Home Energy Model: Explained

The transition from SAP to the new Home Energy Model (HEM) has reached a critical turning point. While originally set to launch alongside the Future Homes Standard (FHS) in early 2026, the government has announced a revised rollout strategy to ensure industry stability.

2026 Compliance Update: With the HEM infrastructure (ECaaS) still being finalised, SAP 10.3 has been introduced as the primary compliance tool for the initial months of the Future Homes Standard. This transition now features a phased approach, including a 24-month dual-compliance period.

The New Home Energy Model Transition Timeline

The road to the Home Energy Model is now split into three distinct phases to prevent a compliance freeze in the housing market. This ensures that developers can continue to meet the 75–80% carbon reduction targets of the Future Homes Standard even while the new digital infrastructure is being stress-tested.

• Phase 1: SAP 10.3 Solo (Minimum 3 Months): Following the introduction of the FHS in early 2026, SAP 10.3 will be the only available tool. It uses the new FHS notional building targets but retains the familiar monthly calculation engine.
• Phase 2: Dual Compliance (24 Months): Once the HEM cloud infrastructure (ECaaS) is approved, both SAP 10.3 and HEM will be valid. This allows the industry to adapt to high-resolution modeling without pausing projects.
• Phase 3: HEM Only: HEM will eventually become the sole regulatory tool for all new-build dwellings and EPC generation

Home Energy Model vs. SAP 10.3

While SAP 10.3 provides a bridge to the Future Homes Standard, it still relies on monthly averages. In contrast, the Home Energy Model is a far more sophisticated tool.

It uses half-hourly timesteps (17,520 per year) to create a high-resolution picture of a home’s energy performance. This shift from a monthly average to a dynamic simulation is underpinned by a massive increase in the data required. Many of the new inputs require specialist evidence from across the entire project team, including architects, plumbers and solar installers.

What’s the Impact on Housebuilders?

The delay in the full HEM rollout provides a “grace period,” but the fundamental shift in compliance remains. Here’s what this means in practice:

1. The ECaaS Shift: HEM is cloud-based (Energy Calculation as a Service). Unlike the standalone desktop software of the past, HEM requires a stable digital connection to the government’s calculation engine.
2. The Dual-Route Strategy: During the 24-month dual-compliance window, you may choose to model “borderline” projects in both tools. HEM’s 30-minute modeling is often more rewarding for heat pumps and battery storage than SAP’s monthly averages.
3. The High Cost of Missing Data: Insufficient evidence, such as a missing data sheet for a tap or an inverter, will force the use of default values. In HEM, these defaults are likely to be far more punitive than in SAP, potentially leading to compliance failures and costly redesigns. Setting up a project data room from the start is highly recommended.

Navigating the Transition: Risk & Cost Management

While the Home Energy Model offers more accuracy, it introduces two significant commercial risks that housebuilders must manage during the transition

1. The High Cost of Missing Data

In SAP, missing information often resulted in a “neutral default”. In HEM, the Energy Calculation as a Service (ECaaS) engine is designed with punitive defaults, meaning that it assumes the worst-performing equipment on the market for any missing inputs. This can easily push a borderline design into a compliance failure, leading to expensive, late-stage redesigns or unnecessary fabric upgrades.

2. Increased Assessment Costs & Time

It is important to be transparent: the Home Energy Model is not a “quick check”.

• Time Impact: In our rigorous testing, a standard house type that takes 20 minutes to model in SAP 10.2 now takes roughly 1 hour and 40 minutes in HEM.
• Calculation Lag: Unlike SAP’s instantaneous results, the HEM cloud engine can take 5–10 minutes just to run a single calculation.
• Fee Structure: Because the assessor’s workload has increased five-fold due to data entry and verification, the industry will see a corresponding increase in assessment fees.

Key New Data Requirements for the Home Energy Model

To illustrate the scale of change, here is a summary of the new data points required by HEM, many of which require new forms of evidence.

External Factors & Building Fabric

• Site & Surroundings: Dwelling altitude and potential for noise nuisance (which may require a formal noise impact assessment) are now required.
• Shading & Materials: Detailed distant shading data from nearby buildings or trees is needed. The solar absorption coefficient of external finishes must also be specified based on their colour and material.
• Windows, Doors & Blinds: Beyond U-values, HEM requires window g-values, frame factors, and precise details of openable areas for overheating calculations. Even internal doors and the presence and thermal resistivity of blinds or curtains must be specified.

Heating, Hot Water & Ventilation

• Heating Systems: For wet systems, you’ll need the thermal mass of the entire distribution system, design flow temperature, flow rates and the specific Ecodesign control class used (from Class I to VIII).
• Hot Water Outlets: SAP only required shower flow rates. HEM now requires flow rates for basin taps, kitchen taps, and baths, plus the volume of baths in litres.
• Pipework: Detailed specifications for both primary and secondary pipework are now needed, including internal and external diameters, insulation thickness, and thermal conductivity.

Renewables & Energy Storage

• Solar PV: You must now provide panel dimensions, the ventilation strategy for the array (e.g. roof-integrated vs. on-roof), and detailed specifications for the inverter.
• Battery Storage: If a battery is installed, HEM requires its capacity (kWh), age, charge/discharge efficiency, location and maximum/minimum charge and discharge rates.

A note on the current HEM test environment

It’s important to note that the HEM software is still in development, and the current test version has some restrictions. Assessors can currently only model a limited selection of technologies, with more options to be added in future releases. Key limitations include:

• Energy Supply: Only electricity can be modelled.
• Heat Generation: Only heat pumps are available.
• Heat Emitters: Only radiators can be selected.
• Domestic Hot Water: Only homes with a hot water cylinder can be modelled.

A new version of the Product Characteristics Database (PCDB) is also being redeveloped to work with HEM.

How to Prepare for the Home Energy Model 🏡

The transition to HEM is a crucial step towards meeting the ambitious targets of the Future Homes Standard. Housebuilders who begin adapting their processes now—even while using the SAP 10.3 bridge—will have a significant advantage.

The long-term benefits are clear: future-proofed homes, a powerful marketing advantage and a genuinely reduced environmental impact.

What’s Next?

Navigate the new Home Energy Model with confidence. We offer expert sustainability consulting to ensure your designs meet the new standards, providing compliance expertise, and early-stage design optimisation.

Contact us today for a free consultation on your new build project.

Expert Training: Book a HEM CPD for Your Team

Want to get your entire team up to speed quickly? Book in a one-hour CPD session on the Home Energy Model, delivered exclusively by our experts.

Perfect for architectural practices, housebuilders and development teams, this focused session can be delivered online or at an in-person workshop. We’ll cover the practical details your team needs to confidently navigate HEM and the Future Homes Standard.