Electrical · Lesson 2 of 13Beginner9 min read

Volts, Amps, Ohms and Watts

Four little words do most of the work in electricity: volts, amps, ohms and watts. Once you know how they fit together, you can read an appliance label, predict when a breaker will trip, and check your electric bill. This lesson gives you the formulas and the reasons behind them.

Push, flow and resistance

Voltage, measured in volts (V), is the electrical push that makes charge move. Current, measured in amperes or amps (A), is how much charge flows past a point each second. Resistance, measured in ohms (Ω), is how hard the path is to push through. A thin wire, a long wire, or the glowing coil in a toaster has more resistance than a short, thick copper wire.

Ohm’s law ties the three together: voltage equals current times resistance. Flip it around and you can find any one from the other two. A heating element with 12 ohms of resistance, plugged into a 120-volt outlet, draws 120 ÷ 12 = 10 amps.

V = I × R (volts = amps × ohms, so I = V ÷ R and R = V ÷ I)

Watts: how fast energy is used

Power, measured in watts (W), is how fast a device uses energy. For any device, power is voltage times current. A kilowatt (kW) is 1,000 watts. For a simple heater, you can also write power as current squared times resistance, or voltage squared divided by resistance. That I²R form is the one that explains why wires get hot.

Most appliances list their watts on a label or nameplate, usually on the bottom or back. If the label gives only amps, multiply by the voltage: a 3-amp shredder on 120 volts uses 120 × 3 = 360 watts. Going the other way, a 1,500-watt electric kettle on 120 volts draws 1,500 ÷ 120 = 12.5 amps. That number matters when you share a circuit (Lesson 4).

P = V × I (watts = volts × amps, so amps = watts ÷ volts)
P = I² × R = V² ÷ R (for a resistor such as a heating element)

Kilowatt-hours and your bill

Your electric company doesn’t charge for watts. It charges for energy: power multiplied by time. The unit on your bill is the kilowatt-hour (kWh), one kilowatt used for one hour. A 40-watt bulb that’s on for five hours uses 200 watt-hours, or 0.2 kWh.

The U.S. Department of Energy gives a simple recipe: multiply an appliance’s watts by the hours you use it each day, then divide by 1,000 to get kWh per day. Multiply by the number of days, then by your price per kWh. A 1,500-watt kettle used one hour a day uses 1.5 kWh a day, or 547.5 kWh a year.

Prices vary from place to place. The U.S. average residential price was 18.31 cents per kWh in July 2026, according to the U.S. Energy Information Administration. At that price, the kettle costs about $100 a year to run. In 2022 the average U.S. home bought about 10,791 kWh of electricity, about 899 kWh a month. Try your own appliances below; your bill shows your actual price.

Wattages are typical ranges from Virginia Cooperative Extension; your appliance’s label is better. The starting price is the U.S. average residential price for July 2026 (EIA); type in the price from your own bill.

Alternating current at 60 hertz

A battery pushes current one way only: direct current (DC). The power at your outlets is alternating current (AC). The voltage swings from about +170 volts to −170 volts and back, 60 times every second (60 hertz, Hz), so the current keeps reversing direction.

Why call it 120 volts, then? Because a swinging voltage delivers the same average heating power as a steady voltage of its peak divided by √2. That “root mean square” value is 170 ÷ 1.414 ≈ 120 volts. When you see 120 or 240 volts for AC, that’s the rms value.

V_rms = V_peak ÷ √2 (170 V ÷ 1.414 ≈ 120 V)

120 and 240 volts from the same service

Most U.S. homes get “120/240-volt” service. Utilities try to hold 120/240 volts at the meter. The service has three main wires: two “hot” legs and a neutral. Each hot leg is 120 volts from the neutral, and the two hot legs are 240 volts from each other, because they swing in opposite directions. When one is at +170 volts, the other is at −170 volts.

