Photo cell principle is based on semiconductor diode which uses photoelectric effect.  It is the emission of electrons or other free carriers when light hits a material. Electrons emitted in this manner can be called photoelectrons.  Photoelectric effect can be attributed to the transfer of energy from the light to an electron. It generates direct current depending on intensity of light hitting PN transition. 

Photovoltaic panel (PV) is combination of serial and parallel connection of photo cells in proper mechanical  casing. The Wp (Watt peak) represents the maximum electric power that can be supplied by photovoltaic panel in standard operating conditions. In other words sunshine of 1000 watts/m² an ambient temperature of 25°C. The higher Wp, the more efficient photovoltaic panel is.

Direct current (DC) generated in PV panels is either directly used in home appliances or accumulated either in variety of battery systems, or in hot water.  Additional important component of photovoltaic system is  DC/AC inverter which can transform DC to alternative current (AC) usually used in public grid systems. Inverter can be designed as modified or pure sine wave inverter. 
Solar regulator/ charge controller/ charge regulator or battery regulator is another important part of PV system. It limits the rate, at which DC is added to, or drawn from battery to grid. It prevents overcharging and can protect battery against overvoltage, which can reduce battery performance or lifetime according to battery technology.

Efficiency of PV system depends on many facts and operating conditions, e.g. no. of panels installed, proper angle, orientation, etc. PV panels can be installed on any type of roof of any cover, optionally monted to building face. PV system can dramatically reduce your dependence on public grid and costs for electricity.  And there are more benefits as well: PV panels have been shown to keep your roof cool by providing shade during daylight as well as warmth at night by helping hold heat inside building. 

Photovoltaic as well as thermal solar power is the fastest-grawing source worldwide and its growth is close to exponential. Especially PV panels are becoming more and more popular due to the minimum maintenance requirements. Such a power is clear, renewable, does not produce smoke and noise and definitelly is the cheapest energy ever.