Energy & power measurement


How is energy measured? There are various units energy is measured in but they all fall into two categories when applied to energy consumption. The units either describe the total energy used to achieve something or the energy use per unit of time. Lets consider speed and distance. When I travel home in my car I might need to travel 10km. To keep to the speed limit I might need to travel no faster than 60km/h.
The 10km I need to travel is the total distance I need to travel in order to achieve my goal of getting home. The 60km per hour that I am traveling describes the rate of movement per hour.

These two units are related but they are not the same thing. With the information I can calculate that if my speed does not change it will take me 10 minutes to get home. If you knew how long you traveled and the speed you could determine how far you moved.

Lets translate this to energy. The unit that energy is measured in is the joule. One joule is roughly the energy required to lift a small apple 1m off the ground. This is not a scientific definition but makes for a good illustration.  Joule is our distance – the total amount of energy required to do something without any consideration for time.


What would be the speed equivalent? Measuring the rate of change of energy is called power. It is measured in the unit watt. Watt is the same as joule per second. So if you had a basket of small apples and were picking them up one by one, and one per second, the power involved would be 1 watt.

So in the same way as the speed/distance example energy/power and joule/watt are related but they are not the same thing. What gets confusing is that the majority of energy measurements are denoted in watt-hours. This is the same as giving distance in the unit of km/h.h or kilometer per hour hours. These are all the different ways of denoting the same thing. So when people talk about watt-hours remember that they are actually taking about joule, the unit of energy measurement.

Example: If I have a toaster rated at 700W and it takes 2 minutes to toast my bread I can determine that running the toaster for 2 minutes requires 84 000 joules of energy.

It is due to the fact the numbers get big so quickly that a different unit was devised but that still describes the same thing. The standard unit for describing electrical energy consumed is watt-hour or kilowatt-hour (a thousand watt-hours). This is the same as 3.6MJ or 3 600 000 joule.

That is a lot of apples.

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