Energy and power: what is the difference?

If the words “power” and “energy” are widely used in our vocabulary, the clear distinction between these very different two notions is often unclear in the general public’s mind.

In order to clarify the difference between power and energy — or put in other words the difference between Watts (W) and Watt-hours (Wh) — I am going to clarify both notions and compare them.



Energy can be expressed in many units : Joules (J), calories (cal) or Watt-hours (Wh) which are all equivalent providing a multiplication factor. For instance, while Joules are often used in physics, calories are far more common when talking about nutrition. They could however also be expressed in Joules, as one calorie is equivalent to 4.18 Joules.

The term “energy” is used very frequently, and for good reason: energy is the engine of every action performed in our Universe. The Sun’s energy is needed by a plant to grow. Us humans and animals need energy from our food to survive. Even our cars need fuel to operate.

Everything comes down to a matter of energy, which is never created from nothing : this is one of the fundamental principles of Physics. It can be transferred and transformed, incurring losses with each transfer.

This is how the Sun and the ground’s nutrients provide energy to a plant, which will eventually serve as a source of energy for a cow. This cow will then have the energy to produce milk, containing a given amount of calories (i.e. energy) needed by a human to perform physical tasks. In this case, we talk about an energy chain: the energy is transferred from one link to the next.



Power is commonly expressed in Watts (W) and is intrinsically linked to energy. It represents the quantity of energy transferred per second: it is a flux of energy.

But, since energy is expressed in Joules, does this mean that power is expressed in Joules per second (J/s)? As a matter of fact, it is indeed the case! A Watt is equivalent to one Joule per second.

Drawing on our previous the car example, power is represented by the pressure on the accelerator. The more you push, the the greater the flux of energy and the bigger the acceleration of the car.

Mistaking power and energy is, by analogy, the same as mistaking speed for distance. Speed is defined as the distance travelled by unit of time. While the highway is often limited to 130km/h, you can still travel thousands of kilometers on it. But if you drive at 130km/h, you will travel 130km in an hour, or 260km in two hours.

Likewise, if you power a 9W LED lighbulb, you will use 9 joules in one second, 18 joules in two seconds and so on. Expressed in terms of Watt-hours, you will then consume 9*1 = 9 Wh in an hour.




Electricity bills are, as you probably understood, expressed in terms of energy. Since Joules or the calories are “small” units compared to the quantity of energy used by a household, consumptions are often expressed in terms of kiloWatt-hours (1000 Watt-hours).

If everything was expressed in terms of power, an instantaneous flow of energy, how could the utility company know how much you consumed over a whole month?

A flat-screen TV which requires 100W to operate and is powered on for 5h a day will consume 100*5 = 500Wh per day. Over a 30-day month, this represents a total of 15000 Wh or 15 kWh.

The table below lists the average consumptions of some everyday appliances found in our homes:

LED light bulb (equivalent to 100W) 25W 0.25 kWh
Incandescent light bulb 100W 1 kWh
Samsung galaxy S8 10W 0.1 kWh
Macbook Pro 15” 80W 0.8 kWh
TV Ultra HD 4K 123 cm 98W 0.98 kWh
Fridge 200L + freezer 300W 3 kWh
Electric keetle 1500W 15 kWh
Cooking hot plate 2000W 20 kWh
Electric water heater 2000W 20 kWh

As you probably noted, the heating systems consume the greater amount of power. However, while an electric kettle needs 1500W to operate, it only operates for about 30 seconds, so the amount of energy consumed stays quite low (1500 W * 30 s / 3600 = 12.5 Watt-hours).

By contrast, an electric hot plate operating for an hour will consume 2000*1 = 2kWh which represents a far bigger amount of energy.

This example demonstrates the importance of the distinction between energy and power, and the need to always consider both notions when evaluating electrical equipment.