It's a combination of hardware and software.

https://www.brultech.com/how-it-works/

Each of my miners is on a dedicated 240V circuits, that have a dedicated Current Transformer connected to it.  The CTs are in turn connected to a monitor that sends readings in real-time to a logging / web server device, that will then allow you to run reports against the data.  It is very accurate (99% +) and takes power factor into account.  You can input your power cost per kWh, and if you have solar, it will offset that, and you can calculate your net cost for running each miner.