I understand that solar panels are rated for a certain wattage but that that is the theoretical maximum power output of the panel. How do I calculate the actual amount of energy that I will get from a panel or array based on this rated wattage and factors such as hours of sun light, direction and angel of the roof etc.? I am assuming these factors are very important and will determine how many panels I need to produce enough power..

Most solar contractor's use a tool called a Sun Eye to evaluate the amount of energy that solar panels will produce on a particular roof or the ground. Since Sun Eye's and solar panel energy efficiency software can be rather pricey, you might want to get a few quotes from reputable solar contractors in your area since they will likely give you these types of calculations for free along with their quote... If you are wanting to figure this our yoursel or just get an estimate before contacting a solar company, I recommend looking up your address on NREL's free website - PVWatts Calculators.

Calculating the actual amount of energy a solar panel will produce on your particular roof or for a ground mount involves many factors so the calculation process is somewhat complex. You can first figure out hte daily energy production by multiplying the panel capacity (watts) by the hours of sunlight. But, as EnergyGuy 805 mentioned, the tilt and orientation of the panels impacts the hours of sunlight so usually a tool like a Sun Eye is used to account for efficiency and the optimal siting location. Then, of course, variations by season need to be calulated, a certain amount of percantage should be factored in for general inefficiencies due to dirt, dust, shading, etc. Finally, you can figure out the annual energy productsion by multiplying the daily energy amount by 365 (days/year).

Although this can be a fun project to do on your own, you might be able to get a solar contractor to even give you an estimate remotely as there are quite a lot of software tools out now used by solar professionals. Best of luck!