There often seems to be confusion between the concept of energy and the concept of power. This problem worsens when we talk about the units of both quantities. Therefore, we dedicate this post to clarifying these aspects.
First, we define power as the relationship between energy and time,
The unit of measurement for power is the Watt (W), which corresponds to the power needed to exchange one Joule of energy with a system in one second. Its multiples are the kilowatt (1 kW = 1,000 W) and the megawatt (1 MW = 1,000,000 W).
For example, a 50kW boiler is capable of transferring 50kJ to the water passing through it every second. Similarly, if the insulation of a wall allows 30J to pass through every second, we say it has losses of 30W. Obviously, a system is in equilibrium when its energy does not change, that is, when the sum of incoming and outgoing powers is zero.
Much of the confusion between both concepts is due to the use of the term kilowatt hour. This is a unit of energy (not power), which corresponds to the energy resulting from maintaining a power of one kilowatt for one hour, i.e., 3,600 seconds. Therefore, 1 kWh = 3,600 kJ.
The use of this unit was introduced to simplify certain calculations. For example, the aforementioned 50 kW boiler, operating for one hour, will transfer 50 kWh of energy to the water. If I store these 50 kWh in a perfectly insulated tank and then consume power at a rate of 10 kW, it will take me 5 hours to deplete the system’s energy.
As can be seen, the relationship between power and energy is simple, and the correct choice of units simplifies calculations, as long as it is done appropriately.
