Watts Calculation Formula: How to Calculate Power

This calculator works out electrical power in watts from voltage and current, or from current and resistance, depending on which two values you enter. You get the power in watts, and the calculator also shows the equivalent in kilowatts for larger loads. The core formula is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes.

Formula

The basic formula for electrical power is:

P = V × I

• P = power in watts (W)
• V = voltage in volts (V)
• I = current in amperes (A)

If you know resistance instead of current, use the equivalent forms derived from Ohm’s law (V = I × R):

• P = I² × R (when you know current and resistance)
• P = V² / R (when you know voltage and resistance)

All three formulas give the same result for DC circuits and for AC circuits with purely resistive loads (like heaters or incandescent bulbs). For AC circuits with reactive components (motors, transformers), the formula P = V × I gives apparent power in volt-amperes (VA), not true power in watts. True power is P = V × I × power factor.

Worked Example

Using the default values from the calculator: voltage = 120 V, current = 5 A.

1. Write the formula: P = V × I
2. Plug in the numbers: P = 120 × 5
3. Multiply: P = 600 W

The calculator shows 600.00 W, and 0.600 kW. This matches a typical US household circuit: a 120-volt outlet supplying 5 amps to a device like a powerful vacuum cleaner.

When to Use Each Formula

Choose the formula based on what you know:

• V and I – most common; use P = V × I. This is what you need when reading a device label that lists volts and amps.
• I and R – use P = I² × R. Useful when you know the resistance of a component (like a heating element) and the current flowing through it.
• V and R – use P = V² / R. Useful when you know the supply voltage and the resistance of the load, and you want to find power without measuring current.

All three are derived from the same two laws: the definition of power (P = V × I) and Ohm’s law (V = I × R).

How to Read the Result

The result in watts tells you the rate of energy conversion. One watt equals one joule per second. A 600 W device running for one hour consumes 600 watt-hours (0.6 kWh) of energy. Your electricity bill is based on kilowatt-hours, so dividing watts by 1000 gives kilowatts, then multiplying by hours gives energy.

For example, a 1500 W space heater running for 3 hours uses 1500 × 3 = 4500 Wh = 4.5 kWh. If your electricity rate is, say, $0.15 per kWh, that costs 4.5 × 0.15 = $0.675.

Common Mistakes and Misconceptions

• Mixing up AC and DC: For AC circuits with motors or transformers, the simple P = V × I overestimates true power. You need the power factor (usually between 0.7 and 0.9 for inductive loads).
• Using peak vs. RMS values: Household AC voltage (120 V or 230 V) is RMS. If you measure peak voltage with an oscilloscope, you must divide by √2 before using the formula.
• Confusing watts with watt-hours: Watts measure power (rate), watt-hours measure energy (total). A 100 W bulb uses 100 J/s; over 10 hours it uses 1000 Wh.
• Unit errors: Always convert milliamps to amps (divide by 1000) and kilovolts to volts (multiply by 1000) before plugging into the formula.

Limits of the Formula

The formula P = V × I is exact for DC circuits and for AC circuits with purely resistive loads. For reactive loads (inductors, capacitors, motors), it gives apparent power in VA. True power is P = V × I × cos(φ), where cos(φ) is the power factor. If you don’t know the power factor, you cannot calculate true power from just V and I.

Also, the formula assumes steady-state conditions. During startup, motors can draw several times their running current, so instantaneous power can be much higher than the steady-state calculation.

Practical Applications

• Sizing circuits: To check if a circuit can handle a load, add up the wattages of all devices on that circuit and divide by voltage to get current. Compare with the breaker rating (e.g., 15 A or 20 A).
• Choosing a generator: Add the wattages of appliances you want to run simultaneously. Choose a generator rated above that total, with extra headroom for startup surges.
• Estimating energy costs: Multiply wattage by hours used per day to get daily watt-hours, then convert to kWh and multiply by your electricity rate.
• Selecting a power supply: For electronics, the power supply must deliver at least the total wattage of all components. For a desktop PC, add CPU and GPU TDPs plus overhead.

Frequently Asked Questions

How do I calculate watts from volts and amps?

Multiply volts by amps. For example, a device that draws 2 amps from a 120-volt outlet uses 240 watts (120 × 2 = 240).

What is the difference between watts and volt-amps?

Watts measure real power (actual work done), while volt-amps (VA) measure apparent power (voltage × current) in AC circuits. For resistive loads they are equal; for reactive loads, watts = VA × power factor.

How many watts is 10 amps at 120 volts?

10 amps × 120 volts = 1200 watts. This is a common maximum for a 120-volt circuit (though continuous loads are limited to 80% of the breaker rating).

How do I convert watts to kilowatts?

Divide watts by 1000. For example, 1500 watts ÷ 1000 = 1.5 kilowatts. This is the unit used on electricity bills.

Does the formula work for three-phase power?

No, not directly. For three-phase systems, the formula is P = √3 × V_line × I_line × power factor, where V_line is line-to-line voltage. This calculator is for single-phase or DC.

Why is my device rated in VA instead of watts?

Devices with power supplies (like computers) often list VA because the power factor is unknown. To get watts, multiply VA by the device’s power factor (typically 0.6–0.9 for computer power supplies).

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in v | Voltage | V | 120 | 0
in i | Current | A | 5 | 0
out p | Power | W | v * i | 2
out pk | Power | kW | (v * i) / 1000 | 3