An off-grid PV system has no connection with any utility and must generate and store enough electricity on its own to run your entire house. An off-grid inverter draws power from either a battery bank or generator and sends it to your main electric panel. It's common to size battery banks to provide power for 3-5 days. An off-grid system often includes an additional source of power such as a wind turbine or generator. If It's cloudy for several days, a back-up generator automatically starts to re-charge the batteries and provide power. Net metering is not available to off-grid homes.
An off-grid system can be financially justified if the cost of bringing power lines to the property is so high it would offset much or all of the cost of the PV system and generator.