Before you pull the trigger and buy a system, take a look at how long the manufacturer guarantees the product for. You want a fairly long warranty because the system will have to pay for itself over time and help you begin to generate a profit. Therefore, make sure the warranty is for at least 25 years.
Remember to account for any extra influences when you calculate how much you can expect your solar panels to output. There are some intended outputs specified on panels, but that is only accurate if they are placed in an area that is sunny all the time. Any obstruction from trees or from a cloudy day will decrease the output.
An overview of how I made a solar pool heater for under $100.00.
Check that your solar-energy installer has the proper licenses and insurance. Installing solar panels on a steep roof is a delicate job, and you should feel confident that your installers know what they are doing. At a minimum, look for a contracting license, workers compensation insurance, and general liability insurance.