Since prehistory, when humanity discovered fire to warm up and roast food, through the Middle Ages in which populations built windmills to grind the wheat, until the modern era in which nations can get electricity splitting the atom. Man has sought endlessly for energy sources.
But what is energy? We use the word so often without knowing what it really means.
So let’s get back to the ancient Greek, where else. The greek word ἐνέργεια, energeia, was used in the antiquity and had a purely philosophical meaning in the sense of living reality and efficiency (see also “Act and Potency”).
As a scientific term, the word energy itself was first introduced to mechanics in 1807 by the physicist Thomas Young. The new quantity of energy should indicate the strength of certain effects that a moving body can cause by its movement.
(Translated from Source, CC-BY-SA-4.0)
Beside the scientific meaning which is defined by the possibility to measure and validate the signals, the word energy is also used in a metaphysical or spiritual context. In this OER we are focusing on the rational scientific facts.
The total energy of a physical system can be subdivided and classified into potential energy, kinetic energy, or combinations of the two in various ways. Kinetic energy is determined by the movement of an object – or the composite motion of the components of an object - and potential energy reflects the potential of an object to have motion, and generally is a function of the position of an object within a field or may stored in the field itself.
Forms of Energy
Beside the fundamental differentiation in potential and kinetic energy there are four different types of energy we have in our everyday life.
Chemical energy is the energy which helps to “glue” atoms together in those clusters called molecules, or chemical compounds. Of special interest to us are substances such as natural gas, or propane, or oil that are capable of releasing some of that energy. When we burn these fuels, we unglue some of the atoms from each other, liberating the chemically-bound energy that held them together. In the process, the chemical energy is changed in form to high temperature heat energy, a form well suited to doing many different kinds of work. This process takes place every time we flick butane lighter. 
Thermal energy involves the microscopic movement of atoms and molecules in everything around us. Thermal energy is often commonly referred to as heat. In fact, there are really two types of thermal energy.
Sensible Energy or sensible heat, is energy that jostles molecules and atoms in substances such as water. The more movement, the hotter the substance becomes. Sensible energy gets its name from the fact that we can sense it, by touching the substance directly or indirectly with a thermometer of some type. When we add heat to water in a kettle, we increase its temperature.
Latent Energy or latent heat, is the energy that is needed to make a substance such as water (a liquid) change to a different form (or phase) of the same substance such as water vapour (a gas). The change of form happens when enough sensible heat is added, and the molecules move too fast to be connected together and eventually separate. It gets its name from the fact that it lies hidden or latent, until the conditions are suitable for it to emerge.
If enough heat is added to liquid water at 100 °C, it eventually boils and becomes a vapour, also called a gas. If enough heat is removed from liquid water at 0°C, it eventually turns into the solid we call ice. Heat will always naturally flow from a higher temperature to a lower temperature. Thermal energy may move in many different ways, between many different substances, and change back and forth between its sensible and latent forms. 
Mechanical energy is the energy of physical movement, such as moving air or water, a ball being thrown, or even a person lifting a piece of wood. As with many forms of energy, mechanical energy eventually ends up being released or lost as thermal energy. A good example of this is the way that the sandpaper and wood convert mechanical energy to sensible energy that you feel as heat. 
Electrical energy involves the movement of electric current through wires. Electrical energy is a very useful form of energy because it can perform many functions. Ultimately, most electrical energy or electricity also ends up as thermal energy in the form of sensible heat. Some devices such as electric heaters convert the energy directly; other devices such as motors convert electricity to mechanical energy which eventually becomes heat. The trick to optimizing electricity use is to maximize the amount of work done by electricity before it is lost as heat. Typically, this also involves optimizing the use of mechanical energy. 
The most important information about energy is that it can’t be created or loose. It just can transform its status from one form to another. For example is a bycicle generator transforming mechanical energy in form of the rotation of a wheel into electrical energy for the lamp to have light at night. Or your body is transforming biochemical energy from your muscle into mechanical energy to run the pedal of your bike. A fire turns chemical energy saved in wood into thermal energy which is heating up his environment.
During the transformation process you always “loose” a piece of energy. This lost energy is called axergy. It is the part of energy which is converted into heat. It cannot be used for moving parts. You feel the axergy when you touch your phone charger. It is always heating up a bit. The productive part of the energy is called exergy. Every transformation contains a information of its energy efficiency which is the relation of exergy to axergy. The less axergy you have the more efficient your transformation process is.
Difference of Power and Energy
The terms “power” and “energy” are often inter-changed in everyday speech but their meanings are very different. Before going any further, we want to point out the difference.
When thinking about a journey, it is clear to everyone that “distance” and “speed” are not the same. After a few moments thought, most people will agree that: Distance (in miles) = Average Speed (in miles/hour) × Time_of_Journey (in hours).
In the world of electricity, “energy” is related to “power” in the same way. So: Energy (in kWh) = Power (in kW) × Time_TheApplianceWasOn (in hours).
If a 2 kW heater (its power rating) were to be switched on for a period of 1 Hour (time), then 2 kWh (of energy) would be consumed. Similarly, if a PV installation were to generate electricity at a constant rate of 1 kW (power), and this situation were to be maintained for a period of 2 Hours (time), then 2 kWh (of energy) would have have been generated.
In the same way that “Speed is the rate at which Distance is covered”, i.e. miles per hour, so “Power is the rate at which Energy is consumed or generated” i.e. kWh per hour, or just kW. To calculate the needed solarpower and battery capacity you need to know how power and energy correalate.
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