Net Metering arrangements for Roof top Solar PV Systems

Most of the Indian cities, towns and villages have nearly 250 to 300 days of sunny days and thus roof top solar PV system can be a very attractive option. The Jawaharlal Nehru National Solar Mission (JNNSM) with the goal to encourage roof top solar PV systems in India has implemented a separate scheme known as “Roof top PV and Small Scale Solar Generation Program”. 

In my opinion, Roof top solar PV systems are going to witness appreciable capacity expansion particularly on the residential and commercial buildings in India and can be seen as a significant business opportunity.

Types and Models of Roof top PV systems:

The roof top solar PV system can be:
1. A grid interactive system or,
2. A stand-alone system. 

In grid interactive system, the DC power produced is converted to AC power, and after due conditioning this power is fed to the captive loads of the premise. The excess power is injected to the utility grid through 11 kV or LT lines, depending on the size of the solar PV system. When sufficient solar PV power is not available, the loads are fed from the grid supply. 

Although the grid interactive solar PV system is not supposed to have a battery pack, but to enhance the reliability a minimum battery backup of 1 hour is technically recommended. There is some minimum capacity constraints ( 1 kWp) below which the system can not be grid interactive.    

In the stand-alone Roof top Solar PV system, the electrical energy produced by the solar modules are fed to the battery bank through a charge controller. A suitable inverter is connected to the battery which converts the DC output of the battery (or the Solar PV) into AC of specified voltage and frequency to run the electrical appliances of the premises. This system is not connected to the utility grid and hence excess electrical energy produced by the solar PV cannot be exported to the grid and similarly if extra energy, in excess of solar PV output, is required cannot be imported from the utility mains. Thus, such a system requires fine balancing of generation, demand and storage capacity.  

Different arrangements of Roof top solar PV system:

Two different arrangements of Roof top solar PV system do exist. They are:

1.      Self owned roof top solar PV system, and

2.      Third party owned roof top solar PV system.

In the self owned arrangement, the roof top owner or the premise owner installs the solar PV system, either of its own or with the aid of a system supplier. The power produced is first used to feed the captive loads within the premise and the excess power is fed to the grid.

The third party roof top solar PV system can be an attractive and viable model particularly for residential sector in India and seems to be a good business and job provider. In this model a developer or intermediate agency called the “third party” design, install and lease out the solar PV system to interested roof top owners who in turn pay them a monthly rent. 

This model offers some great advantages; 
(i) the roof top owner does not have to invest for the PV system, and
(ii) does not have to bear the brunt of the technological risks involved in a fast changing solar PV business.

Net Metering arrangement:

In both the above mentioned roof top solar PV models, the feeding of excess energy to the grid or the drawal from the grid in case of insufficient solar power generation is through a net metering arrangement. 

Two meters have to be installed at the premise and will replace the existing meter. One, the solar energy meter to record the energy produced by the PV system and other the net meter, a bi-directional meter, to record the import/export of energy by the consumer from/to grid. The point of solar power injection may be in between the load and the bi-directional meter. These meters are supposed to have the MRI and AMR facility and should adhere to the standards specified by the CEA regulations. Figure 1 shows a net meter installed for a 2 kW grid tie Roof Top Solar PV system in Bhopal, India.

Fig. 1: A Net Meter installed for a 2 kW Grid-tie Roof Top Solar PV system in Bhopal, India.

The net energy (kWh) injected to the grid during a billing month/period is supposed to be carried forward. At the end of the financial year any net energy credits which is yet not adjusted is paid to the owner of the premise/consumer by the distribution company. To promote the scheme, certain states have waived off the wheeling charge, cross subsidy surcharge and other similar charges/levy for a period of 5 years.  

If the cost of solar energy and the grid energy is kept the same, there may not be any significant financial benefit to the residential consumers, although the commercial consumer paying at a much higher grid tariff may get benefited. To get more, the residential premise owners have to install a much larger PV system. 

Policy on roof top solar PV systems of certain states in India has put a cap on the solar generation which is about 90 % of the annual energy drawal. In the net metered solar PV arrangement the roof top owner  can also avail the available subsidy (currently 30 %) from the Ministry of new and Renewable Energy (MNRE) for which he or she has to approach the state nodal agency.