Quick look at my most recent Lidl purchase for the garden, a PID solar spotlight for £7.99.
While solar panels can continue to work on days when it is not sunny, you should not have them installed if you live in an area that is cloudy most of the time. The amount of energy you save with them will not be worth the amount you paid for them.
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.