Changing sunlight into electricity, along with storing it and controlling it, fall to the main components of the PV system. The two primary types of systems in use today are stand-alone and grid-tied. Understanding their main components, how they work and how they work together makes it easy to understand why the grid-tie approach is by far the most common system in use today.
Both stand-alone and grid-tie systems convert sunlight to electrical energy using PV panels. Although both systems produce electrical energy in the same way, they store it differently. The stand-alone system typically uses flooded lead-acid storage batteries. Lead-acid batteries don't like to be discharged too deeply, charged too fast, charged at too high or too low a voltage, be too hot or too cold, etc. This requires sophisticated charge controllers to continuously monitor and control charging/discharging as well as force the array to run at its maximum power point voltage and current to maximize power.
Grid-tie systems "store" energy by exchanging it with the commercial electric grid. A properly sized system will produce more electrical energy when the sun shines than the system's loads require. The excess is pushed into the grid.
The last critical function of the main PV system components is conversion of direct current (DC), electrical energy produced by the PV panels, to alternating current energy (AC) required by most modern loads and for exchange with the commercial grid. Both systems require an inverter. For the stand-alone system is pretty straightforward — it just converts DC from the battery bank to AC and protects the battery bank from over discharge.
The grid-tie inverter, however, must do much more. It must also sense the critical characteristics of the AC signal on the grid so it can synchronize the AC it is generating from DC with the grid or not produce power if it can't. It must also shut itself down if the grid goes down to prevent "islanding" (independently sending power into a grid that is otherwise dead) and possible damage to the system.
The components of modern PV systems are quite sophisticated, efficient and effective with, of course, the exception of storage. The future will bring continued improvements to the components like more efficient PV panels and better ways to convert DC to AC at the array (e.g. microinverters). Storage, on the other hand, could benefit from significant technical developments.
Significant improvements in storage battery energy density, cost, maintenance and longevity may make stand-alone systems much more attractive as well as battery backed up grid-tie systems. But without better on site storage capabilities, even grid-tie systems will become more problematic as their popularity increases and unpredictable solar energy becomes a larger fraction of net electric energy consumption. The current grid evolved with highly predictable energy sources (coal, nuclear, hydro) and can become unstable when using large fractions of unpredictable sources. The solution to this issue will probably require massive development of an interactive "smart grid."