Rainwater...

Rainwater harvesting meter. By fixing the rainwater collection system in a commercial premise, we have saved the company quite a bit of water already. Sadly the company who installed and claimed to have maintained the system for 10 years weren't doing their job properly. Thus the building owners were receiving £16,000 water bills!
Since fixing the issues only 30 ltrs of 'mains' water has been used and 50,000 litres of harvested water between the tanks! Job well done I say!