There are many benefits. You are in control of the system, and depending on the size of the system you may be able to supply others and charge them for your electricity. It is ideal in a rural locations, where it would be costly to connect to a mains electricity supply. It can be used on homes, offices, portacabins, summerhouses - in fact anywhere you need power. Generally the power generated is for your own use and not sold back to the grid, but it is possible.
The system works the same way as an on-grid system, but the power generated by the PV collectors is stored in batteries for your use as required. The panels are connected to an intellegent battery charge controller. From this the batteries are then connected to an inverter. The batteries are then connected via your fuse board to enable you to use the power.
The system can be easily re-sited if required. Please keep in mind that there are dangers involved as there is current through the system, and extreme caution needs to be taken and set procedures must be followed.
There are FITs tariffs available regardless of the size of the system, unlike on-grid PV systems.An off-grid system could make you a lot of money; a large system would allow you to sell electricity to others, at a reduced rate, and also claiming money back from FITs. It is a simple device, virtually maintenance free. The only outlay would be to replace the batteries every 10 years or so. So you could be making money AND saving the planet at the same time.