There are four forms of solar power panels on the market. Mono-crystalline, Poly-crystalline, Thin-film and the latest being Integrated Circuit CIGS panels (Copper / Indium / Gallium / Selenium ).
Poly-crystalline panels are the least efficient of the crystalline variety, but also the ones nearly all solar supplier will sell you. They were developed in the early 1980’s by Solarex, to address the issues of high energy production cost and low packing density prevalent with the then existing circular “ingot” based Mono-crystalline cells.
Poly crystalline cells are a square “ingot” based technology, that omit the second refinement step necessary to form mono crystalline cells. The material is less pure, and more susceptible to current and voltage drop with increasing cell temperature. They have a characteristic speckled blue appearance as shown below.
Poly-crystalline cells are susceptible to temperature, so they need to be de-rated by at least 20% at cell operating levels of 50 degrees Celsius, which is typical under field conditions (especially in Australia in our summers). Panels are manufactured by joining multiple cells together.
Mono-crystalline panels date back to the 1950’s and early space exploration, which was their main use.They have basically changed very little since that time. The production of Mono-crystalline solar cells is a tedious, convoluted and high energy process and it is unusual to see them being supplied today.
Understanding Panel Specifications
When purchasing solar panels there are a number of terms that you will encounter that unless you have an electronics background will not make much sense to you. To demystify these terms, lets have a look at them one at a time.
|Electrical Characteristics: 190W Hanwa Monocrystaline|
|Maximum power-Pmax||190 W|
|Open circuit voltage-Voc||44.8 V|
|Short circuit current-Isc||5.78 A|
|Maximum power voltage-Vmp||35.8 V|
|Maximum power current-Imp||5.33 A|
|Electrical Characteristics: 65W Shell Showa Thin Film Panels|
|Maximum power-Pmax||65 W|
|Open circuit voltage-Voc||52.7 V|
|Short circuit current-Isc||2.2 A|
|Maximum power voltage-Vmp||36.7 V|
|Maximum power current-Imp||1.78 A|
Open Circuit Voltage (Voc): The voltage at the solar panel terminals when nothing is connected to it (when appliances are connected this is called the “load”). The circuit is “open” and not connected to anything and not supplying any current.
Maximum Power Voltage (Vmp): The voltage at the terminals of the solar panel when the maximum rated power (Imp) is being drawn from it.
Short Circuit Current (Isc): This is the amount of current that flows through the electrical leads when directed connected to each other with nothing in between. They are “shorted”. DO NOT do this unless you want to risk damage to your panels. It is the maximum amount of current that the solar panel can supply.
Maximum Power Current (Imp or Imax): This is the maximum current in amps that flows through the terminals when the maximum power is being supplied by the solar panel. In an off-grid system you use this figure to work out how many panels you need to charge your batteries for a given period of time. More on this later.
Rated Power (Pm or Pmax): This is the maximum rated power in Watts.