Overall system performance in real-world conditions
Generally speaking, there are four kinds of solar photovoltaic (PV) panels, each offering varying degrees of efficiency. (We address these various technologies in a forthcoming article.) These technological considerations aside, a given system's performance will be impacted by the following variables:
Insolation is a fancy word for sunshine. All else equal, the more sunshine you get, the more electricity your system will produce. (I can hear the "duhs," by the way...). The National Renewable Energy Lab (NREL) has compiled monthly insolation data for nearly 250 locations around the United States. To see how many hours of sunshine, on average, you can expect to get over the course of a year, check out their online Solar Radiation Data Manual.
Slope and orientation
You'll notice that slope - or the angle of your roof, assuming you're installing a roof-mounted system - will impact the amount of direct sunlight that hits your panel array. This figure varies, depending on the latitude of the location where the system is installed. A general rule of thumb: the closer to the equator you are, the flatter the optimal slope; the more pole-ward your latitude, the steeper the optimal slope. And, for those of us in the northern hemisphere, arrays are always most efficient when facing South. (These observations have everything to do with the fact that the earth is round and is on a tilted axis.) Anyway, suffice it to say here that south-facing arrays, with slopes between 20 and 60 degrees, are a fairly safe bet in North America.
It goes without saying that if there are trees, buildings or other objects blocking sunlight from reaching your array, your system performance will suffer. Shading is commonly measured in 30-minute increments, throughout the course of the day, in the incredibly scientific units: no sun, partial sun, and full sun. Short of taking a chainsaw to your neighbors hedgerow (which is in no way advised or condoned by Getsolar.com), you should check with municipal and/or state authorities, as some governments have enacted solar access laws to protect property owners' rights to access sunlight.
Dust and dirt
Chances are, you probably won't be up on your roof every week, attacking your panel array with a squidgy and a bucket of warm, soapy water. Even if they're cleaned twice a year, your modules will face a slight performance loss due to the accumulation of dust and dirt. (Cleaning the panels with water and mild, bio-degradable detergent is recommended.)
Total system efficiency factor
No matter how well insulated, wires are prone to a certain amount of electrical loss. This is a function of distance traveled (among other things), and any smart solar energy installer will do his/her best to minimize the total wiring length needed. There is also a loss factor as the DC current from the system's panels passes through the inverter and is converted to AC power. Good inverters are about 95 percent efficient. Finally, while plenty of direct sun is good for boosting total output, solar modules are not efficient at high temperatures. Thus, there is a degree of what's referred to loss due to "module heating." Lump these factors together, and you get an idea of how efficient your system will be.
A good starting point is a total system efficiency factor of 70 percent. Indeed, the California Energy Commission (CEC) guidebook suggests a factor of 71 percent. (This is sometimes referred to as the "derate" factor.) Multiply 0.7 by the DC wattage - listed according Standard Test Conditions (STC) on the back of each of your modules - and you'll get a decent ballpark figure of how many kilowatts your panel will pump out when exposed to full sun. Multiply this figure by the average hours of sunlight per day. Multiply this by 365, and you should come up with the approximate number of kilowatt-hours produced by each module over the course of a year.
GetSolar Blog: "Rules of thumb: Expected performance of PV systems"
GetSolar Blog: "5 Things Homeowners Should Know About Solar Energy"
PV System Performance