Power and the watt

Whenever current flows through a resistance, heat results. This is inevitable. The heat can be measured in watts, abbreviated W, and represents electrical power. Power can be manifested in many other ways, such as in the form of mechanical motion, or radio waves, or visible light, or noise. In fact, there are dozens of different ways that power can be dissipated. 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 again at the diagram of Fig. 1. There is a certain voltage across the resistor, not specifically given in the diagram. There’s also a current flowing through the resistance, not quantified in the diagram, either. Suppose we call the voltage E and the current I, in volts and amperes, respectively. Then the power in watts dissipated by the resistance, call it P, is the product E × I. That is: P = EI.

Fig. 1: Whenever a resistance carries a current, there is a voltage across it.

This power might all be heat. Or it might exist in several forms, such as heat, light
and infrared. This would be the state of affairs if the resistor were an incandescent light bulb, for example. If it were a motor, some of the power would exist in the form of mechanical work.
If the voltage across the resistance is caused by two flashlight cells in series, giving 3 V, and if the current through the resistance (a light bulb, perhaps) is 0.1 A, then E = 3 and I = 0.1, and we can calculate the power P, in watts, as:

(watts) = 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 = 0.855 A. Then

P (watts) = 117 × 0.855 = 100 W

You will often hear about milliwatts (mW), microwatts (μW), kilowatts (kW) and megawatts (MW). You should, by now, be able to tell from the prefixes what these units represent. But in case you haven’t gotten the idea yet, you can refer to Table 1.
This table gives the most commonly used prefix multipliers in electricity and electronics, and the fractions that they represent. Thus, 1 mW = 0.001 W; 1 μW = 0.001 mW = 0.000001 W; 1 kW = 1,000 W; and 1 MW = 1,000 kW = 1,000, 000 W.

Table 1. Common prefix multipliers.

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 power. It’s easiest to remember that P = EI (watts equal volt-amperes), and derive the other equations from this by dividing through either by E (to get I) or by I (to get E).