If you need to work out the Building Performance Index (BPI) for a house, this article shows how you can use software called AccuRate NZ to do so. A correlation between energy calculations from AccuRate NZ and from the Annual Loss Factor (ALF) method is given, and applied to BPI calculation.
You can use the BPI to show compliance with the house insulation requirements of Clause H1 of the Building Code. A house complies with the insulation requirements of H1 if the BPI ≤ 1.55.
The BPI is calculated as follows:
BPI = heating energy / heating degrees total x (floor area + total wall area)
You can work out the heating energy using either ALF or another method that can be correlated with ALF.