One of the most commonly discussed types of renewable energy is solar power.  There are two main types of solar power solar thermal discussed in another section and photovoltaic (PV) power.  The photovoltaic effect is the process of converting light into electricity.  Sunlight is made up of bundles of energy called photons.  Sunlight contains photons within the total spectrum of electromagnetic radiation, which includes infrared, visible and ultra-violet light.  When the photons from sunlight strike a solar panel the photons may either be reflected, absorbed or pass right through.  The photons which are absorbed by the solar cell transfer their energy to an electron in the n-layer of the photocell.  One photon will bump exactly one electron away to the N-type silicon, only photons with energy equal to or greater than the band gap of the cell material can free an electron.  Photons with more energy than the band gap will expend that extra amount as heat, this causes solar (PV) panels to lose efficiency as they heat up.   A photon will also rearrange the electrons on the N-type silicon and will effectively move a "hole" over to the P-type silicon. When N and P-type silicon come into contact, they create their own electric field. This electric field is created when the electrons on the N-type silicon close-by fall into the holes in the P-type silicon. The result is a barrier between the positive and negative sides that allows electrons to travel one way. The barrier is called a diode. The diode allows electrons to travel from the P-type silicon to the N-type silicon, but not the other way around.  When a wire is attached on either type of silicon, the electrons follow the path of least resistance and travel along the wires. This flow of electrons creates a current, while the cell's electric field creates voltage.  The result is an electron pump, which moves electrons from one side to the other.  This flow of electrons is electricity.

 

Figure 1:  Diagram of a photovoltaic panel (http://www1.eere.energy.gov) 

The amount of energy that can be obtained from photovoltaic panels is dependent on the amount of solar radiation that strikes the place where the panels are situated.  The map below supplied by the National Renewable Energy Laboratory (NREL) shows the areas of the US with the highest solar resources.

 

Figure 2:  Solar Resource map (NREL)

There are three main types of photovoltaic panels; monocrystalline, polycrystalline and amorphous.  Monocrystalline solar cells use very pure silicon.  They are the most efficient (on average 16.8%) but also the most expensive kind of solar cells.  Monocrystalline cells have the ability to use the entire visible spectrum, plus some part of the infrared spectrum.  Polycrystalline cells require less pure silicon so they are less expensive but also less efficient (12.7% on average).  Amorphous cells are not crystals, but a thin layer of silicon on a base material.  These cells are much cheaper, but also have a much lower energy efficiency so more square footage is required to produce the same amount of power.  The lifetime of a solar panel array can easily be 30 years or more. 

Generally speaking solar panels can be expected to produce between 5 and 10 Watts per square foot depending on the type in direct sun.  Solar panels should be placed facing south with a tilt angle approximately to the degrees latitude of the location if they are to be stationary.  For panels with an adjustable tilt angle, the tilt should be increased 15 degrees in the winter and decreased 15 degrees in the summer for maximum output.  Panels placed on a roof with an easterly or westerly tilt may also be acceptable but panels should never be placed facing north in the Northern Hemisphere.  Some other factors which may decrease the output of solar panels are a coating of dirt or dust and increasing temperature.   

Solar panel array can range in size from just a few watts to Megawatts depending on how many arrays are strung together.  Solar photovoltaic panels have an average installed cost of about $8000/kW, plus associated system costs (i.e. inverter, controller, bracing, etc.) of approximately $6000.  There is a new federal income tax incentive for installation of solar panels.  Homeowners can receive a tax credit equaling 30% of the total amount that they spend toward the installation of solar photovoltaic panels.  Homeowners utilizing grid tied systems do not need to supply 100% of their power needs, they can start with 50%, 25% or even 10%, then add on later as desired.  Just be sure that other components like inverters are sized correctly if you plan on expanding in the future.  The typical life of a solar panel system can be greater than 30 years.  Pay back periods for this investment vary depending on local conditions, individual power requirements, and local electricity rates.  Please see our RENEWABLE ENERGY CALCULATOR to estimate the required investment, payback and power output for your specific location.