What Is Power in Watts?

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A watt is the unit of power, which is the rate at which energy is used or work is done. NIST defines the watt as “unit of power, or rate of work, equal to one joule per second.” For an ordinary resistive load such as a heater or a light bulb, power in watts equals volts multiplied by amps. A multimeter reads volts and amps, so you can work out watts from two readings.

Quick answer

• One watt is one joule per second. A joule is a unit of energy.
• For a purely resistive circuit, power in watts equals volts times amps (P = V x I).
• A multimeter measures volts and amps. You calculate watts from those readings.
• In AC circuits with a phase difference, the simple formula does not give real power. Use the phase relationship or a power meter.

What a watt means

NIST’s joule entry states that “a watt is a joule/second.” It also defines the joule as the SI unit of energy, described as “the preferred SI unit of heat, energy, or work.” Put together, a watt is a measure of how fast energy moves or is converted. A 60 W bulb converts energy at a faster rate than a 10 W bulb, so it uses more energy in the same time.

NIST also gives an electrical meaning: one watt is “the rate of work represented by a current of one ampere under the potential of one volt.” That is the link between watts and the two values a multimeter reads.

The formula for a resistive load

Fluke’s multimeter glossary defines the watt as “the unit of electrical power in joules per second, equal to the voltage drop (in volts) times the current (in amperes) in a resistive circuit.” In symbols, P = V x I, where P is power in watts, V is voltage in volts, and I is current in amps.

Example: a circuit with 120 V across a load that draws 2 A has a power of 240 W, using P = 120 x 2. The same formula works for direct current and for a purely resistive AC load.

Real power and reactive power

Fluke’s glossary separates the terms. It defines real power as “the power dissipated in the purely resistive components of a circuit measured in watts.” It defines reactive power as “the product of the voltage and current in a reactive circuit measured in volt-amperes (apparent power).” That distinction matters for motors, transformers and some electronics. In those loads, volts times amps is not the same as the real power in watts. The product is the apparent power in volt-amperes.

For this reason, a reading of volts and amps from a multimeter gives watts only when the load is resistive, or when you know the phase relationship. Do not treat a multimeter product as a power meter unless its listing says it measures power.

Practical checks

• Measure voltage across the load with the meter set to AC or DC volts, as the circuit requires.
• Measure current in series only with the meter set to the correct amps range and input jack. Check the fuse rating of the meter first.
• Multiply the two readings. Label the result as apparent power if the load may be reactive.

Work on mains circuits can kill. Panel or service work belongs to a licensed electrician. Choose a meter whose CAT rating suits the circuit. If the listing does not state a CAT rating, do not use that meter on household mains.

How we chose the sources

We opened three sources on 2026-10-27: the NIST entry for the watt, the NIST entry for the joule, and the Fluke digital multimeter glossary. We quote the definitions directly. We did not use retail pages or product listings to define power.

What we could not verify

The sources we opened do not cover power factor values for specific equipment, so this article gives no typical phase angle for any appliance. They also do not give a method for measuring real power with a basic multimeter. Check the manual of any power meter you consider.

Frequently asked questions

Is a watt the same as a volt-amp?

Not in every case. Fluke’s glossary says volt-amperes measure apparent power, and watts measure real power in the resistive part of a circuit. The two are equal only for a purely resistive load.

How many watts is 120 volts at 10 amps?

For a resistive load, P = V x I = 120 x 10 = 1200 W.

Does a multimeter measure watts?

A basic multimeter measures volts, amps and resistance. You calculate watts from the volts and amps. Some models list a power function; check the listing before you rely on it.

What is a joule?

NIST describes the joule as the SI unit of heat, energy or work. A watt is one joule per second.

Why does power matter when I measure current?

Power tells you how much energy a load uses over time. The current and voltage readings together show whether a circuit is within the rating of its fuse or breaker.

Bottom line

For a resistive load, power in watts equals volts times amps. Use that formula only when you know the load is resistive, and check the CAT rating before you measure on any mains circuit.

Related guides

• Best true RMS multimeters, for meters that read non-sinusoidal AC more reliably.
• Best multimeters for home use, for basic models and their safety ratings.

Sources

• NIST, “Watt (W),” https://www.nist.gov/glossary-term/34606 (opened 2026-10-27)
• NIST, “Joule,” https://www.nist.gov/glossary-term/26261 (opened 2026-10-27)
• Fluke, “Digital Multimeter Glossary,” https://www.fluke.com/en-us/learn/blog/electrical/multimeter-common-terminology (opened 2026-10-27)