A current transformer, often called a CT, is a small electrical sensor used to measure how much alternating current (AC) is flowing through a wire. CTs are commonly shaped like a ring or clamp and are installed around an electrical conductor. They do not connect directly to the copper inside the wire; instead, they detect the magnetic field created as AC electricity flows through it.
(Note: Standard CTs only work on AC circuits, meaning they measure the AC electricity used by the home or produced by a solar inverter, rather than the direct current [DC] generated raw by solar panels.)
The CT sends a small measurement signal to an energy meter, solar inverter, or monitoring system. That system combines the CT's current reading with a voltage sample from the electrical panel to convert the signal into useful power data—such as how many watts a home is using or how much electricity a solar system is producing.
In a solar installation, CTs can be placed around the wires carrying AC power from the solar inverter or microinverters. This allows the monitoring system to measure the precise amount of electricity the system is producing.
Production monitoring helps homeowners confirm that their solar system is operating properly. A sudden drop in production may point to shading, equipment problems, a tripped breaker, or another issue that needs attention.
CTs can also be installed around the main electrical service wires to measure how much electricity the entire home is consuming. For accurate readings, CT directionality is critical: CTs have a designated orientation (often marked with an arrow) indicating the direction of current flow. If installed backward, the monitoring system will mistake power exports for imports or vice versa.
When production and consumption are measured together, the monitoring system can accurately calculate whether the home is:
Using power from the utility grid
Sending excess solar power back to the utility grid
Using solar power directly
Charging or discharging a battery system
Consumption monitoring is often required for solar batteries, export-control systems, utility programs, and advanced energy-management features. It also gives homeowners a much clearer picture of when and where their electricity is being used.
Before purchasing equipment that requires consumption CTs, homeowners should have a qualified electrician or solar professional safely verify that the CTs can physically fit inside the main service panel.
CTs must usually be installed around the large main service conductors. Some panels have plenty of room, while others are crowded or difficult to access. Installation can be especially challenging in meter/main combination panels (all-in-ones), where the utility meter and main breaker are housed together. In some meter/main designs, the required conductors may be located in a sealed utility section or may not have enough physical space or clearance around them for standard CTs.
Homeowners should never remove panel covers or enter utility-sealed compartments themselves. Main service conductors remain energized (live) even when the main breaker is turned off and can cause severe injury or death.
CTs are simple devices, but they play an important role in modern solar systems. By measuring current safely through electromagnetic induction and sending signals to a central monitoring system, they help confirm solar production, track household consumption, manage batteries, and control how electricity moves between the home, solar equipment, and utility grid.