choosing cells and output voltage

[Jdubb]

I only have enough room for (30) of the 3x6 cells which is only good for 15 volts. I have been told I must try to fit 36 cells to reach 18 volts. My goal is to keep a 12 volt battery (maybe 8Ah) charged for a 30 watt load. I am trying to understand why 18 volts? Does the voltage drop down to 14-15 when placed across the battery? The frame I built was made out of an old piece of tempered glass from a stereo cabinet door. the dimensions were dictated by that piece at 17 x 38 usable surface area. I planned on using a charge controller but not sure whether MMPT or PWM. First thing is to figure how many and what kind of cells to buy.

[Naptown]

If you can trim the cells to make 36 fit it might work
One thing about cells. no matter what the size they produce about .5V at Imp so smaller cells will bring the voltage up but at the cost of amperage or watts.

[Jdubb]

Trim the cells? Sorry but I've never actually seen one. Please explain which would be better, trimming the cells or just hiding the edge behind the frame a little. I measured and with 36 cells just about 1/4 inch of the outside edges will be behind the frame or hidden from sunlight

[Naptown]

cutting down the cells to fit the area is much better than hiding behind the frame. You will create hot spots and lose much more power by covering the cells