Formula to calculate power from a solar panel?
Have a look at Sustainable Energy, without the hot air, Prof. David Mackay, ISBN 9780954452933.  It's also on the web.  http://www.withouthotair.com

He says:


  Typical solar
  panels have an efficiency of about 10%; expensive ones perform at 20%.
  (Fundamental physical laws limit the efficiency of photovoltaic systems to
  at best 60% with perfect concentrating mirrors or lenses, and 45% without
  concentration. A mass-produced device with efficiency greater than 30%
  would be quite remarkable.) The average power delivered by south-facing
  20%-efficient photovoltaic panels in Britain would be
  
  &nbsp;&nbsp;20% × 110 W/m2 = 22 W/m2.


http://www.withouthotair.com/c6/page_39.shtml

The 110 W/m2 is derived on the previous page, for a average south-facing roof in Britain: raw energy = 1,000 W/m2, with a correction of 32% for orientation and 34% for cloud = 108.8 W/m2.  Tracking systems, non-Earth locations, no cloud would affect all of these.

There are many references in this book for you to follow up.  Wikipedia page Solar Cell Efficiency also has several dozen references.