This project is a Coefficient Of Performance heat meter for an air-to-water heat pump, using an ESP8266 node-mcu board with OLED display, only about $7.

It measures water flow from a hall-effect turbine meter, temperatures across the refrigerant to water heat exchanger with DS18b20 sensors, collects electric power consumption over MQTT from an Emporia Vue-2 monitor which has been ESPhome'd. It calculates and publishes the COP and data to MQTT and the cute little OLED screen. An IOTstack RPI collects and plots the data.

More discussion about the project and the heatpump system: https://www.reddit.com/r/DIYHeatPumps/comments/189mlyy/diy_airtowater_cop_measurement/