Power and the Watt

Whenever current flows through a resistance, heat results. The heat can be measured in watts (symbolized W) and represents electrical power. (As a variable quantity in equations, power is denoted by the uppercase italic letter P.) Power can be manifested in many forms, such as mechanical motion, radio waves, visible light, or noise. But heat is always present, in addition to any other form of power, in an electrical or electronic device. This is because no equipment is 100 percent efficient. Some power always goes to waste, and this waste is almost all in the form of heat.
Look at following Figure.


There is a certain voltage across the resistor, not specifically indicated. There’s also a current flowing through the resistance, and it is not quantified in the diagram, either. Suppose we call the voltage E and the current I, in volts (V) and amperes (A), respectively. Then the
power in watts dissipated by the resistance, call it P, is the product of the voltage in volts and the current in amperes:

P = EI

If the voltage E across the resistance is caused by two flashlight cells in series, giving 3 V, and if the current I through the resistance (a light bulb, perhaps) is 0.1 A, then E = 3 V and I = 0.1 A, and we can calculate the power P in watts as follows:

P = EI = 3 × 0.1 = 0.3 W

Suppose the voltage is 117 V, and the current is 855 mA. To calculate the power, we must convert the current into amperes: 855 mA = 855/1000 A = 0.855 A. Then:

P = EI = 117 × 0.855 = 100 W

Prefix multipliers from 0.000000000001 (trillionths, or units of 10−12) to 1,000,000,000,000 (trillions, or units of 1012).
Prefix Symbol Multiplier
pico- p 0.000000000001 (or 10-12)
nano- n 0.000000001 (or 10-9)
micro- μ 0.000001 (or 10-6)
milli- m 0.001 (or 10-3)
kilo- k 1000 (or 103)
mega- M 1,000,000 (or 106)
giga- G 1,000,000,000 (or 109)
tera- T 1,000,000,000,000 (or 1012)

You will often hear about milliwatts (mW), microwatts (μW), kilowatts (kW), and megawatts (MW). By now, you should be able to tell from the prefixes what these units represent. Otherwise, you can refer to above table. This table lists the most commonly used prefix multipliers in electricity and electronics.

Sometimes you need to use the power equation to find currents or voltages. Then you should use I = P/E to find current, or E = P/I to find voltage. Always remember to convert, if necessary, to the standard units of volts, amperes, and watts before performing the calculations.