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Physics formula

How to calculate electrical power

Electrical power measures how fast a circuit uses or delivers electrical energy.

See the formulaOpen calculator
Direct answer
Formula
P = V * I

Use the table below to match each symbol with the right input. Keep units consistent before you start.

Electrical power formula
Start here when you only need the equation and variable names.
Formula
P = V * I

Electrical power measures how fast a circuit uses or delivers electrical energy.

Variable meanings
Check each symbol, meaning, and unit before you calculate.
SymbolNameDescriptionUnit
PPowerRate of electrical energy transfer.W
VVoltageElectrical potential difference.V
ICurrentElectric current.A
When to use this formula
Check that your situation matches the formula before you trust the result.
  • Fits simple DC circuits and rated devices when voltage and current are known.
Step-by-step method
Follow these steps when you are solving it by hand.
  1. 1Use voltage in volts.
  2. 2Use current in amperes.
  3. 3Multiply voltage by current.
  4. 4Report the answer in watts.
Examples
These sample numbers show the order of operations and units.
Household device
V
120 V
I
2 A
  1. 1.P = 120 * 2
  2. 2.P = 240 W
Result
The device uses 240 W.
Small DC circuit
V
12 V
I
5 A
  1. 1.P = 12 * 5
  2. 2.P = 60 W
Result
The circuit power is 60 W.
Mistakes to avoid
Small input or unit errors can change the answer a lot.
  • Mixing metric and imperial units before solving the electrical power formula.
  • Forgetting squared units such as m^2, m/s^2, or velocity squared.
  • Dropping negative signs when direction matters.
  • Rounding measured values too early in the calculation.
Open the Power Calculator
Open the calculator for the answer without rewriting the formula.

Enter your values in the related calculator, then compare the output with the hand method above.

Open Power Calculator
FAQs
Short answers for common formula questions.
Can I calculate Electrical power by hand?

Yes. Write the units next to each value, convert rates or measurements first, and round only the final answer.

Why does my Electrical power result differ from another calculator?

Most differences come from rounding, unit conversions, rate timing, or a slightly different version of the formula.

When is the Electrical Power Calculator better than hand math?

Open the Electrical Power Calculator to check several scenarios or skip the hand arithmetic.