There are two types of energy: Potential energy (stored energy) and kinetic energy (moving energy). All forms of energy are stored in different ways in the energy sources that we use every day. These sources are divided into two groups, renewable energy and non-renewable energy. Renewable and non-renewable energy can be converted into secondary energy sources like electricity and hydrogen. Secondary sources of energy are used to store, move, and deliver energy in easily usable form. We have to use another energy source to make electricity or hydrogen, e.g. coals.

Energy is measured in joules (J). The work done, or energy used, during a certain time is called power. Power is measured in watts (W). A watt is the use of one joule of energy for one second. A watt is a small amount of energy, thus we measure power in units of 1000 watts for one hour, kilowatts for one hour. 

Non-renewable energy is an energy source that we are using up and cannot recreate in a short period of time. We get most of our energy from non-renewable energy sources, which includes the fossil fuels (oil, natural gas and coal). They are called fossil fuels, because they were formed over millions of years by the action of heat from the earth's core and pressure from rock and soil on the remains (or ‘fossils’ ) of dead plants and animals.  Examples of non-renewable energy sources are coal, petroleum, natural gas, propane, oil and uranium.

Renewable energy is an energy source that can be replenished in a short period of time. Renewable energy sources are solar energy (sun), wind energy, geothermal energy (from inside the earth), biomass (from plants) and hydropower (water). We use renewable energy sources mainly to make electricity. Also referred to as sustainable energy, which means the sources of energy should be replenishable within a lifetime and should not cause any longterm pollution or damage to the environment.

Supercharged geothermal energy could power the planet - The next generation of geothermal plants will unlock more of Earth's bountiful, underground energy and could allow the technology to finally fulfil its promise
Falling rocks can explode so hard that only nuclear weapons beat them - If big rocks fall far enough they can explode with more energy than any non-nuclear bomb – and the ensuing shockwave can snap large trees half a kilometre away
EU will limit the use of palm oil as car fuel but won’t stop it - The European Union will make only minor tweaks to “renewable” energy policies that are actually increasing greenhouse gas emissions and driving deforestation
Why tidal power won't solve the world's renewable energy needs - There are widespread calls for the UK government not to abandon a trailblazing tidal power project, but this energy source is no green panacea, says Hans van Haren

Energy Education Foundation 
US Department of Energy 
Energy Information Administration
European Commission - Energy
Alternative Energy
Sustainable Energy Coalition
Energy Library Archives