Photovoltaics (PV) are what we commonly recognize as solar panels. PV's are silent, non-polluting, and can be used anywhere, which gives them many advantages over other forms of energy transformation. However, they are often expensive in comparison to grid electricity. They make great sense when the alternative is wiring or creating the grid to transport energy to where it is needed, or when used to power intermittent-duty devices, such as trash compactors or wireless transmitters. The cells are typically made of semiconductor materials, such as silicon. When light strikes the cell, it excites the electrons in the atoms of the semiconductor, causing a current to flow. Light energy strikes electrons on the top layer of silicon, energizing them and making them "jump" across the insulator to the other layer. Once the electron has made the jump, it tends to travel back to its original state, doing "work" in the process. In a photovoltaic panel, the electron flows through a circuit to return to its original position. This flow of electrons is what is used to power devices or charge batteries.
Passive solar is another way to use the sun's natural energy to light rooms or heat buildings, reducing energy costs. Often, mirrors and windows are used to direct solar energy to the proper locations.