Miles of wires bring life-saving energy: decoding the power grid

The power grid is changing fast, and those changes could determine how stable, affordable and clean our energy is. Electricity is an essential part of everyday life, but the components that make up the grid, and the terminology that describes it, can often feel overly complex and alienating.

This power grid terminology guide will knock down the barrier between energy experts and SAN readers, empowering everyone to speak the same language.

What are watts and megawatts?

A watt is a measurement of electricity at a given moment. A watt measures how much electricity a device needs in order to run, or how much power a power plant or solar panel can provide. We can think of this as instantaneous power output or capacity.

A watt-hour, on the other hand, measures quantity. It’s the amount of electricity needed to provide a watt of power for one hour. For example, a minifridge needs roughly 50 watts of capacity in order to run. If you plug in the minifridge for an hour and then unplug it, you’ve used 50 watt-hours of electricity.

What is a kilowatt? A megawatt? Gigawatt? Terawatt? 

Each unit of measuring electricity builds off the previous unit, increasing 1,000-fold each time. A kilowatt is 1,000 watts. A megawatt is 1,000 kilowatts. A gigawatt is 1,000 megawatts, and a terawatt is 1,000 gigawatts.

Hourly units follow the same rule. For example, the average U.S. home uses about 900 kilowatt-hours of electricity per month. The average month contains 730 hours. Using overly simplistic math, this means the average home needs 1.2 kilowatts of electricity at any moment — in reality, a home’s energy needs are often higher during the day and evening than at night, depending on consumption habits.

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Unbiased. Straight Facts.^TM

A large data center can use 81,000 times more electricity than the average American home.

Large hospitals, by comparison, can use 50,000 kilowatt-hours of electricity in a single day, according to Advanced Energy Management — about 1.5 gigawatt-hours per month. That 1.5 gigawatt-hours expended per month is the same as about 1,666 homes consuming 900 kilowatt-hours.

Data centers are a hot topic due to the significant amount of electricity they consume. Large data centers can require more than 100 megawatts of capacity. Let’s say a 100 megawatt data center runs at full capacity 24 hours a day for a full month. It would consume 73,000 megawatt-hours of electricity. That’s equivalent to 73 gigawatt-hours and 73,000,000 kilowatt-hours — about 81,000 times more electricity than the average home consumes in a month, or about 48 times more than a large hospital.

What are thermal power plants?

For a long time, America’s electricity came mainly from coal, which was burned inside power plants to produce steam. That steam would rise through a turbine, spinning it to generate electricity. Thermal power plants have advanced in technology and fuel — most are powered by natural gas rather than coal — but the basic mechanics remain the same. In nuclear power plants, the heat is generated from the nuclear reaction of splitting atoms apart rather than burning fuel.

Coal, natural gas and nuclear all fall under the category of thermal power plants that can supply what’s known as base-load power. Base-load power is very stable and — as long as they have an adequate fuel supply or a stable nuclear reaction — these power plants can provide a consistent amount of electricity to the grid 24 hours a day. This is sometimes described as “firm” power.

However, grid operators cannot always count on steady output from power plants: Thermal plants can also shut down due to routine maintenance and mechanical issues.

What counts as renewable energy?

In contrast, wind and solar power are intermittent because their output varies depending on weather conditions. These sources are categorized as renewable energy. Renewables are not dependent on fuel that needs to be mined, drilled or enriched by other industries. Instead, renewables are powered by the earth’s natural features: sunlight, wind and water flows in the case of hydroelectric power.

Hydroelectricity is unique in that it’s renewable and — except in the case of severe drought — it provides firm, base-load power to the grid because water flows through hydroelectric dams at a consistent rate, generating a predictable amount of electricity.

What is dispatchable power? 

Dispatchability is another key feature describing resources on the electric grid that can quickly add or reduce power production. Dispatchable power comes from sources with an electricity output that can be turned up or down, like a dimming light switch.

For example, batteries are completely dispatchable as long as they have sufficient charge. Batteries can be called on to add power to the grid at a moment’s notice.

Gas and coal power plants are also dispatchable. They typically keep operating below their maximum level and can add more power to the grid when needed. However, the time it takes for power plants to ramp up and down varies depending on plant design. Some gas plants take less than two minutes, while coal plants can take hours. Nuclear can also provide dispatchable power, but nuclear plants typically operate at a consistent megawatt output around the clock because it’s more efficient.

Dispatchability is important to the grid because power demands vary throughout the day and can change rapidly. For example, thousands of homes might increase power demand by turning on the lights around sunset, or a power plant tripping offline could lower supply. Grid operators prepare for these scenarios and look to dispatchable sources to make up the difference and ensure electricity demand is met.

However, resources are only dispatchable up to a point. Whether it’s a battery or power plant, each energy generation resource has a maximum megawatt capacity at which it cannot add more power to the grid.

Geothermal power is harnessed from the naturally occurring heat within the earth’s surface. Geothermal holds the unique position of being renewable, dispatchable and providing firm, base-load power. Until recently, geothermal was only possible in a narrow set of geological regions, but technological advances like horizontal drilling originally developed in the oil and gas industry have made geothermal more widely available.

