The common factor is used in determining the gas bill for individual residents where a building has a central hot water system.
It is derived from an estimate of all heat generated in the central hot water system divided by all the hot water heated from 15oC to 65oC (50oC) produced through the hot water system. The value not only takes into account the energy converted to hot water that is used but also the standing heat lost by the hot water equipment and piping.
There are several factors that influence the common factor for a centralised hot water system:
- average daily hot water usage per home unit;
- occupancy rate;
- the energy required to heat the water;
- pipe heat loss;
- water heater maintenance rates (stand-by losses) and effciencies;
- heating augmentation such as solar or co-generation.