Energy is the capacity to do physical Work. That is; lifting, pushing, heating, lighting ... all those practical things we need to exist.
Energy can be stored. Familiar examples include the chemical energy stored in coal or petroleum or in the fats and sugars in our food, the electrical energy stored in our car battery, and the energy stored the mass of water of a hydroelectric dam.
In doing work, energy is transformed. The chemical energy in our food is transformed into our movements, metabolism and heat. When the hydro dam water is released it drives the turbines and generates electricity.
Where does energy come from?
The story of energy is the story of the Universe; it’s mostly about gravity and, significantly for us, about ‘Life’. Gravity built the stars. Each is a nuclear furnace bathing its solar system with unbelievable amounts of energy. The stars manufactured the chemical elements from which our world is built and within it - Life. It is Life which, over the eons, has captured, transformed and stored much of the energy upon which we’ve built our modern civilization. Coal, oil, and gas (the so-called fossil fuels) are the product of once-living things. Fossil fuels are just one way energy is stored, but they provided the ready means for us to create our modern world ... hopefully a stepping stone to the nirvana of a ‘clean’ energy future.
Energy vs Power
We need to be clear about the relationship between energy and power. They are commonly heard terms but they are not the same thing.
Power is the rate at which Energy is transformed. Transformation in this sense includes either changing between different forms of energy (such as burning coal to generate electricity) or using energy directly to do work (such as discharging a battery to start your car engine).
The faster the transformation for a given amount of energy (or work), the higher will be the power. Power is what we “feel”. For example, the energy used to walk 5 km is more or less the same whether we stroll or go at a brisk pace. But, the faster you walk the more effort you put in; the more rapidly you burn up your energy (fuel) and the more power you generate. You certainly feel it.
We can express this mathematically: Power equals Energy divided by the Time during which the energy is used. Conversely, Energy equals Power multiplied by this Time. The faster a process can transform energy, the higher its power. That’s why a powerful V8 uses a lot of petrol!
Measuring Energy and Power
We can’t analyse anything unless we can measure it. For this we need units of measure or simply ‘units’. The basic unit of Energy is the “Joule” and the basic unit of Power is the “Watt” – both named after famous scientists. These units are related by our formula: One Watt equals One Joule of energy transformed during One Second. In everyday experience a Joule is a pretty tiny amount of energy (or work). The energy involved in picking up an apple from the floor, ie., lifting about 100 grams (one tenth of a kilogram) one metre, is about one Joule – not a lot!
There are other units of measure for Energy and Power and the units we choose depend upon the scale or context of the discussion. For example we talk about kilojoules (kJ) ie., one thousand Joules when we discuss the energy content of foods but we use megawatts (MW) ie., one millions Watts to describe the output of a power station. These all refer to the same concepts and it’s simple to convert between them ... Google will find you hundreds of conversion calculators.
We often see the term “kilowatt” (kW) ie., one thousand Watts ... this is a unit of Power. As an example, your car engine may be capable of a peak motive power of say 100 kW. Or, an electric radiator might deliver 1 kW of heat (and some light). A fit human, working hard, can generate only about 200 Watts of power ... just one fifth of a kilowatt!
Most of us also know the term “kilowatt-hour” (kWh) ... this is a unit of Energy. Kilowatt-hours are probably most familiar as the units our electricity utility uses to calculate our bill. One kilowatt-hour is the energy required to deliver one kilowatt of power for one hour. We can see for example that a one kilowatt (kW) radiator, operated for six hours on a winter’s night would consume 6 kilowatt-hours (kWh) of energy ... which would appear as something like $2 on your electricity bill!
There are 3,600,000 Joules in each kilowatt-hour – that’s a lot of apples! Most of the examples and discussion on energyfacts use kilowatt-hours for the units of energy.
How much work can one kilowatt-hour do?
Since we will be talking quite a bit about kilowatt-hours it’s good to have some sort of feel for how much work one kilowatt-hour actually represents.
As a hypothetical example, let’s assume that you had a very high crane driven by a perfectly efficient electric motor. You might be surprised to learn that just one kilowatt-hour would be enough energy for your crane to lift an object weighing 2500 kg (say a typical large 4WD) to a height of almost 150 metres! Sure, that calculation assumes a 100% efficient lifting mechanism, but you get the point ... one kWh does a LOT of work.
If you were winching this thing up there yourself, even if you were more than Olympian fit, it would take you well over five hours to do it. But, if you replaced the electric motor with the 4WD’s 500 kilowatt engine, it could do it in less than 10 seconds.
This example represents the same amount of work as picking up 3,600,000 apples. How long it takes is simply determined by the power available.
