Lesson 1 of 8 · about 14 minutes

Electricity Units: Volts, Watts and Kilowatt-Hours

You'll be able to

Explain volts, amps and watts in plain words
Tell power (kW, MW, GW) from energy (kWh, MWh, TWh) and convert between them
Check a 'homes powered' claim by asking which definition was used

Almost every energy fight you will ever follow turns on two numbers that people mix up: how fast electricity is being used, and how much was used in total. Get those two straight and you can catch errors that slip past council members, press offices and sometimes the utility's own slide deck.

Volts, amps and watts

The water-in-a-pipe picture works well enough for anyone following a power debate. Voltage is the pressure. Current, measured in amps, is how much is flowing. Power, measured in watts, is the work being done each second, and it equals the two multiplied together.

watts = volts x amps

A U.S. household outlet: about 120 volts
A hair dryer drawing 12.5 amps: 120 x 12.5 = 1,500 watts

You will rarely need volts and amps at a public hearing. They matter for wiring, substations and transmission lines, which come up in the next lesson. For most energy questions, the watt and its big siblings do the work.

Power is a rate. Energy is an amount.

A watt is a rate, like miles per hour. A watt-hour is an amount, like miles driven. If you run a 1,000-watt microwave for one hour, you have used 1,000 watt-hours, which is 1 kilowatt-hour (kWh). Run it for six minutes and you used a tenth of that.

The kilowatt-hour is the unit on your bill. The kilowatt (kW) is the unit on the side of an appliance or in a utility's peak demand number. They are related by time, and only by time.

energy (kWh) = power (kW) x time (hours)
power (kW)   = energy (kWh) / time (hours)

Bar height is power. The area under all the bars is energy. A utility plans its wires and plants around the tallest bar, and bills you mostly for the area.

The scale ladder

Every step up is a factor of 1,000. Memorize this table once and you are set for life.

Power (rate)	Energy (amount)	Size of thing you will see it on
watt (W)	watt-hour (Wh)	Phone charger, LED bulb
kilowatt (kW) = 1,000 W	kilowatt-hour (kWh)	Appliances, a home's draw, your monthly bill
megawatt (MW) = 1,000 kW	megawatt-hour (MWh)	A wind turbine, a small solar farm, a factory, wholesale power prices
gigawatt (GW) = 1,000 MW	gigawatt-hour (GWh)	A large power plant, a utility's peak demand, a giant data center campus
terawatt (TW) = 1,000 GW	terawatt-hour (TWh)	A state's or nation's yearly use. You will almost never see TW alone.

For a sense of the top of the ladder: EIA counts about 1,281 GW of utility-scale generating capacity in the U.S. at the end of 2025, and about 4,429 TWh of utility-scale generation during 2025. The Department of Energy's data center report put data center use at 176 TWh in 2023, about 4.4 percent of U.S. electricity.

Worked conversions

Appliance to bill. A 1,500 W space heater runs 4 hours a night for 30 nights. 1.5 kW x 4 h x 30 = 180 kWh. At 13 cents per kWh, that is 180 x $0.13 = $23.40 for the month.
Bill to average draw. The average U.S. residential customer bought 10,791 kWh in 2022, per EIA. Divide by 8,760 hours in a year: 10,791 / 8,760 = 1.23 kW. That is the home's average draw. On a hot afternoon with the air conditioner and dryer running, the same home might pull 5 kW or more.
Plant to year. A 100 MW plant running flat out all year would make 100 MW x 8,760 h = 876,000 MWh, which is 876 GWh, or 0.876 TWh. Real plants make less. How much less is the subject of lesson 4.
TWh to homes. 176 TWh is 176,000,000 MWh, or 176,000,000,000 kWh. Divide by 10,791 kWh per home: about 16.3 million average homes' worth of yearly electricity.

How many homes does a megawatt serve?

This is the most abused number in energy debates, and the abuse is usually innocent. Honest people compute it in several ways, and the answers differ wildly.

