# Energy and Electric power

**Power** is the rate or speed at which work is done. **Electric power** is the multiplication of the electric current (amps) by the voltage (in volts). P = I x V.

For example: If in a resistor, the current I is 0.25 amperes and the voltage V is 3 volts. Then the power is: P = I x V = 0.25 x 3 = 0.75 watts = 750 milliwatts. Using the Ohm’s law, we obtain the following formulas: P = V^{2}/R and P = I^{2}xR.

Using the first formula can get **electric power** without the current value and, using the second formula the power is obtained without the voltage value. These formulas are useful for finding the power in a resistor, where the energy is converted into heat.

But not always electricity is converted into heat. In the case of an electric motor, the power is converted mainly into mechanical motion. In radio and television stations, power largely become, electromagnetic waves. In a stereo, power becomes sound waves. In a bulb power becomes light and heat.

Normally the power converted into heat, is considered lost power or useless power. The main idea is that the power supplied is make the most, and that the power lost in heat and others is minimal.

To find out how well the power is used, we use the term “Performance”.

“Performance” = Output power / Input power.

For example: If we use a 100 watts light bulb, and the power transformed into light is 80 watts, performance is: 80/100 = 0.8 = 80%. The remaining 20% is lost in heat.

The unit of **electrical power** is watt. Multiples and submultiples of watt are:

- 1 milliwatt = 1 watt / 1000.
- 1 kilowatt = 1000 watts.
- 1 megawatt = 1000 kilowatts = one million watts.

Note: 1 HP (Horsepower) = 745.7 watts.