Meter or TA: how to choose the right size for your system

Two solutions for reading energy flows. The choice should be based on the system layout. To manage self-consumption, regulate a battery, or limit feed-in, the inverter needs to know the power exchanged with the grid. This information can come from current transformers connected directly to the inverter or from a compatible external meter. Both solutions can work well. The choice depends on the system architecture, distances, required functions, and the manufacturer's specifications. The key point With a direct current transformer, the power is calculated by the inverter. With an external meter, the calculation occurs within the instrument, and the inverter receives pre-processed data. Where the measure is born Direct CT: the calculation takes place in the inverter The current transformer detects the current flowing through a conductor and provides a proportional signal. In suitable systems, the inverter acquires this signal and combines it with its own voltage measurement to calculate the power and determine the direction of the exchange. It's a compact solution, suitable when the measurement point is reachable while respecting the connection limits. Measurements depend on sensor compatibility, position, orientation, and wiring quality. The fact that the CT detects the current does not mean that the inverter ignores the phase shift: a system designed for this function can take it into account when calculating the active power. External meter: the calculation takes place at the measuring point The meter acquires voltage and current at the point where it is installed, calculates the expected values, and transmits them to the inverter. Current can be measured by direct connection or via external current transformers. Meters and CTs can therefore be part of the same chain. The useful distinction is between sensors connected directly to the inverter and an external instrument that processes the measurement. The comparison, at a glance Accuracy depends on the entire chain A meter is not automatically more accurate under all conditions. The comparison must be made considering the declared accuracy, operating range, any current transformers, wiring, and configuration. The meter can offer an advantage because it acquires voltage and current at the same point. This becomes significant when there is a line with a significant voltage variation between the panel and the inverter. Even a direct current transformer system can provide adequate measurements for the functions for which it was designed. The performance of the actual combination must be verified, including that at low currents and the update times required by the regulation. Management measure and billing measure The presence of a meter does not automatically make the reading usable for billing purposes. Specific features and compliance are required for that use. Distances: Separate the sensor from the data channel The direct CT connection transmits an analog signal. An extension must comply with the manufacturer's specifications: increasing the cross-section does not, by itself, authorize exceeding the specified length. With an external meter, digital data travels to the inverter. Many systems use RS485 with Modbus RTU protocol; others use different connections. The connector or cable type alone isn't enough to identify the protocol. Digital transmission avoids carrying the CT's analog signal all the way to the inverter. Improper wiring can still cause communication errors, delays, or outdated data. Cable, shielding, reference conductor, terminations, and topology must be defined in the device manual. Any wireless bridges must also be expressly compatible and verified under actual installation conditions. No universal distance. The limit to be used in the project is that of the combination of devices and connections actually installed. Which solution to choose A direct CT is a practical choice when the inverter supports all the required functions and the installation complies with the required limits. An external meter deserves special consideration when the exchange point is distant, the circuit requires it, or dedicated measurements are needed. For limiting emissions, the solution approved by the manufacturer, along with its response times, is crucial. The use of direct current transformers cannot be ruled out a priori, nor can any meter be considered sufficient. With multiple inverters, existing generators, wallboxes, or heat pumps, the overall architecture must be verified. A single device is not automatically shared between different devices. Before leaving the construction site Check the measuring point: must include all flows affected by regulation. TA check and phases: model, relationship, orientation and voltage-current association. Check the link: length, route and installation requirements. Verify communication: parameters and devices provided by the supported scheme. Try real flows: controls the withdrawal, input and behavior of configured functions. Three questions to get off to a good start Where to measure? What functions does it manage? What components does the inverter support? The answers define the sensor or meter, its location, connection, and configuration. A well-designed measurement facilitates commissioning and troubleshooting. Do you need support? Our technical-sales office can help you evaluate the measurement scheme, verify the compatibility between inverters, meters and CTs and identify the right components for your system. Technical references: SolarEdge current transformers ; SolarEdge · Modbus meter ; SMA · Energy Meter For installation, use the current manual for the exact model.