How detailed does the sub-metering have to be to account for 'at least 80% metered subsystem'?
If a building has only indoor plumbing fixtures (WC, taps, showers) and landscape irrigation system can a solution be mains water meter (metering the entire building and associated landscape water use) and a water meter for irrigation system? Calculating 'indoor plumbing fixtures water use' as 'total water use' minus 'landscape water use'?
Is plumbing fixtures tend to consume hot water as well. Is it required that hot water use is deducted from 'plumbing fixtures' water use?
Or is it expected that each end fixture has it's own water meter (at least 80% of fixtures)? If this is the case, it can be expensive credit depending on the number of fixtures.