Solar power system consists of solar batteries, solar controller, battery (group). If the output power is 220V AC or 110V, it also needs to configure inverter.
The role of each part as follows:
Solar panels
solar panel is a core and highest valuable part of the solar power system. Its role is to convert the sun's radiation to electrical energy, then make the electrical energy store in batteries or directly power the work load. Solar panels will directly determine the quality and cost of the entire system;
Solar Controller
Solar controller's role is to control working state of the entire system, and play a role of battery over-charge protection as well as over-discharge protection.In areas with large temperature difference, the qualified controller should have  temperature compensation function. Other additional features such as light control switch, timer switch are optional functions;
Battery
lead-acid batteries are most popular in solar lighting system. In small micro-system, nickel metal hydride batteries, nickel cadmium batteries or lithium batteries are also used. Its role is to store energy when there is sunlight, and re-release it when necessary.
Inverter
On many occasions, it needs to provide 220VAC, 110VAC AC power. As the direct solar energy output are generally 12VDC, 24VDC, 48VDC, in order to provide the 220VAC electrical power, the DC issued by solar power system needs to be converted into AC power. So we should use the DC-AC inverter.In some cases, if it needs to use a variety of voltage load, we also should use DC-DC inverter, for example, convert 24VDC to 5VDC power(note: it's not a simple voltage step-down).