Just to clarify, my first 3 points mentioned earlier are some basic PV module characterics that are pretty important for any PV system designer working for an installer company. I am an electrical engineer and I was recently working for a solar installer here in the USA. I was responsible for roof layout design, string-sizing (how many PV panels to connect in series to match the central inverter), and creating the "one-line" system schematic showing the connection of the components I mentioned earlier, as well as the connection to the utility. If you work for a company with many engineers these duties may be split among various people but if you can gain knowledge in all these areas you are more valuable.
As far as "string-sizing", if you can find out what brand of inverter the company likes to use, you can go to the website of the inverter manufacturers
and practice string-sizing using the free interactive tools that most provide. The tools calculate for you the sum of voltages of the series connected PV modules including the effect of temperature coefficient and also the power totals. I think all of these manufacturers have such website tools:
Satcon (for higher power systems)
Again, best of luck.