Some GE and BP panels use 40 cells for a 12V panel.
The 0,58V is 'Open Circuit' with no current flowing.
In operation the voltage per cell will be closer to 0,48.
They panel needs to operate at 17V because of voltage losses.
If current flows, voltage is lost.
Wires, connections, diodes or controller will all cause losses.
A 12V battery is Dead at 11,9V, and Full at 12,6V.
However, it needs charged to 13,7 ~ 14,4V to become Full efficiently.
Cold battery temperatures need higher voltages.
Some panels were made with 32 or 33 cells. They were called "self regulating".
They could not make enough voltage to over charge the battery, but they often could not make enough voltage to overcome the voltage losses, and still have enough voltage to charge a battery.
I do not believe any company makes them now.
I would recommend building 1 panel with 40 cells,
unless you can get 20 or 30 more cells to build 2 panels with 36 cells each.
Some cells will break.
I would recommend a 12V system.
One panel will be about 20 watts at 12V using the standard rating method.
Wire losses will not be large with 'normal' size wire.
One battery. Cheaper and more common inverters, lights, etc.
Instructions on how to build a panel will be different, because everyone has different materials, skills, and ideas.
Read how other people made them, think about what you can do,
pay attention to problems they had after the panel was built.
Search page after page in the 'solar' link at the top.
There is a lot of information here.
When you find something you like, click the users name and look at his other stories and diaries for more information.
Avoid buying booklets.
Where are you?
G-