Solar panels, also called photovoltaic (PV) panels, produce DC power from sunlight to charge batteries and provide electrical power. The panels can be used single or in series or parallel arrays.
Since PV necessarily run at higher voltages than the batteries they charge, a charge controller is placed between the PV and the battery bank.
Many factors will affect output; and panels will rarely generate power equal to their laboratory rating.
There are several types of PV widely available on the market, and many more in research stages. PV are described by how each panel's cells are constructed.
Mono or single crystal cells are made from complete slices of silicon crystal. Mono cells have rounded edges because they were cut from a single cylindrical crystal.1)
Strained metaphor: if poly cells were it were paneling they would be veneer because the visible surface is made from one piece of material.
Poly cells are made up of smaller pieces of slices. Poly cells are rectangular.3)
If poly cells were paneling they would be OSB, because they are made up of many flat pieces of silicon.
[Note: actual thin-film panels are rarer as of 2018. The market has gone to flexible mono or poly5)]
Thin film or amorphous PV uses photovoltaic material deposited on a substrate rather than silicon crystals.
If thin film cells were were paneling they would be colored plastic veneer because it is produced inexpensively and is least affected by ambient conditions.
While most panels are hard mounted horizontally, some are mounted on tilt panels to increase yield, and some are portable.
12V panels are not really 12V; they are called that because they charge 12V battery banks (which aren't really 12v either!). Since they are called 12V we say nominal (ie, “named”) 12v. [yes, it's confusing. – frater secessus]
higher voltage panels (24v = 72 cell, 20V = 60 cell) are usually cheaper by the watt than 12V (36 cell)
higher voltage panels are more common on the used market, as people upgrade residential/commercial installations
MPPT controllers do a DC-DC downconversion that is quite efficient. If Vmp isn't required most will move PV voltage away from Vmp to prevent power from ever getting to the CC.
Higher voltage panels can reduce wire costs; amps are cut in half for the same amount of wattage. They more likely to stay above charging setpoints in poor insolation or high heat.
PV are generally rated by several criteria11)
Note that in this example the Power (W) rating is 190, which is the Vmp x Imp (28.60 x 6.64 = 189.904W).
In real world conditions power output will likely be less than under optimal lab conditions and the Vmp may not be at the voltage given on the label. Vmp will vary due to local conditions like temperature, shade, and sunshine. An MPPT charge controller, if present, will sweep the range of voltages regularly to find Vmp under the existing conditions.
Panels will get closest to their rated output at ambient temperatures around 32f/0f when the panels will be running about 85F. By the time ambient temperatures are 90F panel temps will rise to 145F and power output will drop about 18.45%.12)