What is a Bidirectional meter? How to calculate the energy saved in a Solar PV system

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Bi-directional metering is one of those things that you get to hear for the first time when you plan to install a grid connected solar plant or wind energy system for your house or office building. You are familiar with the meter installed by the electricity company in your house or office for taking the electricity consumption reading but never heard about bidirectional meters till date. So what is a bi directional meter? Why do we need such a meter? Why can’t I use my same old meter with solar PV system?

Before answering all these questions let me answer you your first question

What is a Bi-directional meter?

A general electricity or energy meter installed in our homes is basically a single directional meter. It means this meter can only register increase in consumption of energy by counting the total amount of energy flown through it during a specific period of time.

These meters does not sense weather energy is drawn from the grid to the home or from your solar roof top system to the grid.

So, if you install a general energy meter with your solar system, then it registers only increase in the energy consumption for the amount of electricity you use from the grid as well as you the energy you export to the grid from your solar system.

And it eventually increases the meter reading and increases your electricity bill and charging extra for the energy you supplied to the grid, instead of rewarding you for it.

Whereas, a Bi-Directional meter senses the direction of flow of current and stores the data in separate registers for your consumption and production of electricity. This meter can measure the flow of electricity in two directions. i.e in-coming as well as out-going at the same time.

In the daytime, most of the loads are in off-state. So, the excess energy produced from your solar plant has to be fed back (Export) to the grid in case of a grid tied system. At this point, direction of flow of current is backward from your home system to the grid. The energy meter records the data in export register.

In night time, there is no solar generation and electricity is imported from the grid or electric company – “kWh delivered.” to run the building load. Now, the direction of flow of current is forward and meter records the data in import register.

So basically, the bi-directional meter allows you to get the information of your energy generation – export, import from your Solar PV plant system and grid.

It also measures the difference between the generators production and the customers load demand – “kWh received.”

Our meter does not measure the generators electricity production. The generator can offset a customer’s electric energy usage with any excess electricity produced. As the generator system produces electricity, the kilowatt-hours are first used to meet the customer’s electric requirements such as lighting and appliances.

If more electric energy is produced from the system than the customer needs, the additional kilowatt-hours are measured, fed into the utility’s electric system and utilized by other customers.

When the monthly electric bill is calculated, if the customer uses more electrical energy than is generated, the customer pays only for the net kilowatt-hours (kWh).

If the customer generates more electrical energy than is used from the utility electrical system, then the customer receives a kWh credit, which is applied to future bills. In addition, the customer is required to pay any customer charges and minimums applicable under their rate schedule.

Requirements to meet for using Bi-directional Metering

  1. An application for interconnection of PV Solar Plant with the grid must be submitted to your electric company prior to the generation connection.
  2. Generation of electricity must meet the specifications of the Net Energy Meter policy and specifications which specifies generator capacity and renewable energy generation source.
  3.  Generation equipment must meet utility, product safety and grid interconnection specifications. For example, an external disconnect is required within 10 feet of the meter location.
  4. An electrical inspection is required prior to energizing a generation interconnection with the grid.
  5. After the electrical inspection, a new bi-directional meter will be installed by your electric company.

A performance test* may be completed to verify that the generator source (Solar PV plant) is isolated from the utility’s electrical system in the event the company’s circuit is de-energized or cut-off.

Understanding Your Solar Bi-Directional Meter

Bi-directional meters can be of various kind depending on its functions. Basically, we use four kinds of Bi-Directional Meter according to the connectivity to the grid in India.

  1. Single Phase Two Wire Bi-Directional Meter
  2. Three Phase Four Wire. Whole Current Bi-Directional Meter.
  3. Three Phase Four Wire. CT operated Bi-Directional Meter.
  4. Three Phase Four Wire HT Bi-Directional Meter.

How to take readings in a Bi-directional meter?

An approved bi-directional meter has following features and information in it.

  1. Name Plate: Various electrical information of the meter is displayed on the Name Plate such as the type of connection, rated voltage, current, frequency, single phase or three phase system. Other than electrical information, they display the company name, warranty, serial number on the nameplate.
  2. LCD Display: A large LCD displays, which shows different electric parameters recorded by the meter. The LCD display has various sections for displaying specific information.
  3. Push Buttons: Two push buttons namely MD push button and Scroll push button are there in the meter. By using scroll push button, we can scroll the display. Each display shows different parameter of the system.
  4. 1107 Port: An optically isolated sealable port to enable data to be transferred to and from the meter through an optical cable.
  5. Terminal Block: Input and Output terminals are there in the meter. On the left side, we can see a communication terminal as well.
  6. Connection Diagram: Connection diagram is labelled on the top of the terminal cover block.

