load centers that are hundreds of miles away. The 

current electricity delivery system was designed and built in the 1950s to move large quantities 

of power from generators to consumers at low cost. Despite a recent trend towards more 

"distributed" power in 

which small 

generation facilities 

such as combined heat 

and power systems are 

located near loads, 

most electric power in 

the U.S. continues to 

be generated at 

central-station facilities 

powered by coal, 

natural gas, nuclear, 

and hydropower. 

The North American 

electric system is 

divided into four 

distinct North American 

Electric Reliability 

Corporation (NERC)

interconnection grids in 

the continental United 

States and Canada: 

eastern grid, western

grid, Quebec grid and 

the Electric Reliability Council of Texas (ERCOT) (see Figure 1). The generators, power lines, 

substations, and power distribution system are the responsibility of various utility companies 

working together under regional oversight to keep each grid operational. Each grid has only 

limited connections to the other three, but within them electricity is imported and exported 

continuously among numerous smaller power control areas (PCA). 

PCAs are managed by system operators, or transmission organizations, whose main function is 

to maintain the reliability of the system in their areas (e.g., New England, New York, California). 

They do this by balancing the electricity supplied by the power plants with that demanded by 

customers. This happens in real-time, every day of the year. In other words, energy is 

simultaneously being generated and consumed on each grid in the same quantity. There is very 

little ability to store electricity, and it is difficult for the grid to accommodate large, rapid 

changes in use and generation. 

Figure 1: NERC Interconnections

Source: 

http://www.nerc.com/fileUploads/File/AboutNERC/maps/NERC_Interconnections_color.jpg
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SECTION B.3: HOW THE ELECTRIC SYSTEM WORKS

Figure 2 depicts the flow of power from the generating station, or power plant, to the 

transformer and transmission lines through a substation transformer (that reduces voltage) to 

the distribution lines. It then flows through the pole transformer to the consumer’s service 

box. Electricity transmission typically refers to power flow between the generating station and 

a substation, and electricity distribution most often refers to delivery from the substation to 

consumers. The flow of electricity occurs in accordance with the laws of physics—along paths 

of least resistance — in much the same way that water flows through a network of canals.

Over time in a given location, the consumer demand for power fluctuates significantly. For 

instance, residential electricity demand typically peaks in the morning and evening when 

residents are home and operating electricity-consuming products. In contrast, commercial 

electricity demand typically peaks during the middle of the day while industrial demand varies 

by individual firm and type of industry. System planners have to account for these variations as 

well as other factors, such as weather and the availability of individual power plants, all while 

keeping the system in balance. Fortunately, the aggregate demand of the many jurisdictions 

across a single grid behaves in a relatively predictable manner.

Figure 2: Flow of Electric Power

To meet consumer demand, the grid operators rely on a fleet of power plants with different 

operational characteristics, fuels, and cost structures. Base load plants, such as nuclear and 

most coal plants, operate 24 hours a day and do not readily cycle up and down. They are 

meant to start up and keep running until maintenance is needed. Base load units are also 

characterized by relatively high capital costs, low operating costs, and a ramp-up process that is 

typically slow, expensive, and results in wear on the generating units. As power demand 

increases over the course of a day, intermediate and peaking plants come on line. These plants 

have the physical capability to quickly ramp up power production to meet increasing demand 

 Source: https://reports.energy.gov/BlackoutFinal-Web.pdf
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and to rapidly cycle down once that demand dissipates. These plants are often engines or 

turbines that are fueled by oil or natural gas (see Figure 3).

The decision of which power plants to dispatch and in what order is based in principle on 

economics, with the lowest-cost resources dispatched first and the highest cost resources used 

last. The last resources to be called upon are referred to as the marginal units, which are 

typically the most expensive units to run. In some cases in certain parts of the country, these 

plants can also be among the dirtiest and least efficient of the power plant fleet.

Renewable energy and EE can affect the 

dispatch in different ways, though both 

cause marginal units to run less frequently 

and result in fewer air emissions. In the 

case of efficiency, energy savings occur at 

the point of consumption resulting in a 

reduction in demand on the electric 

system and a corresponding reduction in 

emissions from the power plant fleet. 

In contrast, RE sources reduce the output 

from the marginal unit by producing 

electricity for the power. Thus, a wind 

farm producing electricity displaces the 

need for electricity that would have 

Figure 3: Unit Dispatch in a Power System

Source: http://www.synapse-energy.com/Downloads/SynapseReport.2005-07.PQA-EPA.Displaced-Emissions-Renewablesand-Efficiency-EPA.04-55.pdf
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otherwise been produced by that marginal unit. Since wind power results in zero emissions, 

overall emissions from the power plant fleet are reduced (absent a cap on emissions that 

determines overall pollution levels). 