Ordinary outlets and lights use one hot leg and the neutral: 120 volts. Big loads such as electric ranges, clothes dryers, water heaters and Level 2 electric-car chargers connect across both hot legs: 240 volts. The reason is the same one that drives the grid. For the same power, twice the voltage means half the current, and the wires waste a quarter as much heat. A 4,500-watt water heater draws 18.75 amps at 240 volts. At 120 volts it would need 37.5 amps.

Ordinary outlets use one hot leg and the neutral (120 V). Dryers, ranges and water heaters use both hot legs (240 V). Sources: University of Hawaiʻi Maui College; University of Missouri Extension; OpenStax 20.5 for the ±170 V peaks.

Why AC won

A transformer is two coils of wire wound on an iron core. Alternating current in the first coil makes a changing magnetic field, and that field makes a voltage in the second coil. The voltage ratio equals the ratio of the number of turns. Because the power out nearly equals the power in, a transformer that raises the voltage lowers the current by the same ratio. Transformers need a changing current to work, which is a big reason the grid is AC: it’s much easier to raise and lower AC voltages than DC.

V_s ÷ V_p = N_s ÷ N_p (secondary to primary voltage = ratio of turns; power in ≈ power out)

The units at a glance
Quantity	Unit	Think of it as	At home
Voltage	volt (V)	The push	120 V outlets; 240 V for big appliances
Current	ampere, amp (A)	The flow	A 15- or 20-amp breaker sets each circuit’s limit
Resistance	ohm (Ω)	How hard the path is	Heating elements; thin or long wires
Power	watt (W), kilowatt (kW)	How fast energy is used	A 1,500-W kettle; a 9-W LED bulb
Energy	kilowatt-hour (kWh)	Power × time	What your bill charges for
Frequency	hertz (Hz)	Swings per second	U.S. power is 60 Hz

WhyWhy the breaker trips when the toaster and kettle run together

Each appliance on a circuit adds its own current. A 1,500-watt kettle draws 12.5 amps and a 1,100-watt toaster about 9.2 amps. Together that’s about 21.7 amps, more than a 20-amp breaker allows, so sooner or later it trips. Lesson 4 has a game where you can try it.

Check yourself

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

A 120-volt outlet powers a heater with 12 ohms of resistance. How much current flows?
Show the answer
10 amps. I = V ÷ R = 120 ÷ 12 = 10 amps.
An appliance label says 1,200 watts. How many amps does it draw on 120 volts?
Show the answer
10 amps. Amps = watts ÷ volts = 1,200 ÷ 120 = 10 amps.
What does your electric bill charge for?
Show the answer
Kilowatt-hours of energy. You pay for energy: power multiplied by time, in kilowatt-hours.
U.S. outlet voltage swings between about +170 and −170 volts. Why is it called 120 volts?
Show the answer
120 V is the rms value, the steady voltage that would heat the same. The rms value is the peak divided by √2: 170 ÷ 1.414 ≈ 120 V.
Why do electric dryers and ranges use 240 volts?
Show the answer
For the same power, the current is half, so the wires heat less. P = V × I. Twice the voltage means half the current, and wire heating (I²R) drops to a quarter.

Go deeper

OpenStax College Physics 2e, 20.2 Ohm’s Law: Resistance and Simple Circuits Ohm’s law, with worked examples.
OpenStax College Physics 2e, 20.4 Electric Power and Energy Power, energy and the cost of running a device.
OpenStax College Physics 2e, 20.5 Alternating Current versus Direct Current AC, rms values and why the grid uses high voltage.
U.S. Energy Information Administration: Electricity Explained, Measuring Electricity How electricity is measured.
U.S. Department of Energy, Energy Saver: Estimating Appliance and Home Electronic Energy Use The Department of Energy’s appliance energy calculator.
Science Lab: Circuit Builder Wire a circuit and read real meters.
Being an Adult 101: Reset a Tripped Circuit Breaker The watts = volts × amps math for a tripped breaker.

Sources

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A free course from Teddy Rook Book. It explains how home electrical systems work in general and follows U.S. sources; your local electrical code, your inspector and a licensed electrician are the final word for your home.