Energy and Power.

Power is many times mistaken to be the same as energy. But they are not. They are two different quantities but are very closely related. The first way of differentiating them is through their individual definition.

General Energy vs power definition

In general terms, Energy is defined as the capacity to do work. It is what causes change. And without change, there won’t be movement or work. For example, when you are running (which is work), it is the energy that you have within you that gives you the capacity to make that run. Also, when you are writing (work) with your hand, it is the energy within you that gives you the capacity to write.

Energy is measured in joules.

Power, on the other hand, is defined as the rate at which this energy is used. That is, how energy is used or work is done over time. For example, when you keep running, the rate of energy that is been used over time is called Power. Also, when you keep writing(work), the rate of energy used or work done over time is called power.

Power = Energy/time = work done/time = f*s/t = Joules/second

Energy = work done = Force * distance = F * s

And, t is measured in seconds.

Therefore, Power is measured in joules per second

Also, the formula for power can be written as:

Power (P) = Force * speed, because s/t = distance/time  = Speed

So, energy tells you how much work a body can do while Power tells you how fast work is done or the rate at which energy is used.

Electrical energy vs power

Electrical energy is the energy that is derived from the flow of electrons or charges. In relation to the general definition of energy, electrical energy is the ability for work to be done by charges, as a result of force of attraction and repulsion between them.

Electrical energy can also be defined as the aggregation of power over time. So, what is electrical power?

Electrical power is the rate at which electrical energy is sent to a load or electrical circuit.

Other ways of differentiating Energy and power

There are many other ways power and energy can be differentiated:

Denotation: They have different denotations. For example, electrical power can be denoted by P while electrical energy can be denoted by E.

Conservation/storage: Energy can be stored but Power cannot be stored. For example, a battery can store electrical energy but does not store electrical power; the food you eat stores chemical energy which it gives you, not power.

SI units: SI unit for energy is Joules while power is in Joules per second or Watt. Other units exist for these quantities: Energy can be measured in BTU (British thermal unit), Calories, and Watt-hour while Power in watts, horse-power, Calories/hour, and Btu/hour.

conversion: Energy can be converted from one form of energy to another while Power cannot be converted to any other form.

Time factor: Energy is power accumulated over time while power is at an instant

How Energy and Power are related

Even though energy and power are different, they relate in a way. And this relationship exists through this Energy-power equation:

Power = Energy/time

Where Energy is measured in Watts-hour and time is measured in hours. Therefore, Power is measured in Watt.

And the formula can be re-written as Energy = Power * time

Energy can lead to power and vice versa

In a centralised power system, power is generated at the generating stations. The generated power is a result of the conversion of one type of energy to other types. For example, in a gas power generating station, energy from natural gas and air is used to drive the turbine which then produces mechanical energy. Since the turbine is locked with the generator’s rotor, the turbine’s mechanical energy is also transferred to the generator’s rotor, thereby leading to the generation of electrical power.  Another example is the solar panels that produce electrical power from solar energy.

The electrical power that is generated is only done instantly. That is, it is generated at once or in an urgent manner.  When the instantly generated power is accumulated over time, then it is called electrical energy. This is why electrical energy is said to be the accumulation of electrical power over time.

As the accumulated power is distributed to loads, the loads consume it in form of power. And if the loads keep consuming the power over time, you can say the loads are consuming electrical energy.

In essence, Energy can lead to power, and power can lead to energy.

Why are electrical appliances rated based on power and not energy?

In electricity, what is meant by power and energy is quite different from the general meaning. Although it is still the same ideology.

As implied earlier, electrical energy is simply said to be the summation of generated power over time while electrical power is how much electrical energy is used over time.

Electrical appliances consume power at an instance. It is when it keeps consuming the power that you can say the appliances are consuming electrical energy.

The power rating of your electrical appliances tells you how much power it can consume at an instance. In this case, time is not factored in because there is no electrical energy yet.

For example, a television that is rated 200W would be able to consume 200W at an instance. And if it consumes such power for an hour, we say it has consumed 200W * 1hr = 200Whr of electrical energy.

Why you pay for electrical energy and not power?

You pay for electrical energy and not power because your appliances consume power over time. As your appliances are supplied with electrical energy, it gets to consume the rated power over time. And that is why electrical energy in measured in Watt-hour.

For example, if the utility company supplies 500Wh of energy to your 500W rated appliance, how long will it take to uses the energy?

Energy = power * time

i.e. 500Wh = 500KW * t

t = 500Wh/500W

t = 1hr.

So, it will take 1 hour for the energy to be fully used by the appliance. That is, your appliance would keep consuming 50KW for only one hour.

The utility companies don’t just care about how much power you are using but for how long you use them. That is why they bill you based on your electrical energy consumption.