Storage devices like batteries will certainly help. As a PV array is a device for generating electricity from sunlight, so a battery is a device for generating electricity from electricity. You can work out if the battery is big enough by performing a worst case analysis of total energy supply and demand. However if you do this you will probably be (extremely) dismayed at the size of battery most real systems will require. Present day batteries probably cease to become cost effective after they have stored a day's worth of energy, though it  depends on the alternative source of electricity they are competing with. Of course, all depends on the battery technology and price.

Batteries, in addition to the needed area, is considered in general the bottleneck of PV systems. Sure the use of batteries will definitely affect the negatively the investment made for such systems. Today, the trend is the use of grid-connected PV systems, where the storage battery is the public grid, saving this way the high cost of traditional batteries.    

Hi Ali and Ahmed, I agree that the public electricity grid behaves like a battery but, despite the assertions of the utility companies, it is no such thing really. It 'stores' electrical energy by delaying the generation of energy. A battery will give up 80-90% of the energy you put in, equivalent to generating electricity from electricity with that level of efficiency. If the grid is powered mainly by fossil fuel, the efficiency of the store is the efficiency with which the fossil fuel can be turned back into the electricity you originally put in, which is nowhere near 90%.

Dear Tony and Dear Ali

Thank you very much for your answers. One question: Are solar Pv's used as base load power plants? Does their nondispatchable nature let them to serve as a base load power plant? Or they only serve during peak loads?

Ahmad

They are used when sun is shining