Electrical Power and the Power Equation

Energy is the potential to do work. However, work is done only when the energy is released. A disconnected battery is not doing work, but it has the potential to do work and is therefore a source of energy. The difference in electron supply at the battery terminals creates electrical force and is sometimes called the potential difference. In the case of a standard charged automotive battery, it has a potential of approximately 12 volts. Tapping this potential means turning one form of energy, the battery’s chemical energy, into another form of energy, electrical energy.

FIGURE 36-28
Watt’s law triangle.

Turning one form of energy into another is called energy transformation. The amount of energy transformed is the amount of work done. When a person’s legs turn the pedals of a bicycle, chemical energy (from oxygen and food) is being turned into mechanical energy. A motorcycle engine turns chemical energy into thermal energy, and then into mechanical energy. In each case, work is being done, but there is a difference with the motorcycle—it does the work more quickly, delivering more mechanical energy faster. That difference is called power. Power is the rate at which work is performed. It is also known as the rate of transforming energy. In an electrical circuit, power refers to the rate at which electrical energy is transformed into another kind of energy.

The unit of electrical power is the watt. One watt is produced when 1 volt causes 1 amp of current to flow. From this comes the power equation: P, the power in watts, equals V, the voltage in volts, multiplied by A, the current in amps. This calculation is applied similarly to Ohm’s law and is typically represented as a triangle Figure 36-28.

When current flows in a circuit with a resistor in it, the resistor becomes hotter as it converts electrical energy into heat energy. If this circuit is powered by a 12-volt battery with a current of 2 amps, using the power equation (P = V × A) we can determine that the resistor is using 24 watts of power.

It is also possible to simplify and transpose the power equation. If power equals voltage times current, then:

• Voltage equals power divided by current: V = P ÷ A
• Current equals power divided by voltage: A = P ÷ V
Solving Power Equations

Electrical power is a measurement of the rate at which electricity is consumed or created. When used in relation to loads, it is a measure of electricity consumed. A light bulb uses a certain amount of electrical power, but the power used is not an indication of brightness; it is a measure of power consumption. When used in relation to generators, it is a measure of electrical power produced. One volt pushing 1 amp equals 1 watt of electrical power. To calculate power, the current flow in the circuit is multiplied by the voltage used. So,

P = A × V, or Watts = Amps × Volts

This is demonstrated by the diagram in Figure 36-30. For reference purposes, 746 watts equal 1 horsepower.

If a 12-volt circuit with a single light has a current flow of 5 amps, then applying the formula will yield:

• P = A × V
• P = 5 A × 12 V
• P = 60 W

The power consumed by the circuit is 60 watts, and the bulb will carry a rating of 60 watts. This rule can be applied to any circuit where the voltage and current flow are known. However, if the values of voltage or current flow are not known, then Ohm’s law can be used to determine the missing value. As an example: V = A × R. By expanding P = A × V, it can be said that

P = A × (A × R), or P = A² × R.

Similarly, by applying A = V ÷ R, we have

P = (V ÷ R) × V, or P = V² ÷ R.