Always use non abrasive cleansers on your solar panels. An abrasive cleaner can scratch the surface of the panel and cause permanent damage and affect how well the solar panel works. There are commercial cleaning solutions to use or you can use a pool skimmer with a very soft cloth on it.
See how to test your solar panel for open voltage and current reading. Results will vary depending on sun strength, time of day, angle of light and temperature.
When researching for a good solar panel installer, don't forget the Internet. Yes you may start out in the phone book, but the Internet will give you a lot more information as well. You may find testimonials from other clients and ideas for contractors that you may not have realized are out there.