Method	Math for 1 MW	Homes
Average draw (a plant running 24/7)	1,000 kW / 1.23 kW per home	about 810
Annual energy from a solar farm (EIA 2025 solar capacity factor 24.4%)	1 MW x 8,760 h x 0.244 = 2,137 MWh; / 10.791 MWh per home	about 198
Peak demand (assume 5 kW per home on the hottest hour)	1,000 kW / 5 kW	about 200

Same megawatt. A fourfold spread. And it gets worse, because the "average home" moves around. EIA's own FAQ shows Louisiana homes using about 14,774 kWh a year and Hawaii homes about 6,178. A Northwest home with electric heat and a Southwest home with heavy air conditioning have different peaks at different times of year. A press release that picks the friendliest method and the smallest home can double its homes figure.

When anyone says a project "powers X homes," ask three things: which homes (whose average usage, what year), peak or annual energy, and did they use nameplate megawatts or expected output. Quote the megawatts and the expected megawatt-hours. The homes number is a translation, and when you repeat it at a meeting, in testimony or online, say whose.

Common mix-ups

Claim: "The new data center will use 300 megawatts a year." What the evidence shows: Megawatts are a rate, so "per year" has no meaning attached to them. The speaker probably means 300 MW of demand, which at a steady 80 percent load would be 300 x 8,760 x 0.8 = 2,102,400 MWh, about 2.1 TWh a year.
Claim: "kW and kWh are basically the same." What the evidence shows: They differ by time. A 10 kW load for 6 minutes uses 1 kWh. A 0.1 kW load for 10 hours also uses 1 kWh. Utilities charge some customers separately for each, as lesson 7 shows.
Claim: "A gigawatt plant powers a gigawatt of homes." What the evidence shows: There is no such thing as a gigawatt of homes. Translate to homes only with a stated method, or leave it as megawatts and megawatt-hours.

Reporting angle

Ask the utility for its own average residential use per customer in the latest year. It is often in its annual report or its filings with the state commission. Use that number, not the national one, in local stories.
When a large customer applies for service, the application usually states requested capacity in MW. Ask for the expected load factor (how close to that maximum it will run on average) so you can turn MW into MWh.
Keep a scratch file of conversions you have checked. Editors will trust you faster when your math is shown.

Research angle

EIA FAQ on home electricity use: National average household use, to check against your utility's own figure.
EIA generation, capacity and sales: Plain definitions and U.S. totals for megawatts, megawatt-hours and sales.
EIA Electric Power Monthly, Table 6.7.B: Monthly capacity factors for non-fossil plants, for load and output math.
DOE report on data center demand: Federal estimates of how much electricity data centers may need.

Check yourself

1. A 1,500-watt space heater runs for 4 hours. How much energy does it use?

2. Which of these is a unit of energy, not power?

3. How many megawatt-hours are in one terawatt-hour?

4. The average U.S. home bought about 10,791 kWh in 2022. What is its average draw across the year?

5. A developer says a 100 MW solar farm will 'power 80,000 homes.' What is the best first question?

6. Watts equal volts multiplied by what?

Terms in this lesson

Volt: The unit of electrical pressure, the push that moves current through a wire.
Amp (ampere): The unit of current, how much electric charge flows past a point each second.
Watt: The unit of power, the rate at which energy is used or produced. Volts times amps.
Kilowatt-hour (kWh): A unit of energy: one kilowatt of power used for one hour. What your bill charges for.
Terawatt-hour (TWh): One billion kilowatt-hours. Used for the yearly output of states, nations and whole industries.

Full glossary

Sources

EIA FAQ: How much electricity does an American home use? checked 2026-10
EIA Energy Explained: Electricity generation, capacity, and sales in the United States checked 2026-10
EIA Electric Power Monthly, Table 6.7.B: Capacity factors for generators not primarily using fossil fuels checked 2026-10
U.S. Department of Energy: DOE releases new report evaluating increase in electricity demand from data centers checked 2026-10