This document shows the correct way to input the Generator Seasonal Efficiency for a hot water system in the Part L Program.
A default factor of .4 (i.e. 40%) is assigned to the Hot Water System by ticking the "Use default" checkbox, highlighted below.
A generator season efficiency other than the default can be set by un-ticking the checkbox and entering a factor into the input area. A factor of .91 would represent a seasonal efficiency of 91%