Net zero energy buildings (nZEBs) are understood as grid-connected buildings which do not require net inputs of non-renewable energy over a defined period of their life cycle. Energy requirements of nZEBs have until now been assessed based on the impact buildings have on the existing energy system. This paper introduces a new approach to nZEB energy balance that takes into account the actual amount of energy nZEBs require. Energy balance methods previously proposed for nZEBs are illustrated in a new way and expressed in a series of equations based on a common terminology. Taking a different standpoint on the very logic that lies behind energy calculations; this article presents a new approach to energy balance in nZEBs. The paper highlights the important difference between preventing an increase in the demand for grid energy and ensuring that a building requires no net non-renewable energy. The authors argue that an energy payback approach constitute a more adequate way to tackle the environmental challenges nZEBs are meant to help solving, and to abide to a definition which stipulates that nZEBs should require no net non-renewable energy. (c) 2013 Elsevier B.V. All rights reserved.