What is the difference between AC and DC electric currents?

The power grid has multiple layers. The outermost is the interconnection — a vast area covering multiple states in which all of the regional power grids are connected on alternating current power lines. It helps to understand the difference between alternating current (AC) and direct current (DC).

In simple terms, alternating current means the power lines share electricity as it flows in both directions; direct current means electricity is sent in one direction on a power line. Think of AC as an ocean wave in which water is constantly rocking back and forth, while direct current is a river sending water in one direction. Nikola Tesla pioneered AC power while Thomas Edison favored DC.

The power that flows from a wall outlet in the typical American home is AC power. But some devices like laptops run on DC power, which is why many laptop chargers have a small box that transforms the electric current from the wall from AC to DC, before sending it to the device.

How is the power grid organized?

The U.S. has two large grids with synchronous AC connections: the Western Interconnection and the Eastern Interconnection. Most of Texas operates on its own grid, which does not share any AC connections with its neighbors. The Texas grid, however, is able to import some electricity from its neighbors on DC connections.

Within these interconnections, there are numerous independent system operators, or ISOs, and regional transmission organizations, RTOs. These are two different types of organizations — typically nonprofits — that operate the grid. The main difference is that ISOs play a more active role than RTOs in creating the market where electricity is bought and sold within the system.

Whatever acronym they use, the grid operators’ mission is to balance electricity supply and demand within their region. A mismatch between the amount of electricity flowing on the grid and consumer demand can cause problems in the frequency and voltage of the grid that may break critical equipment and cause blackouts.

The grid operators continuously monitor the grid and create an efficient system for buying and selling electricity to ensure the lights never go out. That’s more challenging than it might seem, because operators have to closely monitor how much power is flowing through the system.

Each regional grid contains two separate systems for moving electricity: transmission and distribution. The transmission grid connects electricity generation to areas where the power is consumed on large transmission lines — the towering power lines that can cover long distances. But before electricity goes into homes and businesses, it must travel on the distribution grid — your typical neighborhood power lines.

Transmission lines have a higher voltage capacity than distribution lines, so they can carry more electricity on one power line.

Substations connect the transmission and distribution grids by converting the power from transmission lines to a lower voltage that the distribution system can handle. All of this infrastructure is typically managed by local and regional utility companies.

Why is balancing the grid important?

Power grid operators need to maintain a delicate balance of frequency and voltage; otherwise, critical equipment can be damaged.

The frequency is the speed — measured in hertz — at which electric current changes direction in an AC system. Hertz measures how many times the current changes direction in one second.

It’s the same concept as frequency in music, where plucking a guitar string sends a wave back and forth. The low-frequency vibration of the grid creates an audible hum that can be heard around transmission lines and substations.

Voltage is more like the pressure utilized to move those electrons through the current. In the same analogy, the voltage is the amount of force used by a musician to pluck the string.

If the voltage becomes too high, it can damage or even fry electrical equipment. If the voltage is too low, the system cannot deliver sufficient power to meet demand.

The electrical grid needs to maintain a stable frequency at 60 hertz, and keeping the grid stable is the main responsibility of ISOs and RTOs. They do this by using dispatchable power to align supply and demand.

Thermal power plants naturally fit into the frequency of 60 hertz. Because they utilize big spinning turbines, the power plants have a natural inertia that can absorb slight changes in the frequency. That absorption gives the grid operator a little more flexibility to keep the system stable during variations in supply and demand.

However, wind, solar power and batteries — sometimes referred to as inverter-based resources because they need an inverter to convert DC power they generate into AC current — do not possess the same natural inertia to absorb frequency fluctuations. Luckily, new technologies are increasingly being deployed alongside inverter-based resources to decrease the risk of blackouts from a lack of inertia.

Why do blackouts happen?

The factors behind any individual power outage can be as simple as a tree falling on a power line or complex enough to warrant thousands of pages of reports from state and federal regulators. Small outages are usually fixed by local utility company crews, clearing debris and making repairs. 

In severe storms, thousands of small outages can compound into a major crisis, even though the cause of each outage remains relatively simple. For example, in 2024, Hurricane Beryl caused lengthy outages across the Houston area. The utility company CenterPoint faced thousands of outages from debris hitting power lines, leaving more than 2 million people without electricity — some for upwards of a month. 

Weather is also often a factor in more complex outages. The February 2021 Winter Storm, commonly known as Winter Storm Uri, brought power outages across multiple states.

Texas was the worst hit. During this storm, electricity demand to heat homes surged at the same time that various generation resources, including some wind farms and gas power plants, failed to operate in the frigid conditions.

The grid came dangerously close to a frequency imbalance that would have severely damaged grid infrastructure and caused a widespread blackout on the entire system run by the Electric Reliability Council of Texas, ERCOT. To avoid that cataclysm, the grid operators at ERCOT initiated rolling blackouts.

Because the electricity supply dipped too low, ERCOT shut off power to parts of the grid. That lowered the demand on the system and kept the grid frequency at a stable level around 60 hertz. The grid operator avoided the worst-case scenario of blacking out the entire system for an extended period, but some parts of Texas remained without power for weeks.