How to take the reading from your Bi-Directional Meter?

Lets check the LCD display of a Bi-Directional Meter.

The top part of thhe LCD display numbers, which basically denotes the value of various electrical parameters. The bottom right corner part shows the unit of the electric parameter currently shown in the display.

It is interesting to note that we can store and retrieve values of all the electrical parameters of the grid in a smart electric meter. But for the convenience of the consumer and the meter reader all meters come with a certain number or important only parameters.

By pressing the push button we can change the display and read the required value of various electrical parameters like Voltage (Volt), Current(Amps), Import Power(KWh), Export Power(KWh), etc.

Take the example of bi-directional meter provided by secure company which is approved by many state governments across India.

1.   Press the first push button to the left. A text ‘Test’ is written on the display. Where we can start to take the Meter Reading.
2.   When you press the push button the second time, the display shows all the digits. This means the meter is working properly.
3.   Pushing the push button for the third time, the display shows its serial number. Each meter has its physical address called serial number. These serial number can be used in future to register any problems happens in meter.
4.   Pressing the Push button for the fourth time, Display shows the parameter date. When you push the button again it shows the current date.
Next the display shows the current voltage of the grid and solar. The value is always in the range of 230V – 240V (Indian grid voltage)

Next Parameter is Current, the Net value of the current will be displayed in the electrical meter. Which may be export or import depends on your load and consumption.

In the next step you get the Imported energy from the grid so far.

The text in first display is “EnI” – It means ‘Energy Imported’ so far from the grid. The unit of electric energy is kWh.

If we again, press the push button, we get a display of energy export.

It shows as “EnE”  on the LCD display meaning the net ‘Energy export’.

Now, we are done with the display part of the meter. The next big thing is to calculate the energy consumed by us and the energy generated from the Solar PV system and fed back to the grid.

So, How to calculate the net electricity bill saved by the solar PV system?

To calculate the total electricity bill saved by your solar PV plant in a month, follow the steps

On a specific date of the calendar note down the EnE or Total Energy Exported to the grid reading and the readings a month earlier or you can take the readings after a one month on the day of your billing.

Say, ENE on previous billing = 4000 KWh

ENE on current billing date = 5000 KWh

So, total energy exported to the grid in that month = 1000KWh

Now take the EnI or the energy imported from the grid reading similarly.

Say, EnI on previous billing date = 1500KWh

EnI on the current billing date = 3500KWh

Now, the net energy imported from the grid during the month is = 2000KWh.

So, the total savings or earnings = the net energy exported to the grid during this month – total energy exported to the grid in that month =  2000KWh – 1000KWh = 1000KWh

Total savings for this customer by exporting solar energy to the grid is = 1000KWh = or 1000 Units (as 01 KWh is counted as 01 Unit for billing purpose)

Now say, Feed in Tariff at the time of Commissioning of the solar project is = 6.8 i.e Rs 6.80 for 01 Unit.

So, the savings in the month from exporting solar energy to the grid  = Rs. Rs 6.80/Unit  x 1000 Units.

= Rs. 6800.00

So this amount for the energy supplied by you to the grid will be reflected in your billing and balanced against your outstanding bill for your next billing cycle.

To calculate total savings, we need to consider total energy production and savings from self-consumption.

You can follow the same procedure to check the readings and calculate the energy saved in case of a three phase bi-directional meter.

6 COMMENTS

  1. Hi,

    Nice article and full details. I have one question to understand.

    If I install 5kw system with net metering then surely it will not run my full home load. For rest of the load I need to utlize wapda directly.

    My question is once I have net metering then how my other load directly run on wapda because my rest of the load will be directly connected to meter and also inverter directly connected to meter and both are on same line.

    So when inverter supply power back to grid how my remaining home load get power?

    For example if I turned on non inverter AC which is connected directly with meter. I turned it on then it will start getting power from wapda and on the same time my inverter will be providing power to the grid. How this two way flow of current work?

    • Hello Saad Akhtar,

      Current will never flow in two ways in a grid-tied solar pv system. Its like the system will draw power from solar PV system first. When the demand goes beyond, or in cloudy or rainy or night conditions, when there is not enough generation of solar power, it will switch to the grid power automatically to meet the energy demand.

      The bi-directional meter records both the units drawn from the Solar system and the grid system. These units are shown in the monthly electrical bill.

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