Solar energy sounds like magic.  It certainly feels like magic when you look up at your panels and realize that they are generating your electricity while lying perfectly still and quiet, but it’s really just physics.

Solar panels, or photo voltaic (PV) panels convert the sun’s energy (photons) into electrical energy.  Photons from the sun hit the panels and excite electrons in the panel’s semiconductor surface (silicon usually, but sometimes gallium arsenide or copper indium diselenide) which has been treated so that one side of the cell is positive, and the other is negative.  If conductors (wires) are attached to each side of the cell, then these electrons will follow the path of least resistance through the panels, which happens to be through the wire and into your home.  At this point, the current is DC.  Most appliances and devices in your home use AC current so an inverter is necessary to – you guessed it – invert the current from DC to AC.  A small amount of power is lost in the form of heat during this transformation.   

The amount of current produced by a solar panel is dependent on the amount of photons (sunlight) hitting the panel.  

The power of solar panels is measured in kilowatts (kW).  This is called a nameplate rating and it describes the maximum power the panels can produce under ideal conditions.  The energy the panels produce is measured in Kilowatt hours (kWh).  These are the same units the utility company uses to bill you.  The amount of kWh’s a panel will produce varies according to the orientation of the panels, and the amount of sun they receive, and even depends somewhat on the temperature of the panels.  The hotter it is, the less efficient the panels are.     

Solar water heaters work by using the sun’s heat to warm a coolant (counter intuitive, but true) that flows through pipes on one’s roof (or some other convenient surface, sometimes panels are mounted to the side of a house).  The super heated coolant is then piped into the hot water tank.  The coolant never mixes with the water in the tank, it is sealed inside pipes and is dyed dark purple so you would know if any leaked into your water system.  In any case, I’ve been assured that it is non toxic, but I prefer to trust the dye to warn me that it’s leaking into my drinking supply rather than rely on the FDA’s testing.  Once the water in the hot water tank is heated it is then pumped into your conventional hot water heater.  In some cases (a sunny summer day) the water is hot enough for you to use and your hot water heater won’t even turn on.  In other cases (winter time) the system functions to pre-heat the temperature of tap water (approximately 50 degrees) before it flows into the conventional water heater, greatly reducing the amount of energy needed to heat the water.  I saw one system that skipped the step of pumping the water into a conventional tank entirely.  Instead, this system used small tankless water heaters (http://www.tanklesswaterheaterguide.com/) close to the taps that needed hot water (kitchen, shower, etc) in order to supply any additional water heating above and beyond that provided by the solar collector. 

Coal works by turning chemical energy into mechanical energy and then into electrical energy.  Byproducts of this process are:  carbon dioxide, sulfur dioxide, methyl mercury, and destruction of natural habitat by coal mining, including mountaintop removal.