36 THE CLIMATE CHALLENGE HEM – the dawn of a new era


As the industry adjusts to the Future Homes Standard, Jon Ponting of AES Sustainability Consultants examines why the long-delayed Home Energy Model will be a transformative break from the long-established SAP method.


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or almost two decades, the Standard Assessment rocedure SA has served as the foundation for measuring the energy performance of new homes in the UK. Introduced in the 1990s and made mandatory from 2006, SAP has been refreshed several times as each new Part L update has arrived. Yet despite these periodic revisions, its core structure remains rooted in assumptions and methodologies that are now signifi cantly outdated. As energy effi ciency has grown from a peripheral consideration into a central component of modern construction, SAP’s limitations have become increasingly apparent, prompting the need for a more sophisticated and transparent system – cue the Home nergy odel H.


HEM REPRESENTS A COMPLETE REDESIGN, WHICH FUNDAMENTALLY RETHINKS HOW WE MODEL, TRACK AND INTERPRET ENERGY PERFORMANCE WWW.HBDONLINE.CO.UK


HEM is not simply another iteration


of SA, nor a rebranding exercise. It represents a complete redesign, one that fundamentally rethinks how we model, track and interpret the energy performance of homes. The overnment has consciously avoided the label new SA’ because the underlying differences are so substantial. While both SA and H produce results related to energy use and effi ciency, almost everything about how H calculates those results is new. Its level of detail, level of accuracy and capacity to incorporate modern and


emerging technologies mark a dramatic advance beyond anything SA can currently offer. Perhaps the most transformative change is the shift from monthly modelling to half-hourly modelling. SA analyses homes’ performance using  month averages, meaning it can only approximate conditions such as how solar panels generate across a typical month or how energy demand shifts throughout the day. H, by contrast, operates using -minute intervals, resulting in roughly , data points over the course of a year. This dramatic increase in resolution enables H to account for daily variation in sunlight, cloud cover, peak- hour electricity use, overnight demand, battery storage behaviours and how energy systems interact with one another in real time.


ually important is H’s transparency. Many of SAP’s assumptions and euations date back decades, often without clear documentation on their original rationale. Over time, this has led to a model that behaves in ways that can feel opaue, even to experienced assessors. HEM, however, is built on a fully traceable, research linked methodology. very calculation and variable maps back to defi ned evidence or modelling work, enabling


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