A rooftop solar PV system could be with or, without having grid interaction.
In grid-interactive system( includes on-grid and grid interactive system), the DC power generated from solar PV panels is converted to AC power using power conditioning unit and is fed to the grid depending on the system installed at institution, commercial establishment or, residential complex.
It used to generate power during the daytime which is utilized fully by powering the captive loads and feeding excess power to the grid as long as grid is available. In cases, where solar power is not sufficient due to cloud cover etc. the captive loads are served by drawing power from the grid.
The grid-interactive rooftop solar PV systems thus work on ‘net-metering’ basis wherein the beneficiary pays to the utility on net meter reading basis only. Ideally, grid interactive systems do not require battery backup as the grid acts as the back-up for feeding excess solar power and vice-versa. However, to enhance the performance reliability of the overall systems, a minimum battery-back of one hour of load capacity is strongly recommended.
In grid-interactive systems, it has, however to be ensured that in case the grid fails, the solar power has to be fully utilized or, islanded immediately feeding to the grid (if any in excess) so as to safe-guard any grid person/technician from getting shock (electrocuted) while working on the grid for maintenance etc. This feature is termed as ‘Anti-Islanding Protection’.
Non-grid (or off-grid system) interactive systems ideally require a full load capacity battery power back-up system. However, with the introduction of advanced load management and power conditioning systems, and safety mechanisms, it is possible to segregate the day-time loads to be served directly by solar power without necessarily going through the battery back-up.
As in the previous case of grid-interactive systems, minimum one hour of battery back-up is, however, strongly recommended for these systems also to enhance the performance reliability of the systems.
The non-grid interactive system with minimum battery back-up is viable only at places where normal power is not available during daytime. In case the solar power is to be used after sunshine hours, it would require full load capacity battery back-up which will increase the cost of system which may not be economically viable even with support from Government.