A grid tied inverter is connected with the LT panel of your building utility. The LT panel is a common bus bar where grid inputs and DG inputs are also connected. Depending on the requirement the surplus electricity generated by solar plant is fed back to the grid and the consumers receive Kwh credits in their electricity bills equivalent to the net electricity fed in to the grid. The net electricity fed in to the grid is evaluated based on the import/export meter under net metering provisions. The grid acts as a battery bank and consumers can take back electricity fed to the grid depending on their daily/seasonal requirement.
While the electricity generated in case of off grid system as to be consumed in real time with in the applications, the surplus electricity generated by solar plant needs to be stored in battery bank. Hence an off grid inverter requires larger batter bank system, so that the electricity generated is available while there is no other source of electricity. In case of off grid system the consumer is completely relined on the electricity stored in the battery bank hence the off grid systems are becoming more expensive as compared to grid tied systems.
The grid tied inverters are typically used in the situations where the grid outages are minimal and consumers take the advantage of net metering and reduce his electricity bill through captive solar consumption. However the grid tied inverter goes off, if the grid power is not available. Some of the grid tied inverters have a provision to charge battery also, so that in case the grid power goes we can consume electricity through batteries. In case grid outages failures are very high (more than 30%) and you have the loads which can run on solar, it is advisable to adopt off grid inverters in order to achieve self-sufficiency and overcome power outages.
Many times consumers ask the question that, how they know that the electricity they consume in case of grid tied inverter is from solar or grid. It may be noted that when we connect the grid tied inverter with LT panel, the output voltage from grid tied inverter is generally 1 volt higher than the grid voltage which allows the solar electricity to get consumed in your system on priority. This means in case the consumer do not have any load requirement, the solar electricity generated is relatively at higher voltage so that it can be fed to the grid.