Electric Power and Energy
Electric power is defined as a product of voltage and current. The corresponding equation is given below.
p = v x i … (1)
where,
v – voltage in ‘volts’ at a given instant of time
i – current in ‘amperes’ at that instant of time
p – power in ‘watts’ at that instant of time
The calculation of electric power is straightforward when the voltage and current are constant with respect to time. In such a case, we can write the equation for power as given below:
P = V x I … (2)
where,
V – voltage in ‘volts’ (constant w.r.t. time)
I – current in ‘amperes’ (constant w.r.t. time)
P – power in ‘watts’ (constant w.r.t. time)
In practice, this is valid only for direct current (DC) systems. DC circuits are commonly used to present the electrical theory and concepts in introductory courses in high school and university. It is not appropriate for AC systems.
Note: Equation 2 is often used for AC power systems under some conditions. It is not appropriate to use Equation 2 for AC power calculations without being aware of its limitations.
In the case of alternating current (AC) power systems, the voltage and current vary sinusoidally with respect to time. Hence, it is only possible to calculate an ‘average’ power over a given period. The aim of future blogs in this series is to provide a good concept of power in AC systems and to derive equations for practical power systems.
Electrical energy is calculated as a product of power and time as given below:
E = P x t … (3)
where,
P – Power in ‘watts’
t – time in ‘seconds’
E – Energy in ‘joules’
As per above, 1 joule of energy is equal to 1 watt-second. This is a very small value in practice. For example, 1 joule approximately corresponds to the energy required to lift a small apple (about 100 gm) to a height of 1 m. Hence, it is common to use the term ‘kilo joules (kJ)’ or ‘Mega joules (MJ). Note that ‘mJ’ is for ‘milli joules’!
In electric power systems, joule is rarely used directly. It is a common practice to express the electrical energy in ‘watt-hours’ or more commonly in ‘kilo watt-hours (kwh)’. 1 kwh is equal to 3,600,000 joules or 3.6 Mega joules!
