How do we calculate power?

Power (P) is the rate of doing work (W), so its fundamental formula is:

P=W/t

That is:

Power = Work divided by time.

Clear explanation: Power is equal to:

How much work is done (in joules) divided by the time (in seconds) it took to do that work.

For example:

Problem: If a construction worker pushes the wheelbarrow, expending 50 J of work in 2s, how much power does he use?

Answer: 50 J divided by 2 s is 25 W so, the power is equal to 25 watts.

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For electrical engineering and electronics, power is calculated as:

P = I2R, or

P= V2/R, or

P= VI

where P is power, I is current, R is resistance, and V is voltage.

The big power formulas are:

Formula 1 - Electrical (electric) power equation:

Power P = I × V = R × I2 = V2 ⁄ R

where power P is in watts, voltage V is in volts and current I is in amperes (DC).

If there is AC, look also at the power factor PF = cos φ and φ = power factor angle (phase angle) between voltage and amperage.

Formula 2 - Mechanical (mechanic) power equation:

Power P = E ⁄ t = W

where power P is in watts, energy E is in joules, and time t is in seconds. 1 W = 1 J/s

There is much potential for confusion between the value for Work (measured in joules but often written in formulas as W) and Power (measured in watts and often written in formulas as W). However, symbols for quantities (work) are italicised, W,whereas symbols for units are upright.

In these formulas, W always refers to watts, although units should never be written in equations.

Scroll down to related links and look at "Calculation: electrical voltage, current, resistance, and power". Look at the written italic letters and normal letters. The SI unit of "work or energy" is the same. One joule is the amount of work done when an applied force of 1 newton moves through a distance of 1 metre in the direction of the force. That is energy.