2. In physics, energy is one of the basic quantitative properties describing
a physical system or object's state. Energy can be transformed
(converted) among a number of forms that may each manifest and be
measurable in differing ways. The law of conservation of energy states
that the (total) energy of a system can increase or decrease only by
transferring it in or out of the system. The total energy of a system can
be calculated by simple addition when it is composed of multiple noninteracting parts or has multiple distinct forms of energy. Common
energy forms include the kinetic energy of a moving object, the radiant
energy carried by light and other electromagnetic radiation, and various
types of potential energy such as gravitational and elastic. Energy is
measured in SI units of joules (J). Common types of energy transfer
and transformation include processes such as heating a material,
performing mechanical work on an object, generating or making use
of electric energy, and many chemical reactions.

3. Hydroelectricity is the term referring to electricity generated
by hydropower; the production of electrical power through
the use of the gravitational force of falling or flowing water.
It is the most widely used form of renewable energy,
accounting for 16 percent of global electricity generation –
3,427 terawatt-hours of electricity production in 2010, and is
expected to increase about 3.1% each year for the next 25
years.
Hydropower is produced in 150 countries, with the AsiaPacific region generating 32 percent of global hydropower in
2010. China is the largest hydroelectricity producer, with 721
terawatt-hours of production in 2010, representing around
17 percent of domestic electricity use. There are now three
hydroelectricity plants larger than 10 GW: the Three Gorges
Dam in China, Itaipu Dam across the Brazil/Paraguay border,
and Guri Dam in Venezuela.

4. Tidal power, also called tidal energy, is a form of hydropower that
converts the energy of tides into useful forms of power - mainly
electricity. Although not yet widely used, tidal power has
potential for future electricity generation. Tides are more
predictable than wind energy and solar power. Among sources of
renewable energy, tidal power has traditionally suffered from
relatively high cost and limited availability of sites with
sufficiently high tidal ranges or flow velocities, thus constricting
its total availability. However, many recent technological
developments and improvements, both in design (e.g. dynamic
tidal power, tidal lagoons) and turbine technology (e.g. new axial
turbines, cross flow turbines), indicate that the total availability of
tidal power may be much higher than previously assumed, and
that economic and environmental costs may be brought down to
competitive levels. Tidal energy is a renewable energy source.

5. Efficient energy use, sometimes simply called energy efficiency, is
the goal to reduce the amount of energy required to provide products
and services. Installing fluorescent lights or natural skylights reduces
the amount of energy required to attain the same level of illumination
compared with using traditional incandescent light bulbs. Compact
fluorescent lights use one-third the energy of incandescent lights and
may last 6 to 10 times longer. Improvements in energy efficiency are
generally achieved by adopting a more efficient technology or
production processes or by application of commonly accepted
methods to reduce energy losses.
There are many motivations to improve energy efficiency. Reducing
energy use reduces energy costs and may result in a financial cost
saving to consumers if the energy savings offset any additional costs
of implementing an energy efficient technology. Reducing energy use
is also seen as a solution to the problem of reducing carbon dioxide
emissions. Energy efficiency and renewable energy are said to be
the twin pillars of sustainable energy policy and are high priorities in
the sustainable energy hierarchy.

6. Efficient energy use, sometimes simply called energy efficiency, is
the goal to reduce the amount of energy required to provide products
and services. Installing fluorescent lights or natural skylights reduces
the amount of energy required to attain the same level of illumination
compared with using traditional incandescent light bulbs. Compact
fluorescent lights use one-third the energy of incandescent lights and
may last 6 to 10 times longer. Improvements in energy efficiency are
generally achieved by adopting a more efficient technology or
production processes or by application of commonly accepted
methods to reduce energy losses.
There are many motivations to improve energy efficiency. Reducing
energy use reduces energy costs and may result in a financial cost
saving to consumers if the energy savings offset any additional costs
of implementing an energy efficient technology. Reducing energy use
is also seen as a solution to the problem of reducing carbon dioxide
emissions. Energy efficiency and renewable energy are said to be
the twin pillars of sustainable energy policy and are high priorities in
the sustainable energy hierarchy.