Source: https://sites.google.com/site/earthforsolar/what-is-tab-and-bus-wire-used-for
Tab or Tabbing wire is used to connect your solar cells in series to get your volts to your desired need,when making a solarpanel with solar cells, you need to connect your solar cells in rows. Make sure your tab wire is around 18-20mm thick to carry the current better.
Anything thicker can be too thick and stiff and harder to work with. Good tab wire is made out of solid copper coated with solder for easy flow.
Just because its coated with solder does not mean you don't have to use solder when connecting your cells together. All tab wire on the market today requires you to use solder with it. Some tab wire is just made out of tin which does not adhere well so if you don't know what it is made out of ask before you buy.
Bus wire or Connecting wire looks like tab wire but is much wider. Bus wire is used to carry the current across each row. It can range from 2.5mm to 5mm depending on the power of the panel and size of the solar cells that you are using. It is also made out of copper coated with solder. Some bus wire is made out of tin as well, which does not carry the current as well as copper. So again If you don't know what it is made out of ask!
Here is an example of using Bus wire; Say you have 36 cells to build your panel. You have 9 solar cells in a row connected inseries. You would then need to connect all the rows together with bus wire on the tab wire sticking out at the ends of therows. You would have 4 rows of your nine this would be 36. This could make a 20 volt panel. Open volts would give you thevolts to push the power to charge a 12 volt battery. Normal panels made for charging a 12 volt battery normally put out 15volts or higher.
Below are some pictures that show you the process of soldering cells together
youtube video for tabbing and connection: https://www.youtube.com/watch?v=kUZYjwwvFvk
