It is very important for anyone who plans to install Solar Power Plant to understand the Solar Metering Policy applicable in the region. The metering policy play's a key role in deciding the returns which you can reap from the Solar Power Plant Investment. The recent uproar in the Solar Industry against the government's plan to impose gross-metering is also due to the same. We will cover, net-metering, net-feed in tariff, zero export mode, and gross-metering as applicable for a Grid-Tied or Hybrid Solar Power plant in this blog.
Please read our blog on different types of Solar Power Plant, if you want to understand the difference between Grid-Tied, off-grid and hybrid Solar Power Plant.
In Normal metering, the energy meter installed by the power utility is a uni-directional meeter. It has the capability to measure the energy consumed by building. If you install a Grid-Tied Solar Power Plant with Normal uni-directional meter, it will record the EXPORTED power also as consumption. This will lead to an increase in your Energy Bill, as the utility will charge you for the solar power your EXPORT to the grid.
Net-Metering
With net-metering your utility energy meter will read the energy you consume (as IMPORT) and the excess solar energy you send back to the utility grid as EXPORT. The customer electricity bill will reflect the net or difference between the IMPORT and EXPORT at the end of the month. Let us understand net-metering in detail by looking at an example. Let's assume that your house has two lights of 50 W each and a Solar Plant of a capacity 100 W.
Scenario 1: Let's assume that your Load is 100 W and Solar Production is 60W, in such a scenario, the excess power of 40 W will be imported from the Grid. The power of 60 W directly consumed from Solar is not recorded in net-meter and is referred to as 'Self Consumption' in the Solar PV Industry. The power of 40 W drawn from grid will be recorded as an Import in net- meter.
Scenario 2: Let's assume that your Load is 50 W and Solar Production is 70W, in such a scenario, the excess power of 20 W will be exported to the Grid. The power of 50 W directly consumed from Solar is not recorded in net-meter, and is referred to as 'Self Consumption' in the Solar PV Industry. The power of 20 W pumped back to the grid will be recorded as an Export in net- meter.
At the end of the month/billing cycle, the utility will bill the customer for the difference between Import and Export. For example, for a customer with a Solar Generation of 200 units, if Import is 100 units, and Export is 90 units, he will be billed for (100 - 90 = 10 units) ( Please note that 110 units is Self-Consumption). Let's assume that the customer has a tariff of INR 8 Rs per unit. Then his/her energy charges will be 8 x 10 units = 80 Rs. (Please note that electricity bill has a fixed charge component for connected load/contract demand in addition to energy charges).
What happens if your export is 110 units, which is more than import (100 units)? In the case of most of the utilities, the excess power pumped to grid at the end of the month beyond import ( 10 units) will be settled to customers account at a weighted tariff of approximately 3 Rs per unit (30 Rs benefit). Kerala State Electricity Board is an exception, which allows the consumer to bank the excess power for one year. In such cases, the excess solar energy will be adjusted in next months energy bill.
Feed-in Tariff
In feed-in tariff regime, the metering arrangement and function is very similar to that of net-metering scheme. However, for the exported power the customer is compensated at a different tariff called a feed-in tariff, which is low compared to the customer's utility tariff. The feed-in tariff is approximately 3 Rs per unit. The customer can use solar power for self-consumption at their utility tariff.
For example, for the same customer, with Solar Generation of 200 units, Import of 100 units, Export of 90 units, and power tariff of INR 8 Rs per unit, let's calculate the net-energy charges under feed-in tariff regime: (100 units x 8 Rs per unit) - (90 units x 3 Rs per unit) = 530 Rs.
In India, TamilNadu has implemented feed-in tariff for a certain category of consumers. In such cases, it would be better to design the Solar Power Plant and operate loads in such a way that you can maximize self-consumption. Complementing Solar with some storage, to maximize self-consumption, is another option, once Lithium-based batteries cost becomes competitive.
No net-metering
Unfortunately in many parts of India, net-metering is not given to all customers. In such cases, the customer may prefer what Industry terms as a zero-export-option. The zero-export grid-tied system design has additional CTs' (current transformers) monitoring customers consumption, and the solar production will be reduced to the value of the consumption by adjusting MPPT (Maximum Power Point Tracking). This will minimize export to the grid. It is important to minimize export in such cases, as normal energy meter will read exported solar power also as consumption.
For example, for the same customer, with Solar Generation Potential of 200 units, net-generation after export limitation will be only 110 units, as the inverter will limit generation to self-consumption when operated in zero export mode. With Import of 100 units, and power tariff of INR 8 Rs per unit, lets calculate the energy charges in zero export mode: (100 units x 8 Rs per unit) = 800 Rs.
Gross-Metering
In gross-metering, the utility pays the customer a fixed charge for the solar energy produced from the Solar Power Plant. It is not a good option compared to net-metering for most of the customers, because of the low tariff decided by utilities for gross-metering. However, for residential property owners with low monthly consumption, who consider Solar as a green investment, it may be a better option. The tariff for gross-metering ranges from 3.5 to 5 Rs. In India, many utilities are now moving from net-metering to gross-metering or feed-in tariff.
For example, for the same customer, with a Solar Generation of 200 units, in this case, the Gross-meter will read 200 units, as it is measuring the Solar Generation. The customer's consumption meter will read 210 units (100 units Import + 110 units self-consumption in earlier case). Let's calculate the net-energy charges under Gross-Metering Regime after assuming a weighted tariff of INR 3 Rs per units for Gross Generation: (210 units x 8 Rs per unit) - (200 units x 3 Rs per unit) = 1080 Rs.
Conclusion
For most of the customers with high energy tariff, net-metering is a better option than feed-in-tariff and gross-metering. In our sample customers case the energy charges in electricity bill under each regime is summarised as below:
Net-metering: 80 Rs
Net-Feed in Tariff: 530 Rs
Zero-Export Mode: 800 Rs
Gross-metering: 1,080 Rs
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