Watts Formula: How to Calculate Power

This calculator works out electrical power in watts using the standard formula P = V × I, where you enter the voltage in volts and the current in amps, and it returns the power in watts. It also gives the result in kilowatts for convenience. The formula is the fundamental relationship between electrical pressure, flow, and power, and it applies to any DC circuit and to AC circuits with a power factor of 1 (like resistive heaters and incandescent bulbs).

Formula

The formula to calculate watts is:

P (watts) = V (volts) × I (amps)

Where:

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

This is often called Watt’s law, and it is the electrical equivalent of the mechanical power formula P = F × v (force times velocity). It works for direct current (DC) and for alternating current (AC) when the load is purely resistive, meaning the voltage and current are in phase. For AC circuits with motors, transformers, or other inductive loads, the formula becomes P = V × I × power factor, where the power factor is a number between 0 and 1 (often around 0.8 for induction motors).

Worked Example

Using the default values from the calculator above (120 V and 2 A):

1. Start with the formula: P = V × I
2. Plug in the values: P = 120 V × 2 A
3. Multiply: P = 240 W

The calculator shows 240.00 W, which matches. To convert to kilowatts, divide by 1000: 240 ÷ 1000 = 0.240 kW. This is the power consumed by a device drawing 2 amps from a standard 120-volt US household outlet.

When to Use This Formula

Use P = V × I whenever you know any two of the three quantities (power, voltage, current) and need the third. It is the most common calculation in basic electrical work:

• Determining how much power a device uses (e.g., a space heater rated at 1500 W on a 120 V circuit draws 12.5 A).
• Checking if a circuit breaker will trip (a 15 A breaker on 120 V can supply up to 1800 W).
• Calculating energy consumption for billing (multiply watts by hours used to get watt-hours, then divide by 1000 for kWh).
• Sizing solar panels or batteries (a 12 V battery supplying 5 A delivers 60 W).

The formula also works in rearranged forms: V = P ÷ I and I = P ÷ V. For example, a 60 W light bulb on a 120 V circuit draws 0.5 A (60 ÷ 120).

How to Read the Result

The watt is a unit of power, which is the rate of energy transfer. One watt equals one joule per second. So a 240 W device converts 240 joules of electrical energy into heat, light, or motion every second. In one hour, that device consumes 240 watt-hours (0.24 kWh). For comparison:

• A typical LED bulb: 9–12 W
• A phone charger: 5–20 W
• A laptop: 45–100 W
• A microwave oven: 800–1500 W
• An electric water heater: 3000–4500 W

When the result is in kilowatts, multiply by hours used to get kilowatt-hours, which is what your electricity meter measures. For example, a 0.24 kW device running for 5 hours uses 1.2 kWh.

Common Mistakes and Misconceptions

A frequent error is mixing up units. Always convert milliamps (mA) to amps (A) by dividing by 1000. For instance, 500 mA = 0.5 A. Another common mistake is applying the DC formula to AC circuits without considering the power factor. For a motor rated at 240 V and 5 A with a power factor of 0.8, the real power is 240 × 5 × 0.8 = 960 W, not 1200 W. The 1200 W figure is called apparent power (measured in volt-amperes, VA), and it is what the utility must supply, but the real power is less.

Also, do not confuse watts with watt-hours. Watts measure instantaneous power; watt-hours measure energy over time. A 100 W bulb uses 100 watt-hours in one hour, but it is still 100 W at any instant.

Limits of the Formula

The simple P = V × I is accurate for DC circuits and for AC circuits with purely resistive loads. It does not account for:

• Power factor in inductive or capacitive AC loads (motors, fluorescent lights, transformers).
• Harmonics in non-linear loads (computers, LED drivers) that distort the current waveform.
• Voltage drops along long wires, which reduce the actual voltage at the load.
• Temperature effects on resistance (a light bulb’s resistance is much lower when cold).

For AC circuits with reactive loads, use P = V × I × pf, where pf is the power factor, or measure with a wattmeter that directly reads true power.

Frequently Asked Questions

How many watts is 1 amp at 120 volts?
Using P = V × I, 1 amp at 120 volts equals 120 watts. At 240 volts, 1 amp equals 240 watts. This is why high-voltage transmission lines carry power more efficiently: for the same power, higher voltage means lower current, which reduces losses in the wires.

How do I calculate watts from volts and ohms?
If you know resistance (ohms) instead of current, use the combined formula P = V² ÷ R. For example, a 120 V device with 60 ohms resistance draws 120² ÷ 60 = 14400 ÷ 60 = 240 W. You can also use P = I² × R.

Is watts the same as VA?
No. Watts measure real power (actual work done), while VA (volt-amperes) measure apparent power in AC circuits. They are equal only when the power factor is 1 (pure resistive load). For most devices with motors or electronics, VA is higher than watts. Power supplies and UPS units are often rated in VA, while actual consumption is in watts.

How do I convert watts to amps?
Rearrange the formula to I = P ÷ V. For a 1500 W space heater on 120 V, the current is 1500 ÷ 120 = 12.5 A. On a 240 V circuit, the same heater would draw 6.25 A. Always check the device’s rated voltage before doing this.

What is the difference between watts and watt-hours?
Watts is the rate of energy use at a given moment (like speed in km/h). Watt-hours is the total energy used over time (like distance traveled). A 60 W bulb running for 10 hours uses 600 watt-hours (0.6 kWh). Your electricity bill charges per kilowatt-hour, not per watt.

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