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.
Once your panels are in place, keep an eye on them and monitor them regularly. Get to know what you daily output of energy is, what everything looks like when operating optimally and even keep a record of such things. This can help you quickly notice when something isn't working correctly and help you ascertain where the problem is.
Solar Energy in our Atmosphere notes.
Use winter sunshine hours instead of summer hours whenever you are calculating how many solar panels you will need. This is because winter sunshine hours only add up to about half that of summer. It is better to overestimate how many panels you will need rather than underestimate. If you underestimate, you will end up losing power when you need it.