An off-grid or standalone photovoltaic system is when your solar photovoltaic system is not connected to the utility grid and you are producing your own electricity via solar, wind, microhydro, generator, etc. These systems will generally have a battery bank in order to store the electricity for use when needed.
Off Grid Solar Systems do not need to be connected to mains power. As there is no ready source of reserve power, they rely on generators or storage batteries to provide electricity on days when the solar panels generate less electricity than is required, e.g. on cloudy days or at night time.
The idea is that at times when the system provides more electricity than required the surplus can be used to recharge the batteries.