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.
To realize even better savings, purchase grid tied panel systems. If your panels are connected, you might be able to sell extra power to the local utility. This should help you pay for your solar energy system and will also give you the option to draw power from the main grid in case your panels are not functioning.
Hot Sun Industries, maker of the Powerstrip solar pool heating system explains basic system operation as well as some plumbing guidelines.