Water usage can be measured in many different ways. Some water meters have a magnet on the rotating dial, but most standard water meters don’t have that (or any other way of monitoring water usage in an automated way).
So the only option was to add a 2nd water meter that did have an output; a reed switch actuated by a magnet on the rotating dial inside the water meter. Those water meters aren’t that hard to find so I bought 2 of those and put 1 of them in series with the water meter and the other one near the central heating; this way I could monitor both total water usage but also the DHW (Domestic Hot Water) usage for bath, shower and so forth.
The hardest part was done – providing a pulse. Picking up the pulse from the reed switch can be in many different ways – with Arduino, Rapberry Pi, whatever you like actually. Since I had a HA7Net near the other the choice was simple:
The primary task of the HA7Net was monitoring some temperature values of our central heating but by adding a 1-Wire counter to the 1-Wire bus it was relatively easy to count the pulses of the hot water meter. For the cold water meter I used an RFXCOM RFXMeter which was already monitoring gas usage and power usage.
Water usage is stored in a database on a hourly basis, meaning that for every hour that water has been used in our house, a record (row) is stored in a database table. With this it is possible to create very detailed charts of our (hot & cold) water usage.