Electrical power is the rate at which electrical energy is transferred through a circuit. It’s measured in watts (W), where one watt equals one joule of energy transferred per second. If you know the voltage and current in a circuit, you can calculate power with a simple formula: power equals voltage multiplied by current (P = V × I). That single relationship underlies everything from the rating on a light bulb to the output of a power plant.
Power vs. Energy
Power and energy are related but fundamentally different. Power describes how fast energy flows at any given moment. Energy describes the total amount of work done over time. A useful analogy: energy is the water in a tank, and power is the flow rate coming out of the tap. You can store energy for later use, but power is always a real-time measurement of how quickly that energy is being used or converted.
The relationship between them is straightforward: power equals energy divided by time (P = E/t). Flip that around and you get energy equals power multiplied by time. This is exactly why your electric bill is measured in kilowatt-hours (kWh) rather than kilowatts. A kilowatt-hour represents one kilowatt of power sustained for one hour. Running a 1,000-watt space heater for one hour uses 1 kWh of energy. Running it for three hours uses 3 kWh. The heater’s power rating stays the same, but the energy consumed grows with time.
One kilowatt-hour equals 3.6 million joules, which gives you a sense of how much energy everyday appliances actually consume.
How Power Is Calculated
The core formula is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amps. A 120-volt outlet supplying 10 amps of current delivers 1,200 watts of power.
By combining this with Ohm’s law (V = I × R, where R is resistance), you get two additional versions of the power formula that are useful when you don’t know all three values:
- P = I² × R: useful when you know the current and resistance but not the voltage
- P = V² ÷ R: useful when you know the voltage and resistance but not the current
All three formulas give the same answer. Which one you use depends on what information you have.
Units and Scale
One watt is a very small amount of power, so larger units are used depending on the context. One kilowatt (kW) equals 1,000 watts. One megawatt (MW) equals 1,000 kilowatts, or one million watts. One gigawatt (GW) equals 1,000 megawatts, or one billion watts.
To get a feel for these scales, here are some common reference points:
- LED light bulb: roughly 10 watts
- Laptop: about 50 watts
- Refrigerator: around 725 watts
- Portable space heater: 750 to 1,500 watts
- Utility-scale solar farm: typically 50 to 100 MW or more
- Large nuclear power plant: around 1,000 MW (1 GW)
In 2022, U.S. solar installations averaged approximately 73 MW per facility, with more than half of all installed capacity coming from systems larger than 100 MW. Total U.S. utility-scale solar capacity is projected to grow from 148 GW in 2024 to over 900 GW by 2050, according to the National Renewable Energy Laboratory.
Power in AC Circuits
The formula P = V × I works cleanly for direct current (DC), where voltage and current flow steadily in one direction. Alternating current (AC), which is what comes out of your wall outlets, adds a wrinkle. In AC circuits, the voltage and current rise and fall in waves, and they don’t always peak at the same moment. When they’re out of sync, not all of the electrical power does useful work.
This creates three distinct types of power in AC systems:
- True power (P): the portion that actually performs work, like heating an element or turning a motor. Measured in watts (W).
- Reactive power (Q): the portion that gets absorbed and returned by components like motors and transformers without doing useful work. Measured in volt-amps-reactive (VAR).
- Apparent power (S): the total combination of true and reactive power, representing everything the utility has to supply. Measured in volt-amps (VA).
For AC circuits, true power is calculated as P = V × I × cos(φ), where φ (phi) is the phase angle between voltage and current. When voltage and current are perfectly in sync, cos(φ) equals 1 and all the apparent power is true power. The more out of sync they are, the more reactive power exists and the less efficiently the circuit operates.
What Power Factor Means
Power factor is the ratio of true power to apparent power. It ranges from 0 to 1, where 1 means every bit of power the utility delivers is doing useful work. A power factor of 0.7 means 70% of the supplied power is being used productively.
This typically matters only for commercial and industrial customers. Homes generally have a high enough power factor that utilities don’t track it on residential bills. But factories and large commercial buildings with many motors, compressors, and similar equipment often have lower power factors because those devices generate significant reactive power. Some utilities charge a penalty when a facility’s power factor drops below 0.90 or 0.95, since the utility still has to generate and transmit all that apparent power even though the customer isn’t using all of it productively.
Improving power factor doesn’t reduce actual energy consumption, but it can lower demand-related charges on a commercial electric bill.
How Power Is Measured
A wattmeter measures electrical power by combining the functions of an ammeter and a voltmeter. It contains two internal coils: one that senses current flowing through the circuit and one that senses the voltage across the load. The meter multiplies these readings together to display the power being consumed.
In AC circuits, the wattmeter automatically accounts for any phase difference between voltage and current. Its reading is proportional to V × I × cos(φ), so it displays true power rather than apparent power. This makes it a practical tool for determining how much power a device or circuit is actually using. Consumer-grade versions (often called “kill-a-watt” meters) plug into a standard outlet and let you measure the real-time power draw of any appliance in your home.