How does power reach your plug, and why does it sometimes fail?

Follow power from plant to lamp through each voltage step, then send a storm through the wires

F6 of Energy Systems, Step by Step: follow electricity from a power plant to a socket through each voltage step, see why lines run at high voltage, send a storm through a town’s feeders, choose a repair order and measure the outage with SAIDI and SAIFI.
  • Foundations · L1
  • 20 min guided · about 10 min practice
  • After lesson 3

The idea in one table: eight stops from a power plant to a socket

Stop Prairie Town (illustrative) Typical in the U.S. What it does
1 Generator 18 kV 5–34.5 kV Makes alternating current, which reverses direction many times a second
2 Step-up transformer 18 → 345 kV up to 115–765 kV Raises the voltage for the long trip
3 Transmission line 345 kV 115–765 kV Carries power far, in loops
4 Substation 345 → 12.47 kV down to below 34 kV Steps down; breakers protect each feeder
5 Distribution feeder 12.47 kV 4.8–34.5 kV Runs through the streets; laterals reach each block
6 Service transformer 12.47 kV → 240/120 V 240/120 V Steps down for a few homes: two live wires and a neutral
7 Service drop and meter 240/120 V 240/120 V Brings power into the home and counts the kWh
8 Socket 120 V 120 V (240 V for big appliances) Powers the lamp: 120 V between a live wire and the neutral, 240 V between the two live wires

Then a storm on Prairie Town's east side (30 homes, illustrative)

What failed Device that opened Homes dark Repair
A branch on Oak Street's lateral Its fuse 6 2 h
A limb on the north feeder's far half Recloser R1 10 3 h
A snapped pole on the south feeder The south feeder's breaker 10 6 h
A toppled tower on one line of the high-voltage loop None: in this drawing, power goes round the loop 0 12 h, by a separate crew
  • Losses. Sending 100 MW through a three-phase line with 5 ohms in each wire loses 10.5% at 69 kV but 0.42% at 345 kV: five times the voltage, one twenty-fifth of the loss. All U.S. lines and wires together lose about 5% (2018 through 2022).
  • Repairs. One crew, best order: north feeder's far half, then Oak Street, then the south feeder, for 10,200 customer-minutes.
  • Measures. The storm's SAIDI is 10,200 ÷ 30 = 340 minutes; its SAIFI is 26 ÷ 30 = 0.87 interruptions per customer (each dark home lost power once). A crew twice as fast halves SAIDI; SAIFI stays the same.
  • The real U.S. Among customers of utilities using the IEEE method, 2025 averaged 269.2 minutes without power and 1.34 interruptions, or 125.3 minutes without major event days. Over 90% of interruptions start on the distribution system.

With JavaScript on, you work through this step by step: the path, a voltage slider, a network you can break, a storm, a repair order and the real U.S. numbers.

  • Voltages (real). DOE's grid backgrounder (November 2023, p. 2) and distribution baseline report (2016, pp. 27, 38).
  • Losses (real). EIA, updated 7 November 2023.
  • Reliability (real). EIA, Electric Power Annual, Table 11.1, released 21 October 2026: utilities using the IEEE 1366 method.
  • Illustrative. Prairie Town's voltages within those ranges, its east-side network and 30 homes, the line's 5 ohms per wire, the storm and its repair hours.

Need a refresher? Lesson 3 keeps the grid balanced second by second; F1 places feeders and microgrids on the five scales.

What you just did, as diagrams

Voltage steps
Transformers raise the voltage from 5–34.5 kV at the plant to 115–765 kV for transmission, then lower it in steps to 240/120 V at the home.
Why high voltage
The same power at a higher voltage needs less current, and the heat lost in a wire grows with current squared.
Meshed and radial
Transmission lines form loops, so if one fails, power can take another path when the others have room; most feeders are radial, with one path to each home.
Protection
In this example, protection is set so the closest fuse, recloser or breaker upstream of a fault opens first, darkening only the section below it.
SAIDI and SAIFI
SAIDI is customer-minutes without power ÷ customers served; SAIFI is customer interruptions ÷ customers served, and a customer interrupted twice counts twice.
Where outages start
Over 90% of U.S. interruptions start on the distribution system, mostly in bad weather (DOE, 2017).
Go deeper · AnalystSAIDI, SAIFI, CAIDI and major event days

Utilities report three yardsticks to EIA each year (Form EIA-861). All three count only non-momentary interruptions.

Yardstick What it measures Unit
SAIDI Minutes without power, per customer, in a year minutes
SAIFI Interruptions per customer in a year; a customer interrupted twice counts twice interruptions
CAIDI Average minutes to restore one interruption: SAIDI ÷ SAIFI minutes

A major event day (IEEE 1366) is a day whose SAIDI exceeds a threshold set from the past five years of daily values. Reports give the yardsticks with and without them:

U.S., IEEE method 2024 2025
SAIDI, all events 662.6 min 269.2 min
SAIDI without major event days 131.6 min 125.3 min
SAIDI, loss of supply removed 646.1 min 246.8 min
SAIFI, all events 1.531 1.34
SAIFI without major event days 1.065 1.08

Source: EIA, Electric Power Annual 2025, Table 11.1 (released 21 October 2026). The minutes without major event days stay near two hours a year; the stormy days swing the total.

  • Try it. CAIDI for 2025: 269.2 ÷ 1.34 = 201 minutes per interruption.
  • Read with care. Utilities that do not use the IEEE method decide their own major events, so EIA gives the IEEE group on its own and all utilities together (“any method”).
  • Loss of supply. That column can cover a different group of utilities (its 2020 value is above all events), so its gap from all events is not the share caused upstream.
Go deeper · AnalystN−1 is more than a second path

The loop in this lesson only checks that a path is left. Planners check more for N−1: after any one line fails, can the remaining lines carry the power within their limits?

