The Home Energy Model (HEM) is being developed to replace SAP. For architects, builders and developers, the interesting part is what it will calculate differently, and whether that changes how you approach a design.
These are three differences worth understanding. Our full HEM guide covers the wider background and the transition from SAP.
1. Energy use calculated every half hour
SAP calculations use a monthly energy balance. HEM can follow a home through the year in 30 minute steps, carrying information from one step into the next. That makes it possible to model the difference in how a design uses energy throughout the day.
Why does that matter? Think about a sunny spring afternoon. The solar panels are generating electricity, but the heating is off. Later, the heating comes on when the panels are producing very little. The daily generation total does not tell you how much electricity was available when the house needed it.
SAP broadly accounts for solar generation and estimates how much is used in the home. HEM’s shorter time steps are a more detailed way to represent that overlap between supply and demand.
What to consider: look beyond annual generation figures when choosing systems. How will the proposed design use the electricity it generates, and when will it still need energy from the grid?
2. A different way to model the building’s temperature
Insulation slows heat loss. Thermal mass describes the ability of materials to absorb and release heat. Both affect how a room warms up and cools down, but they do different jobs.
SAP accounts for thermal mass, while HEM represents the temperature response of building elements in more detail. This allows the simulation to follow heat being stored and released over time.
HEM also controls heating using operative temperature, which accounts for the surrounding surfaces as well as the air. That is relevant to comfort: sitting beside a cold surface can feel different from sitting in a room with warmer surfaces, even when the air temperature is the same.
Two homes with similar U-values can respond differently to sunshine and heating schedules. The location of insulation and the materials exposed to the inside are therefore worth considering alongside the headline U-values.
What to consider: keep the actual wall, floor and roof specifications available. Discuss glazing, shading and heating controls together, particularly in rooms with large areas of glass. More thermal mass is not automatically better for every design.
The government’s HEM heating and cooling technical paper explains the temperature and heat balance calculation.
3. Heating, generation and storage working together
The first change is about the time steps used. This one is about how the systems interact.
In SAP 10.2, heat pump performance is assessed using a separate simulation outside the main monthly calculation. HEM brings the heat pump into the home’s changing conditions, alongside other systems. The government’s HEM overview identifies this as a reason for changing the methodology.
That gives the model more scope to examine combinations of heating, solar panels, batteries and controls. For example, electricity could charge a battery for later use or heat a cylinder to store energy as hot water. Those choices affect where the energy goes and what is available afterwards.
A battery is not a guarantee of lower costs, and a larger solar array does not automatically make the whole design work better. Equipment, capacity and controls need to suit the building.
What to consider: agree how the systems are intended to operate before finalising their specifications. A collection of individually efficient products still needs to work together.
Do you need to change your plans now?
These modelling changes do not mean every project immediately needs a HEM assessment. The government has confirmed that SAP 10.3 will initially support the Future Homes Standard while HEM is delayed.
For now, confirm which requirements apply to your project and involve your assessor while the design can still be adjusted. Our new build SAP service includes advice and reasonable revisions as the specification develops.
