Power or energy? What is the difference between kW and kWh?

Switch a home’s appliances on and watch power and energy part ways

Lesson 1 of Energy Systems, Step by Step: switch appliances on and off and watch power (kW) and energy (kWh) part ways, place real amounts of energy from a phone charge to the U.S. year on a ladder of units, estimate the country’s average power, and test a wind-farm claim.
  • Foundations · L1
  • Lesson 1 of 14
  • 15 min
  • No prerequisites

The idea in one table

Amount of energy Rung What it is
19 Wh Wh Charging a smartphone once
29.6 kWh kWh An average U.S. home's day
10.8 MWh MWh An average U.S. home's year
7.0 GWh GWh Prairie Town's day in lesson 0 (3 September 2025)
299 TWh TWh The Southwest Power Pool's year (2025)
4,200 TWh TWh The whole United States' year (2025)

Power is how fast energy flows (kW); energy is how much has flowed (kWh = kW × hours). A 1.5 kW hair dryer for 10 minutes and a 0.05 kW laptop for 5 hours both use 0.25 kWh.

With JavaScript on, you switch a home's appliances on and off, place these amounts on a ladder of units, and test a wind-farm claim.

Appliance powers are typical values, rounded and illustrative. Prairie Town's day is lesson 0's: the real Southwest Power Pool shape of 3 September 2025 (EIA-930) scaled to the town, with lesson 0's hourly MW, which add up to 6,957 MWh. Every other number has a source and a date below.

Need a refresher? Toolbox K1 (units and prefixes, kW against kWh) and K2 (rates, such as the average power a home buys).

What you just did, as diagrams

Lesson 0 used both units without stopping on them: MW on every plant, MWh in the peaker question. Here they come apart.

Power (kW)
How fast energy flows, right now: the flow from a tap.
Energy (kWh)
How much has flowed over a stretch of time: the water in the tub.
Energy = power × time
1.5 kW for 10 minutes and 0.05 kW for 5 hours both give 0.25 kWh.
A ladder of ×1,000
Wh, kWh, MWh, GWh, TWh: from a phone charge to a nation's year.
Average and peak
The U.S. averaged about 479 GW in 2025. In one grid, the Southwest Power Pool, the 2025 peak hour needed about 1.6 times its average.
Go deeper · AnalystEnergy is the area under the power curve

When power changes in steps, energy is the sum of power times duration over the steps, in kWh when power is in kW and time in hours:

\[E = \sum_i P_i\,\Delta t_i .\]

The two jobs: \(1.5\ \text{kW} \times \tfrac{10}{60}\ \text{h} = 0.25\ \text{kWh}\) and \(0.05\ \text{kW} \times 5\ \text{h} = 0.25\ \text{kWh}\). Same area, very different shapes: a tall thin block and a long low one in the top panel of your chart.

Going the other way, average power is energy divided by time, \(\bar P = E / T\). For the U.S. in 2025: \(4{,}200\ \text{TWh} \div 8{,}760\ \text{h} \approx 479\ \text{GW}\).

Peak against average needs one data set. The Southwest Power Pool used 298.6 TWh in 2025, an average of 34.1 GW; its peak hour needed 54.4 GW, about 1.6 times the average. Lesson 2 reads both from the hourly data.

Physicists measure energy in joules: \(1\ \text{kWh} = 1{,}000\ \text{W} \times 3{,}600\ \text{s} = 3.6\ \text{MJ}\).

Go deeper · ModelerTime steps: where MW meets MWh in a model

In an hourly model a plant’s output \(p_t\) is in MW and each time step lasts \(\Delta t = 1\) h, so its energy in hour \(t\) is \(p_t\,\Delta t\) MWh: the same number, which hides the units.

\[E = \sum_t p_t\,\Delta t\]

With 15-minute data \(\Delta t = 0.25\) h. Leaving it out makes these energy-based terms four times too large.

Storage needs both kinds of number: MW (how fast it charges) and MWh (how much it holds); lesson 9 shows it. Lesson 10 writes a whole model down.

Go deeperWhat the board leaves out
  • Appliance powers are typical nameplate values; your own appliance's label gives its real one.
  • Many appliances cycle: a refrigerator or a heater switches on and off under its thermostat, so its average power is below its rating.
  • Devices on standby still draw a few watts, all day.
  • The board ignores losses in the home's wiring.

Check yourself

Myth or fact? “My electricity meter measures kilowatts per hour.”

  • Myth
  • Fact
Answer

Myth. A kilowatt-hour is kilowatts multiplied by hours: power kept up over time. “Per hour” would describe how fast the power itself changes.

Myth or fact? “The appliance with the highest power rating always costs the most to run.”

  • Myth
  • Fact
Answer

Myth. Cost follows energy, power × time: a 50 W laptop for 5 hours uses as much as a 1,500 W hair dryer for 10 minutes.

Myth or fact? “A 100 MW wind farm powers 100,000 homes.”

  • Myth
  • Fact
Answer

Myth, as stated: it divides 100 MW by 1 kW per home.

U.S. wind ran at 33.8% of its capacity in 2025. An average home bought 10,791 kWh a year (about 1,232 W on average), so the farm's yearly energy matches what about 27,400 homes buy.

That is an illustrative equivalent: on calm hours the farm makes little.

A 2 kW heater runs for 3 hours. How much energy does it use?

  • 6 kW
  • 6 kWh
  • 0.67 kWh
  • 5 kWh
Answer

6 kWh: energy = power × time = 2 kW × 3 h. “6 kW” names a power, not an amount.

Prairie Town used about 6,957 MWh on lesson 0's day. About how many GWh is that?

  • 0.7 GWh
  • 7 GWh
  • 70 GWh
  • 7,000 GWh
Answer

7 GWh: one GWh is 1,000 MWh, so divide by 1,000. Each rung of the ladder is a factor of 1,000.

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
19 Wh Electricity to charge a smartphone once (0.019 kWh). EPA, Greenhouse Gas Equivalencies Calculator · updated 4 Aug 2026
10,791 kWh Electricity bought by an average U.S. home in a year: about 29.6 kWh a day, an average of about 1,232 W bought from the grid. EIA, Frequently Asked Questions · data for 2022
4,200 TWh U.S. electricity use in 2025 (4.20 trillion kWh), the highest on record. EIA, Energy Explained · data for 2025
759 GW Record peak demand of the Lower 48 states, 29 July 2025. EIA, Today in Energy · as of August 2025
33.8% Capacity factor of U.S. utility-scale wind in 2025: its energy as a share of running flat out all year. EIA, Electric Power Monthly, Table 6.07.B · released 23 Oct 2026

Next on the starter path

Lesson 2 · Why does electricity demand rise and fall?

In the real world

Your bill counts kWh.

EIA's own worked example: a 40 W bulb used for five hours uses 200 Wh, or 0.2 kWh.

Many business tariffs also charge for the month's highest power, in $/kW. Analyst lesson A4 covers tariffs.

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.