If you think of the flow of electricity like the flow of water, then a diode can most closely be related to a one-way valve in a sewage pipe. It lets electricity flow one-way only and blocks it from coming back in - just like a one-way valve operates in a sewage pipe - letting sewage out, but not back into your house.
A solar blocking diode (or low voltage drop diode) is a diode that is used in the electrical schematic of a solar panel to restrict or block the electrical current coming out of the solar panels to reverse or flow backward through the solar panels. If the current were allowed to flow back into the solar panel, the solar panel could get damaged by the current and suffer from thermal destruction. So you do not want the power coming out of a solar panel to ever flow back into it.
It is claimed that if you have a charge controller and a single panel or single string of panels, that you do not need a blocking diode - but I still recommend that you put a blocking diode on every solar panel you create or that you purchase solar panels with blocking diodes to prevent any possibly damage to your solar panels.
It is important to note that the rating on a blocking diode - for example; 8 amps, 45volts - is rated on a free-air system. The effeciency of a blocking diode decreases as the temparature around the diode increases (as is the case when blocking diodes are installed inside solar panels and solar panel junction boxes, so make sure that you purchase blocking diodes from manufacturers that allow a safety factor when selling diodes to you. A diode I like to use is a Schottky blocking diode. The amps and volts depend on the application. Some sellers advertise an 8amp 45 volt blocking diode for sale, but the diode is actually rated at 16 amps. The company is good about selling this diode with a safety factor so that you can use it correctly in your application. Always check for the full rating of a diode and how temparature affects its performance if that information is available.
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