My water provider uses a water meter with a metal butterfly that spins 1 revolution for each litre of water spent.
So I decided to build a water sensor to get readings into Home Assistant 
There are similar projects here in the community using ESPhome and Zigbee Sensors, but my meter is not in the main house. I don’t have power available at the meter location, so I build the project around a RPi Pico W and a bit counter that will only turn the Wifi On for brief moments every 16 litres (this is configurable), thus saving energy.
In normal state the system draws 5mA, and peaks at 93mA when Wifi is transmitting, hopefully the battery can run the systems for some days.
Please check all the details at GitHub - dgomes/pico_w_water_meter: Water Meter using Raspberry Pico W