$4-$6 per watt (DC) is a ballpark range for an installed solar electric (PV) system before incentives. A one-time federal tax credit of 30 percent of the total system cost is available for both residential and commercial solar electric systems. It may be carried forward if not completely usable in the installation tax year. This credit is available through 2016. Depending on your local utility, there may be solar rebates available. The average 5-kW residential system will cost $20,000-$30,000 before the tax credit.
An average PV system consists of modules that produce approximately 1,000 watts (1k) per one-hundred square feet of space. So a standard 2k system needs 200 square feet of unshaded, south or south-west facing space. The direction the system faces and angle at which it tilts can be adjusted with minor losses in productivity.
Figure on 4-6lbs per square foot of PV panels for standard multi-crystalline modules.
Due to module supply shortages plan on at least 12 weeks from the day you sign a contract to the system installation. This can be faster if your vendor has modules on hand, or slower if modules are in short supply or your local permitting authority is slow to grant permits. It helps to flexible about the exact equipment you'll install.
Estimate how much electricity you use, on average, per month. Of the kWh you purchase, how many are billed at Tier 2 , Tier 3, Tier 4 rates by your utility? Those are the kWh you are trying to offset with your PV system (if you’re focused on the economics).
Do you have unshaded south-facing space on a newish roof or structure?
If you have an old roof, consider installing the PV system in conjunction with a new roof.
Once you know your average kWh use per month, and have confirmed you have an installation location (pole mounted, shade structures, carports, roofs, etc.), it's time to call an installer for a quote.
The CEC website can direct you to local vendors. We suggest getting at least two quotes and checking the installers figures against the Clean Power Estimator. Also, since you will have a long-standing relationship with this vendor (PV systems last 30+ years) your comfort level with the person/company is more important than minor price differences.