Energy is defined as the ability to do work. It is vitally important to develop more efficient and environmentally friendly techniques for energy sources to keep modern society moving forward. There exist different forms of energy sources such as petroleum, waves, bioenergy, and wind, all of which are from the Sun. Many scientists have been working hard to directly harvest the energy from the sun, especially to generate electricity using  solar cell. It is not only because the solar energy has the biggest power reserves compared with any other sources, but also because it is a clean energy that doesn't emit any pollution. This video is going to introduce the definitions of energy, work, and power and how they are related, to understand how much energy Hong Kong gets from the Sun, how the solar cell works and the recent developments in this field.