This theory of “economic dispatch” predicts that any new resource will shift upward all 

resources above it in the dispatch order, thereby reducing demand on the marginal unit (the 

most expensive unit needed to meet demand). Actual plant dispatch, however, is frequently 

more complicated than the representation in Figure 3 for three main reasons: 

Transmission constraints may require system operators to dispatch certain units that 

are more expensive than other available units. 

It is time consuming to start and stop many types of large generating units. Limitations 

on unit “ramp-up rates” also force system operators to keep some units running during 

periods when they are not needed (in order to have the units available when they are 

needed). These are referred to as load following, or intermediate units, and are often 

running at a lower and less efficient rate while not producing any power for input into 

the grid. 

System operators do not treat generating units as single entities in the dispatch process. 

Instead, plant owners in competitive markets typically bid the power from an individual 

generating unit into a smaller number of “blocks” that are instead bid into the grid. 

Because actual unit dispatch often looks very different from the ideal shown in Figure 3, 

environmental regulators and others should be aware of how these electric-system realities are 

represented in control-measure estimates of emission reductions.

SECTION B.4: ROLES AND RESPONSIBILITIES

In the electric system, four entities have key roles and responsibilities:

State energy offices

Public utility offices or service commissions

Vertically integrated utility companies

Regional transmission organizations and independent system operators

State energy offices perform a number of functions:

Assist in achieving state energy-related environmental goals

Ensure that the needs and issues of industry, business, and residential energy 

consumers are considered during energy policy and program development 

Aid businesses in using energy effectively, modernizing industry, and retaining and 

creating jobs 

Help residential and other low-level energy consumers meet their energy needs through 

cost-effective and energy efficient solutions 
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Demonstrate the application of cost-effective advanced EE/RE and other clean energy 

technologies

With other state agencies, deploy technologies to reduce energy consumption and meet 

energy-related environmental goals 

Public utility or service commissions act as governing bodies that:

Regulate the rates and services of a public utility that provide essential services, 

including energy

Oversee and evaluate EE/RE policies 

Ensure reliable utility service at fair, just, and reasonable rates 

Ensure that the facilities necessary to meet future growth can be financed on 

reasonable and fair terms

Encourage harmony between utility companies and their customers

Foster planned growth of public utility services

Coordinate energy supply facilities with the state's development

Cooperate with other states and the federal government in providing interstate and 

intrastate public utility service and reliability of energy supply 

Vertically integrated utility companies:

Oversee the entire chain of power delivery

Produce electricity through the operation of power plants

Deliver power to residential, commercial, and institutional customers

Regional transmission operators and independent system operators provide several services:

Serve as grid operators, coordinating the power grid to ensure reliable delivery of twothirds of the electricity used in the United States to two-thirds of its population

Match generation to load instantaneously to keep supply and demand for electricity in 

balance

Provide non-discriminatory transmission access, and facilitate competition among 

wholesale suppliers to improve transmission service and provide fair electricity prices

Schedule the use of transmission lines

Manage the interconnection of new generation and monitor the markets to ensure 

fairness and neutrality for all participants 

SECTION B.5: LOCATION OF EMISSION REDUCTIONS RELATIVE TO THE 

SITING OF CLEAN ENERGY RESOURCES 

The goal of clean energy policies in the SIP planning context is typically to reduce emissions

within the jurisdiction where the policies are implemented. To achieve this goal, all (or a 

portion of) the emission reductions from EE/RE must occur in a location that affects air quality 
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in the implementing jurisdiction. The environmental regulator can take steps to ensure that the 

analysis supporting such a policy accounts for the interconnected and dynamic nature of the 

power system, and that it examines the possibility that the benefits of clean energy policies 

may not be completely realized within the jurisdiction of interest. 

This can be illustrated by the example of a state with a renewable portfolio standard requiring 

utilities to buy a fixed percentage of their electricity from renewable energy facilities. If a local 

utility signs an energy-purchase contract with the nearest renewable facility, the state may find 

it difficult to correlate wind power produced by that wind farm to a corresponding reduction in 

electric output and emissions from specific fossil-fuel generators. The implementing state 

needs to ensure that the emission reductions occur at an upwind or nearby facility, which

affects the implementing state’s air quality. For this reason, it is critically important to 

understand and accurately predict how the regional power grid is likely to behave when 

assessing the emission benefits from clean energy resources.
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REFERENCES

EPA (2010). Assessing the Multiple Benefits of Clean Energy: A Resource for States. February 2010. 

Available online at <http://www.epa.gov/statelocalclimate/resources/benefits.html>

EPA (2011). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009. April 2011. Available 

online at <http://epa.gov/climatechange/emissions/usinventoryreport.html>
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United States

Environmental Protection

Agency

Office of Air Quality Planning and Standards

Outreach and Information Division

Research Triangle Park, NC

Publication No. EPA-456/D-12-001c

July 2012