During the day, sunlight provides two valuable forms of energy: heat and light. Heat is also called thermal energy, and light is considered a form of radiant energy, or energy that travels as electromagnetic waves. The sun, generating both heat and light, produces a form of energy that we call solar energy. Solar energy is a renewable resource, a source of energy that is constantly "renewed," or replaced, by the environment. As long as renewable resources are not used up more quickly than they can be restored, their supply is essentially limitless.

Solar energy can be harnessed passively or actively. An example of a passive use of solar energy is a home that has most of its windows facing south to receive the rays of the daylight sun, as well as thick walls and floors to absorb the heat during the day and release this heat at night when the earth cools down. An example of an active use of solar energy is an array of solar panels installed on a home for heat or electricity.