On-Grid Solar Electric Systems

Governments are encouraging households and group housing societies and associations to install On-Grid Solar Electric Supply by offering tax cuts, subsidies, etc. This is being done with the intention of bringing about reduction of electricity generation from fossil fuels which will reduce carbon emission.

A home with solar electric supply installation has to face eventualities of receiving inadequate or no power at all from it when the sun is not shining in the sky; an alternative supply has to be available in order to maintain continuity of electric supply. This then results in what is referred to as on-grid solar electric system. It is also called as grid-tied or grid-intertied solar electric supply system. The two alternative schemes in the on-grid solar systems are described here.

On-Grid Solar Supply System Without Battery

Fig. 1 shows one such on-grid solar electric supply system schematically. The figure shows the solar panel mounted on the roof. The inverter is synchronized with the grid supply so that it works in conjunction with the grid supply to provide electricity to the household. The energy meter in this system is a bi-directional meter; it permits flow of energy from the grid to the household loads or from the solar electric system (through inverter) to the grid. The solar system supplies load upto its capacity. When the solar electric system generates more electricity than required by the household load, it supplies the extra power it generates to the grid; and when the household load is more than what the solar system can supply, the grid supplies the balance power.

In this scheme the surplus power generated by the solar system and supplied to the grid system earns a credit from the utility.

You will notice that in this scheme if the grid supply fails at any time, for any reason, this home will not get power even for emergency purposes from anywhere. This scheme does not have a back-up which could have been provided if a battery was provided to feed the inverter.

This scheme of on-grid solar power supply without a battery is cheaper than the next alternative we shall see, but is not very popular due to the above mentioned drawback.

The schematic in fig. 1 shows only the basic system components. The other minor but essential components such as the DC switch between the solar panels and the inverter, the various MCBs required for the various loads, the main utility breaker, etc. are not shown.

On-Grid Solar Supply System With Battery

In areas where continuity of grid supply cannot be depended upon, a battery backup is desirable. Fig. 2 shows the schematic of such a system. In the normal course, the battery gets charged from the solar electric system and remains in a standby mode. When the grid supply fails the battery can supply at least the essential loads of the household. (The last blog dealt with the requirements regarding the charge controller and the battery).

These on-grid solar supply schemes are called net-metering systems. Different states and utility companies have different regulations for operation of net metering systems. Obviously the utility does not give credit for the power you supply at the same rate at which the energy drawn from the utility is charged.

Therefore generally such systems are designed so that the capacity of the solar electric system is about half of the total load of the household. During the daytime when the solar electric system is able to generate to its full capacity, it supplies most of the power requirement of the household with the grid supplying a smaller proportion; rest of the time the grid supplies the entire power requirement.

The initial cost of solar electric systems is quite high. But its operational cost is very low. Moreover the solar system has a life of about 25 to 30 years. Therefore this On-Grid Solar Electric system tries to derive the benefit of reduced grid supply charges over a long period of time by making a large initial investment.