Water energy

The basic process of utilizing water energy to produce electricity is described as water flowing through the  turbines that spin in order to transfer kinetic energy in water into electricity that supplied to the grid. In fact, the water power represented as Hydro power, Wave and Tidal. In this case, we mainly focus on the hydro power which is widely used around the world.

Hydro Energy

Approximately 1/3 solar incident upon earth  produces evaporation from sea and land. When condensation occurs in the atmosphere, most of this energy is given up in the form of latent heat and re-radiated to earth. Approximately 0.1% is available due to precipitation onto high land which could be harnessed as hydro energy source when it falling through hydro turbines (As shown in video below).

According to the Hong Kong Observatory in 2012, the global annual rainfall is about 1.4767 m. The surface area of the earth is about 5.11 x 10¹⁴ m². So the annual precipitation is about 7.5459 x 10¹⁴ m³.
The energy that is available to the power plant is equal to the potential energy, PE (Joules) in the stored water, give by the equation:

PE = mgh 

Where

m = mass of the water (kg)

g = acceleration due to gravity (10 m²/s )

h = average height of the land above sea level (800 m)

Therefore the total potential energy  = 6.0367 x10²¹  J

Divided by number of second in 1 year:

Power = 1.914 x 10¹⁴ W

Multiply by number of hours in one year:

Power = 1.6767 x 10¹⁸ Wh = 1676.7 TWh

It is equal to nearly twice mankind's primary energy consumption. However, only a small part of it could be used as energy source.

Reference

Nickl Elsa. Etc (2010)Changes in Annual Land-Surface Precipitation Over the Twentieth and Early Twenty-First Century

http://www.weather.com/outlook/weather-news/news/articles/yearly-rainfall-2011_2012-01-06
http://www.kidzworld.com/article/1375-hydro-energy#

http://www.hko.gov.hk/wxinfo/pastwx/ywx2011.htm

iha_activityreport_2012_web.pdf