In his case, he used a YHDC SCT-013-030 sensor that can measure up to 30 amps and costs about $12 USD. It outputs a voltage between ... data he collects with his ESP32 energy monitor.

This one is a complete ESP32 which fits inside a USB-A connector. The brains of the projects is the ESP32-D2WD which is a dual core chip with 2 Mb of memory, making it more than capable.