A self-generator that exports surplus energy needs a revenue metering system that meets the utility's requirements for generation connection points. The meter must be bidirectional, of the accuracy class that corresponds to the size of the metering point, with a product conformity certificate and calibration in an accredited laboratory. Requirements for backup meters, remote reading and verification vary by jurisdiction and by whether the system exports; a system that does not export usually keeps its existing meter.
What the bidirectional meter is for
The settlement of a self-generator is based on two quantities: the energy it imports from the grid and the surplus it delivers. If both pass through the same point, a conventional meter can only subtract or, worse, add them. That is why metering codes require that at points where flows in both directions occur or are expected, bidirectional meters be installed that determine the flow in each direction independently.
With those two readings, closed at each billing period, the retailer applies the local net-metering or export rules, which are explained in surplus and energy credits for self-generation: typically an energy credit up to the value of the import and a different value for any excess.
Do not confuse the bidirectional meter, which measures energy in both directions, with the advanced meter of smart metering, which communicates two-way with the utility. They are two different features.
What the metering requirement looks like
Rules vary by country, but most codes separate three cases:
| Case | Metering requirement |
|---|---|
| Self-generation without export | Keeps its existing meter, with a verified anti-backfeed (zero-export) system |
| Small self-generation with export | Bidirectional revenue meter that meets the generation connection point requirements, often without a backup meter or third-party verification |
| Larger distributed or commercial generator | All requirements for a generation connection point: backup meter, third-party verification, remote reading |
For consumption, the metering system meets the minimum requirements of the metering code for the type of customer. Who reports the readings to the market operator, and by when, depends on the rules of each market.
The class depends on the size of the point
Metering codes classify metering points by monthly energy or installed capacity; if the two criteria give different categories, the stricter one applies. The accuracy class follows the category. As an orientation, with IEC and ANSI equivalents:
| Point size | Active energy meter | CT | VT |
|---|---|---|---|
| Large (about 1 MVA and above) | IEC 62053-22 class 0.2S or 0.5S; ANSI C12.20 class 0.2 | 0.2S or 0.5S | 0.2 or 0.5 |
| Medium (about 0.1 to 1 MVA) | IEC 62053-22 class 0.5S; ANSI C12.20 class 0.5 | 0.5S | 0.5 |
| Small (about 10 to 100 kVA) | IEC 62053-21 class 1 | 0.5 | 0.5 |
| Very small (below about 10 kVA) | Class 1 or 2, direct connected | — | — |
Instrument transformers follow IEC 61869-2 and -3, or IEEE C57.13 in North America. The exact thresholds are set by the local metering code. A small self-generation system usually falls in the medium, small or very small rows. The full classification logic is in commercial boundary and metering system.
What else a generation metering point asks for
- Product conformity certificate of the meter and the instrument transformers, from an accredited body.
- Calibration before entering service in a laboratory accredited under ISO/IEC 17025.
- Bidirectional reactive energy measurement, which many codes require at generation points.
- Hourly (or finer) recording: points that report to the market operator record at least the transactions of each hour, with a clock synchronized within the allowed drift.
- Seals, equipment records and maintenance at the frequency of the code.
- Remote reading only where hourly telemetered reporting is mandatory; where it is not, it usually cannot be required between the self-generator and the retailer, although the parties may agree on remote interrogation.
- Does the system deliver surplus to the grid?
- No: Keeps the existing meter, with a verified anti-backfeed system
- Yes: Is it small self-generation under the local export threshold?
- Yes: Bidirectional revenue meter for a generation point, with no backup meter or third-party verifier
- No: Full generation point, with backup meter and third-party verification
When the system has batteries
Recent storage rules in several markets add requirements when a self-generator has batteries: the technical verification also follows the storage regulation, and the metering scheme may need separate points for charging, discharging and auxiliary consumption, especially for standalone storage and hybrid resources. See battery storage regulation. If the project adds batteries, confirm with the utility, before buying, which metering scheme it will accept at the inspection visit.
Who buys, who verifies and who represents
In most markets the parties own the metering elements and may buy them on the market, provided they meet the technical characteristics of the code. The retailer that represents the connection point registers it and is responsible for settlement. The utility checks the metering system at the connection visit, which is described in small-scale self-generation and in energization with the utility. Where the market allows it, the self-generator can choose the agent that represents its export point.
To verify the meter’s accuracy on site, it is compared with a portable reference standard, such as the Calmet TE30.
Common mistakes
- Unidirectional meter or one configured to add both directions.
- Reversed polarity in the connection or in the instrument transformers, which records surplus as consumption.
- Insufficient class from classifying the point by monthly energy and not by installed capacity.
- Instrument transformers shared with protection without checking the burden.
- Calibration without an accredited laboratory or a certificate for a different reference.
A polarity or ratio error is detected with tests on the instrument transformers before energization; see current transformer and energy meter and multifunction meter.
How we help
At Tiza Group we define the point and the type of metering from the connection feasibility. In systems with instrument transformers, we perform the current and voltage transformer tests: ratio, polarity, burden and accuracy, so the metering point passes the utility’s verification. Field tests are done in Colombia, or elsewhere on request. Utility connection paperwork will soon be available as a service; register here if you are interested. For your project, write to us.
Related guides
- Calmet TE30, Caltest 10 and C300B: meter calibration
- Solar photovoltaic energy: standards and equipment
- Surplus and energy credits for self-generation
- Battery storage: regulation
- Commercial boundary and metering system
- Small-scale self-generation
- Energy meter and multifunction meter
- Current transformer
- Energization and commissioning with the utility
- Smart metering (AMI): what it is and how it works
References
- IEC 62052-11 and IEC 62053-21 / -22, electricity metering equipment: general requirements and particular requirements for static meters.
- IEC 61869-2 and -3, instrument transformers: current and voltage.
- ANSI C12.1 and ANSI C12.20, electric meters and accuracy classes; IEEE C57.13, instrument transformers.
- ISO/IEC 17025, general requirements for the competence of testing and calibration laboratories.
Frequently asked questions
What accuracy class of meter does a solar self-generation system need?
The one that corresponds to the metering point category set by the local metering code or the utility, usually defined by monthly energy or installed capacity. As an orientation, IEC 62053-22 meters of class 0.5S or 0.2S are used for larger points with CTs of class 0.5S or 0.2S, and class 1 meters (IEC 62053-21) for small points; in North America the equivalent meters are ANSI C12.20 class 0.2 or 0.5.
Does the utility have to supply the bidirectional meter?
Not necessarily. In many jurisdictions the parties own the metering system and may buy the meter on the market, provided it meets the technical characteristics of the metering code. The utility and the energy retailer check that it complies before commissioning. Confirm the rule with the utility before buying.
Must a system that does not export change its meter?
Usually not, as long as it has a certified anti-backfeed (zero-export) system that prevents injection into the grid. Check the utility's requirements, because some require a bidirectional meter anyway to detect unintended export.