A bidirectional meter measures electricity moving both ways across a grid connection. It records import when a building draws power from the grid and export when eligible rooftop solar sends surplus power out. The import and export registers are readings, not an automatic promise that every exported unit receives the same credit as an imported unit.
Bidirectional meter explained simply
A conventional meter counts one way. A bidirectional meter keeps two counts: what you take from the grid and what you send back. On a sunny day the rooftop system may first supply the building’s own load, and only the surplus crosses the meter outward. The DISCOM then applies the approved billing arrangement to the two readings. See the net versus gross metering guide for how the same meter can be used under different settlement rules.
How to read it
The display or utility bill may show separate import and export energy totals in kWh. Record the register label and date when comparing readings. If the labels are unclear, ask the DISCOM for its meter-reading guide; display codes and billing formats differ between meter models and utilities.
| Register | What it measures | Billing question |
|---|---|---|
| Import | Energy received from the grid | Which tariff and fixed charges apply? |
| Export | Energy sent to the grid | How is export credited under the approved arrangement? |
How the meter works
- The meter measures voltage and current at the service connection through internal sensors, or through external current transformers on larger connections.
- A processor samples voltage and current many times per cycle and multiplies them to get instantaneous active power.
- The sign of the power gives the direction: one sign for import, the other for export.
- Energy in each direction is accumulated into its own register at each interval.
- Interval data may be stored in the meter and, in a smart meter, sent to the utility’s system over a communication link.
Accuracy class
Accuracy class describes the permitted measurement error under defined conditions. The CEA metering regulations specify, for consumer meters, class 1.0 or better up to 650 V, 0.5S or better above 650 V and up to 33 kV, and 0.2S above 33 kV. The class that applies to your connection is stated in your DISCOM’s specification, and the regulations are amended periodically, so check the current text.
| Class | Permitted error (indicative) | Typical use |
|---|---|---|
| 0.2S | Tightest | High-voltage and large industrial connections |
| 0.5S | Tighter | Larger low-voltage commercial or industrial connections |
| 1.0 | Standard for small consumers | Residential and small commercial single-phase and three-phase connections |
Active and reactive measurement
A four-quadrant meter records active and reactive energy in both directions. The usual convention is: Q1 active import with reactive import (an inductive load), Q2 active export with reactive import, Q3 active export with reactive export, Q4 active import with reactive export. Quadrant labelling varies between manufacturers, so rely on the meter’s own manual. A household net-metered connection mostly needs active energy only. A larger commercial connection may need reactive energy for billing, which links to power factor and the reactive power entry.
Communication
Smart meters send data to the utility through a communication link such as RF mesh, power-line carrier or cellular, depending on location. Indian smart meters use the DLMS/COSEM data model, which IS 15959 adapts as a companion specification. The smart meter entry covers what a smart meter adds, such as time-of-day registers and two-way communication.
Who selects and installs the meter?
The serving DISCOM sets its metering specification and connection process. Ask it who supplies, tests, seals and pays for the meter in your application. Do not buy one based only on a generic online price list. Meter price, accuracy class and replacement terms depend on the utility specification and the device supplied.
Where it is used
- Net-metered rooftop solar at homes and small commercial buildings.
- Gross-metered arrangements where all generation is measured and settled separately.
- Virtual net metering, where one generating site is allocated across several consumers.
- Captive plants and commercial sites with grid backup, where both export and import are measured.
Common problems
- Export not registered. The meter is not set up to record export, so surplus energy is lost to billing. Audit the first bills.
- Wrong register read. Import and export labels are confused, or generation from the inverter app is compared with the export register.
- Current sensors reversed on a CT-operated connection, which inverts the direction and corrupts both readings. Check it at commissioning.
- Communication gaps, where the meter reads correctly but the utility has not received recent data. Ask the DISCOM about its fallback billing rule.
- Seal or tamper alarms after cleaning or pest control, which usually need a utility visit.
Best practices
- Confirm the meter make and model are on the DISCOM’s approved list before purchase, where the DISCOM expects you to supply it.
- Witness the installation and note register readings, seal numbers and serial numbers on the commissioning record.
- Compare the first few bills with inverter monitoring data to confirm the export register is working.
- Keep a copy of the single line diagram showing meter location and sensor placement.
- Ask in writing how import and export will be settled before you rely on any credit calculation.
Unidirectional, bidirectional and four-quadrant meters
| Feature | Unidirectional | Bidirectional (active) | Four-quadrant bidirectional |
|---|---|---|---|
| Active import | Yes | Yes | Yes |
| Active export | No | Yes | Yes |
| Reactive energy | Not measured | Limited or none | Measured in both directions |
| Typical use | Ordinary consumption | Residential net metering | Larger commercial and high-voltage connections |
Indian context
The Central Electricity Authority’s metering regulations (Installation and Operation of Meters, 2006, as amended) set the framework for consumer meters, including accuracy classes and smart-meter requirements. The most recent amendments make smart meters the norm in areas with communication coverage, so what your DISCOM installs depends on the date and area. IS 16444 (Part 1) sets requirements for direct-connected smart meters of class 1 and 2, and IS 16444 (Part 2) covers transformer-operated smart meters of classes 0.2S, 0.5S and 1.0S. The Smart Meters (Quality Control) Order, 2023 requires BIS certification for such meters. The Revamped Distribution Sector Scheme supports smart prepaid metering. See the DISCOM entry for who to approach and the PM Surya Ghar entry for how the scheme interacts with commissioning. Sources: CEA metering regulations (consolidated) (CBIP copy of the 2006 Regulations with the amendments to 2019; the 2022 amendment is not in it), checked 14 October 2026.
For on-grid inverters, the metering arrangement affects settings such as export limit. See the Qbits on-grid range and the zero export entry.
Standards and references
| Standard | Scope |
|---|---|
| IS 16444 (Part 1 and Part 2) | Smart meters, direct-connected (class 1 and 2) and transformer-operated (0.2S, 0.5S, 1.0S) |
| IS 15959 | DLMS/COSEM companion specification for data exchange |
| IEC 62052-11 | General requirements for AC electricity metering equipment |
| IEC 62053 series | Particular requirements and accuracy classes for static meters |
| CEA metering regulations | Accuracy classes and smart-meter requirements for consumer meters in India |
Common mistakes
- Assuming generation in the inverter app is the export register.
- Accepting a meter that is not configured to record export.
- Not recording the seal and serial numbers at installation.
- Buying a meter that is not on the DISCOM’s approved list.
- Not checking the first bills against monitoring data.
- Treating the meter reading as a credit promise before the settlement terms are confirmed.
Key takeaways
A bidirectional meter records import and export separately, and the DISCOM’s approved arrangement turns those readings into a bill. Check the register labels, the class and the configuration at installation, keep the commissioning record, and audit the first bills. Check the current local metering and billing order before using a price or export credit in a purchase calculation.