The Big Picture: From Power Plant to Outlet

It starts with a spinning magnet

Most power plants make electricity with a generator. Moving a magnet inside a coil of wire makes an electric current flow in the wire. A power plant does the same thing on a giant scale: a turbine spins a magnet past coils of wire, and current flows.

What spins the turbine depends on the plant. In a turbine generator, a moving fluid (water, steam, hot combustion gases or air) pushes a series of blades. Steam comes from burning coal or natural gas or from a nuclear reactor; dams use falling water; wind turbines use the wind. Solar panels are the big exception: they make electricity straight from sunlight with no moving parts (Lesson 10).

In 2022, natural gas plants made about 40 percent of the electricity from large U.S. power plants, coal about 19 percent, nuclear about 18 percent, wind about 10 percent, hydropower about 6 percent and large solar plants about 3 percent.

The journey to your house

Electricity travels from power plants to homes through a connected network of lines and equipment called the grid. The U.S. grid has thousands of miles of high-voltage power lines and millions of miles of low-voltage lines. Here’s the trip, step by step:

1. Step up. Next to the power plant, transformers raise the voltage. For long-distance transmission it’s raised to over 200,000 volts (200 kilovolts, kV), sometimes as high as 700 kV.
2. Transmission. High-voltage lines, the ones strung between tall metal towers, carry the power over long distances.
3. Substation. Near the towns that use the power, transformers in substations step the voltage back down.
4. Distribution. Lower-voltage lines run along streets on poles or underground, usually at 5 to 13 kV.
5. The last transformer. A transformer on a pole or in a box on the ground drops the voltage to 120 and 240 volts for homes.
6. The service. Wires run from that transformer to your house, overhead (the service drop) or underground. They pass through the electric meter and into your main panel, which splits the power into branch circuits for each part of the house.

Along the way, some energy turns into heat in the wires. The U.S. Energy Information Administration estimates that about 5 percent of the electricity sent out was lost in transmission and distribution in 2018 through 2022.

Why such high voltage?

Power is voltage times current: P = V × I. To send the same power, you can use high voltage and a small current, or low voltage and a big current. The wires don’t care about voltage, but they do care about current. A wire with resistance R wastes power as heat at a rate of I² × R. Double the current and you get four times the heat.

So the grid sends power at high voltage to keep the current small. The OpenStax physics textbook works an example: 100 megawatts sent at 200 kV needs only 500 amps, and through lines with 1 ohm of resistance it loses 250 kilowatts, just 0.25 percent. Send the same power at 25 kV and the current jumps to 4,000 amps, and the lines waste 16 megawatts, 16 percent of it. Try it below.

This is also why the grid uses alternating current (AC). Transformers can raise and lower AC voltages easily and efficiently, often better than 99 percent. Doing the same with direct current is much harder. You’ll see the math in Lesson 2.

A map of the house

Inside the house, the electrical system has a few main parts. The meter measures how much energy you use. The main panel holds the main breaker and a breaker for each circuit. Branch circuits carry power from the panel to outlets, switches, lights and appliances. Big appliances such as a dryer, range or water heater get their own 240-volt circuits. And a set of protective parts, the ground wires and grounding electrode, GFCIs, AFCIs and alarms, is there to keep people and the house safe when something goes wrong.

Point at a part of the house below to see what it does, and click to open its lesson. Switch between Power, how electricity gets where it’s going, and Protection, what keeps it from hurting anyone.

Check yourself

Tap an answer to check it, or open “Show the answer.”

1. What does a power plant’s generator do?

   • Stores electricity for later
   • Turns motion into electricity by moving magnets past coils of wire
   • Changes AC into DC
   • Raises the voltage for transmission
   Show the answer

   Turns motion into electricity by moving magnets past coils of wire. A turbine spins a magnet past coils of wire, and the changing magnetic field makes current flow.

2. Why is power sent over long distances at very high voltage?

   • High voltage travels faster
   • For the same power, the current is smaller, so much less is lost as heat in the wires
   • Homes need high voltage
   • Transformers only work above 200 kV
   Show the answer

   For the same power, the current is smaller, so much less is lost as heat in the wires. Power is V × I, and heat lost in a wire is I² × R. Higher voltage means less current and far less loss.

3. In the OpenStax example, how much of 100 MW is lost in 1-ohm lines at 25 kV instead of 200 kV?

   • 0.25 percent
   • About 1 percent
   • 16 percent
   • None
   Show the answer

   16 percent. At 25 kV the current is 4,000 A, and 4,000² × 1 Ω = 16 MW, which is 16 percent.

4. What is the service drop?

   • The main breaker
   • The overhead wires from the utility’s last pole to your house
   • A wire inside your walls
   • The meter display
   Show the answer

   The overhead wires from the utility’s last pole to your house. The service drop is the overhead service wiring from the utility’s pole to the building. Some homes get underground service instead.

5. About what share of U.S. electricity was lost in transmission and distribution in 2018 through 2022, according to EIA?

   • About 5 percent
   • About 25 percent
   • About half
   • Less than 0.1 percent
   Show the answer

   About 5 percent. EIA estimates annual transmission and distribution losses at about 5 percent.

Go deeper

• U.S. Energy Information Administration: Electricity Explained, Delivery to Consumers How the grid delivers electricity, from the EIA.
• U.S. Energy Information Administration: Electricity Explained, How Electricity Is Generated How power plants make electricity.
• OpenStax College Physics 2e, 23.7 Transformers Transformers and the transmission system, with the math.
• Science Lab: Circuit Builder Build your own circuits and watch the current flow.
• Being an Adult 101: Reset a Tripped Circuit Breaker Find your panel and reset a breaker.

Sources

• U.S. Energy Information Administration: Electricity Explained, How Electricity Is Generated
• U.S. Energy Information Administration: Electricity Explained, Delivery to Consumers
• U.S. Energy Information Administration: Electricity Explained, Batteries, Circuits, and Transformers
• U.S. Energy Information Administration FAQ: How Much Electricity Is Lost in Transmission and Distribution?
• U.S. Energy Information Administration: Electricity Explained, Measuring Electricity
• OpenStax College Physics 2e, 23.7 Transformers
• OpenStax College Physics 2e, 20.5 Alternating Current versus Direct Current
• OSHA 29 CFR 1910.399: Definitions (Electrical)
• Lawrence Berkeley National Laboratory: Sizing Electric Service Panels and Utility Infrastructure for Residential Electrification (technical brief, July 2024)
• OSHA Fact Sheet: Working Safely with Electricity (FS-3942, 2018)
• OSHA Fact Sheet: Working Safely Around Downed Electrical Wires (FS-3941, 2018)