How to choose a Solar Panel

MUCH more information will give you a much better answer

Find average daily insolation by month from Gaisma

Watt hours max per day != Watts rate max x sunshine hours from Gaisma.

If storing in a battery and retrieving the average wWattage you can run continually ~~=

Watts = Panel_peak_Watts x Sunshine hours per day / k

k is a mimimum of about 30 and can be 60+.
Depends on how well you match panel to battery etc.

What are you operating?
what Wattage?
What location?
More ...?

Olin says black side up.
Sometimes its pretty blue side up :-)
Depends on technology used in cells.

I try my best to put up a list, please feel free to add and elaborate.

Solar Panel selection criteria:

• Watt
• Dimension
• Efficiency
• Technology (Thin Film, Multi-junction)
• Operating voltage
• Operating setup (With Battery or Without Battery)
• Cost

Solar Panel Distributor normally sell by Watt, quotation normally comes in Watt. From what i got from a few manufacturer, averagely they are selling USD4/Watt