Electronics Basics: Electrical Power
The three key concepts you need to know before working with electronic circuits are current, voltage, and power. Current is the organized flow of electric charges through a conductor, and voltage is the driving force that pushes electric charges to create current.
The third piece of the puzzle is called power (abbreviated P in equations). Simply put, power is the work done by an electric circuit.
Electric current, in and of itself, isn’t all that useful. It becomes useful only when the energy carried by an electric current is converted into some other form of energy, such as heat, light, sound, or radio waves. For example, in an incandescent light bulb, voltage pushes current through a filament, which converts the energy carried by the current into heat and light.
Power is measured in units called watts (abbreviated W). The definition of one watt is simple: One watt is the amount of work done by a circuit in which one ampere of current is driven by one volt.
This relationship lends itself to a simple equation. Fortunately, this one is pretty simple:
P = E × I
In other words, power (P) equals voltage (E) times current (I).
To use the equation correctly, you must make sure that you measure power, voltage, and current using their standard units: watts, volts, and amperes. For example, suppose you have a light bulb connected to a 10volt power supply, and onetenth of an ampere is flowing through the light bulb. To calculate the wattage of the light bulb, you use the P = E × I formula like this:
P = 10 V × 0.1 A = 1 W
Thus, the light bulb is doing 1 watt of work.
Often, you know the voltage and the wattage of the circuit and you want to use those values to determine the amount of current flowing through the circuit. You can do that by turning the equation around, like this:
For example, if you want to determine how much current flows through a lamp with a 100watt light bulb when it’s plugged into a 117volt electrical outlet, use the formula like this:
Thus, the current through the circuit is 0.855 amperes.
Here are some final thoughts concerning the concept of power:

The term dissipate is often used in association with power. As the energy carried by an electric current is converted into another form such as heat or light, the circuit is said to dissipate power.

Did you notice that current and voltage are represented by the letters I and E, not the letters C or V as you might expect, but power is represented by the letter P? Sometimes you wonder if the people who make the rules are just trying to confuse everyone.
Maybe the following table will help you keep things sorted out:
Concept Abbreviation Unit Current I amp (A) Voltage E or EMF volt (V) Power P watt (W) 
The definition of a volt is simple: One volt is the amount of electromotive force (EMF) necessary to do one watt of work at one ampere of current.

The P = E × I formula is sometimes called Joule’s Law, named after the person who discovered it.

Calculating the power dissipated by a circuit is often a very important part of circuit design. That’s because electrical components such as resistors, transistors, capacitors, and integrated circuits all have maximum power ratings.
For example, the most common type of resistor can dissipate at most 1/4 watt. If you use a 1/4watt resistor in a circuit that dissipates more than 1/4 watt of power, you run the risk of burning up the resistor.