  • Read next. Analyst lesson A17 checks a plan against grid physics; A9 plans for hazards; F8 puts a large new load on the town’s grid, and A6 checks its substation with one line out.
  • Later. M1 writes a small network’s rules for one case.
Go deeperWhat this lesson leaves out
  • Reactive power and voltage: the slider's three-phase line carries real power only; real power flow also needs voltage and reactive power (Analyst lesson A17).
  • Protection settings: how fast each fuse, recloser and breaker acts. This lesson assumes they are coordinated, so the closest one opens first.
  • Momentary interruptions: a recloser that trips and closes again within seconds, which SAIDI does not count.
  • Underground cables, rooftop solar pushing power back up a feeder, and switches that reroute feeders.
  • Crews, travel time, parts and priority loads such as hospitals: this lesson's crew weighs only the number of homes.
  • Supply and line limits: the network checks only connection, not how much each line can carry; keeping supply equal to demand is lesson 3.

Check yourself

Practice, about 10 minutes: three myths, then six questions.

Myth or fact? “Most power outages start at power plants.”

  • Myth
  • Fact
Answer

Myth. DOE found that over 90% of U.S. power interruptions start on the distribution system, mostly in bad weather (2017). Plant and transmission failures are rarer, but can darken many customers at once.

Myth or fact? “A higher-voltage line wastes more energy.”

  • Myth
  • Fact
Answer

Myth. For the same power, a higher voltage needs less current, and the heat lost falls with the square of the current: five times the voltage, one twenty-fifth of the loss.

Myth or fact? “SAIDI counts every flicker of the lights.”

  • Myth
  • Fact
Answer

Myth. EIA's SAIDI counts non-momentary interruptions only. A recloser that trips and closes again within seconds, after a branch brushes a wire, causes a momentary one.

A utility serves 10,000 customers. A fault darkens 400 of them for 90 minutes. How much does it add to the year's SAIDI?

  • 3.6 minutes
  • 36 minutes
  • 90 minutes
  • 0.04 minutes
Answer

3.6 minutes: 400 × 90 = 36,000 customer-minutes, ÷ 10,000 customers. Its SAIFI share is 400 ÷ 10,000 = 0.04 interruptions per customer.

A branch falls on a lateral that has its own fuse. What opens?

  • The substation transformer
  • The lateral's fuse
  • The feeder's breaker at the substation
Answer

The lateral's fuse. When protection is coordinated, as here, the closest device upstream of the fault opens first, so only the homes on that lateral go dark.

Double the line's voltage and send the same power. What happens to the heat lost in the wire?

  • It doubles
  • It halves
  • It falls to a quarter
  • It stays the same
Answer

A quarter: the current halves, and the loss goes with current squared, (½)² = ¼.

Which voltage does a typical U.S. wall socket give?

  • 120 V
  • 12.47 kV
  • 345 kV
  • 5 V
Answer

120 V. Homes get 240/120 V service; 12.47 kV is a common feeder voltage, and 345 kV a transmission voltage.

In the lesson's high-voltage loop, one of the three lines falls. How many homes go dark?

  • All 30
  • 10
  • 6
  • None
Answer

None: power takes the other way round the loop, which the lesson assumes can carry it. Only when both lines into the substation fall do all 30 homes go dark.

In 2025, SAIDI for U.S. utilities using the IEEE method was 269.2 minutes with major event days and 125.3 minutes without them. About what share came on major event days?

  • About 47%
  • About 53%
  • About 27%
  • About 215%
Answer

About 53%: (269.2 − 125.3) ÷ 269.2 = 0.53. In 2024, a year of hurricanes, it was about 80%.

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
115–765 kV Typical U.S. transmission voltages; plants make electricity at 5–34.5 kV. DOE CESER, grid backgrounder, p. 2 · November 2023
4.8–34.5 kV The four common U.S. distribution circuit voltages: 4.8, 12.47, 22.9 and 34.5 kV. PNNL for DOE, distribution baseline report, p. 27 · July 2016
240/120 V The service a typical U.S. home gets from its local feeder. PNNL for DOE, distribution baseline report, p. 38 · July 2016
about 5% Electricity lost in U.S. transmission and distribution, average of 2018 through 2022. EIA, Frequently Asked Questions · updated 7 November 2023
269.2 min U.S. SAIDI in 2025, utilities using the IEEE method: minutes without power per customer, all events (1.34 interruptions per customer, SAIFI). EIA, Electric Power Annual, Table 11.1, IEEE method · released 21 October 2026
125.3 min U.S. SAIDI in 2025 without major event days, utilities using the IEEE method. EIA, Electric Power Annual, Table 11.1, IEEE method · released 21 October 2026
over 90% Share of U.S. power interruptions that start on the distribution system, mostly in bad weather. DOE, Quadrennial Energy Review, Chapter IV, p. 4-2 · January 2017

Next

F7 · What is a microgrid? From a hospital that keeps its lights on to a village with no grid

In the real world

U.S. utilities report SAIDI and SAIFI to EIA every year. In 2024, hurricanes Beryl, Helene and Milton made it the worst year in ten: 11 hours without power on average, 80% of them from major events (EIA, December 2025).

Sources and further reading

Spotted an error, or stuck? Write to hlli@ksu.edu. Corrections are made in place and dated here.

Text licensed CC BY 4.0, code MIT: teachers and students may reuse and adapt it, with credit to Honglin Li.