There often seems to be confusion between the concept of energy and the concept of power. This problem is exacerbated when we talk about units of both magnitudes. Therefore, we dedicate this post to clarify these aspects.

First, we define power as the relationship between energy and time,

The unit of measurement of power is the Watt (W), which corresponds to the power required to exchange the energy of one Joule 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 that passes through it every second. Similarly, if the insulation of a wall allows 30J to pass through every second, we say that 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, that is, 3,600 seconds. Therefore 1 kWh = 3,600 kJ.

The use of this unit was introduced to simplify certain calculations. For example, the previous 50 kW boiler, operating for one hour, will provide the water with an energy of 50 kWh. If I store these 50 kWh in an insulated tank, and later consume it with a power of 10 kW, it will take 5 hours to deplete the energy of the system.

As can be seen, the relationship between power and energy is simple, and the correct choice of units simplifies the calculations, as long as it is done properly.