What is a diode?
Simply put, a diode is a device that allow electricity to flow in one direction only
Why do you need them with solar panels?
Solar panels generate electricity when the sun shines on them. If you connect a solar panel direct to a battery and the panel is a generating a higher voltage than the battery voltage, electricity will flow to the battery and charge it up.
However, if the battery is a higher voltage than the panel (for example at night) then electricity will flow the other way and slowly discharge your battery.
For this reason, we normally put a diode in the cicuit to ensure that electricity can only flow in the direction we want (into the battery) Its normal to put the diode on the back of the solar panel on the Positive (+) terminal with the 'arrow' pointing away from the panel.
I heard Diodes lose a bit of my voltage - is that correct?
Yes, all diodes will have some 'voltage drop' - A 'Standard' diode may lose about 1.5 volts. Often this isn't a problem, but for solar we want every volt and amp we can get. Consequently solar panels typically use 'Schottky' Diodes. These were specially designed to have as low a voltage drop as possible. Typical about 0.3V drop across the diode
Do I always have to have a diode?
No. If you are using a panel to directly power a fan in the sun, you wouldn't need a diode as there is no battery in the circuit to get drained. Many people use panels to directly charge devices like phones or mp3 players. These will already have a diode built in to the device so would not need a diode (though it wouldn't hurt to have one in)
There are so many diodes out there...! Which diode do I need for solar
I recommend a Schottky diode with a voltage rating and current rating about triple what you need. So if I had a 300ma 5v panel I'd probably pop a 1amp 20V Schottky diode on it. You don't need to be too specific and a 2amp one would also be fine.
OK, I get it, but where do I put it?
You put it on the positive connection of the solar panel with the stripe 'away' from the panel pointing in the direction of the current flow