If you're reading this before 2026, good on you, you're one of the proactive ones preparing for the Future Homes Standard and Energy Calculation as a Service before it becomes mandatory. If you're reading it after, chances are you're trying to make sense of what’s changed, and why everything suddenly feels more complex. Either way, stick with this for the next 5 minutes, and you'll be glad you did.

What is Energy Calculation as a Service (ECaaS)?

ECaaS is a cloud-based API service that performs building energy calculations using the Home Energy Model (HEM), a new, more sophisticated simulation engine. It was designed to ensure uniformity, scalability, and future-proof compliance for standards like the Future Homes Standard (FHS) and Part L of the Building Regulations.

Unlike traditional SAP-based systems where each software vendor creates their own compliant engine, ECaaS centralises the calculation logic. This ensures consistent, tamper-proof outcomes, reduces fragmentation, and offers a single source of truth for all regulatory energy calculations.

Why Is Energy Calculation as a Service Needed Now?

Over the years, the building performance assessment process has faced persistent inconsistencies. Under the existing SAP and RdSAP frameworks, each software provider was required to independently develop and validate their own calculation engines. As a result, the process became fragmented, time-consuming, and prone to errors, with delays often caused by discrepancies between tools or incomplete test cases.

By introducing ECaaS:

• The government removes ambiguity between competing software tools.
• Compliance becomes faster, more reliable, and more transparent.
• Developers and assessors can focus on UI/UX and data input, while the core calculations remain government-controlled.

Ultimately, ECaaS aims to streamline the entire regulatory machinery, reducing delays, duplication, and interpretation differences.

How Does ECaaS Work in Practice?

ECaaS isn’t a standalone application you log into, rather, it’s an API-based service running behind the scenes. Here’s how it typically works:

• First, a user interface (developed by an approved accreditation body) collects the building’s input data.
• That data is then sent via a secure API to the ECaaS backend.
• The HEM engine runs dynamic simulations based on physical models and standardized weather data.
• Finally, a BRUKL-style report or EPC is generated, confirming whether the building complies with FHS or Part L.

Because the engine is centrally managed, the output is consistent every time, assuming the inputs are accurate and complete.

Understanding the Home Energy Model (HEM)

At the core of ECaaS is HEM, a detailed simulation model rooted in BS EN ISO 52016-1:2017, capable of:

• 30-minute time-step resolution
• Multi-zone modelling
• Full solar gain simulation by orientation and shading
• Part-load efficiency calculations
• Dynamic thermal mass tracking

Unlike SAP, which uses static correlations, HEM solves physics-based heat balance equations at each timestep. This realism allows it to better represent actual energy performance, even in non-standard homes or next-gen tech (e.g., heat batteries, PV diverters, and smart tariffs).

Benefits of Energy Calculation as a Service ✅

When considering the advantages of ECaaS, several key benefits stand out:

Finally, alignment with Net Zero: ECaaS supports accurate modelling against long-term climate goals by enabling compliance with more ambitious carbon targets.

First and foremost, uniformity: Everyone gets the same result from the same inputs, reducing inconsistencies across the industry.

Secondly, future-readiness: The model is built with smart homes, heat pumps, and evolving control strategies in mind, making it well-suited to the demands of modern residential design.

In addition, simplified software development: Since calculation logic is handled centrally, developers can focus on building better interfaces rather than duplicating engine development.

Moreover, an open-source philosophy: The Rust engine and Python wrappers are freely available to inspect, adapt, or build upon, encouraging transparency and collaboration.

Challenges to Watch ⚠️

While ECaaS brings several improvements, it’s not without operational challenges:

• Downtime dependency: If the central service experiences outages, assessments may be delayed, introducing a single point of failure.
• Opaque calculation engine: Although the engine is technically open-source, its complexity can make it difficult for most in the industry to fully understand or review.
• Slower computation: Earlier versions of the HEM model took up to 10 minutes per run. Although performance has improved, it still doesn’t match the near-instant results of legacy SAP tools.

Therefore, allowing time for processing and avoiding last-minute spec changes will be more important than ever.

Realism in Modelling: Beyond the Basics

HEM isn’t your typical EPC engine. It simulates:

• Thermostatic controls, using daily and weekly setpoint schedules to reflect real-world heating patterns.
• Zone-by-zone heat transfer, accounting for individual thermal behaviours and avoiding oversimplified assumptions.
• Part-load boiler performance, including cycling degradation, which captures the efficiency losses when systems operate below optimal output.
• Variable occupancy and internal gains, allowing user behaviour, lighting, and appliance use to fluctuate throughout the day.
• Solar shading by obstructions, factoring in nearby buildings or features that reduce direct solar gain.
• Dynamic thermal memory, which ensures that stored heat — such as from solar gain during the day — continues to influence indoor conditions overnight and beyond.

Together, these elements make HEM more capable of reflecting how buildings actually perform, especially under the demands of the Future Homes Standard.

Access and Usage

Who can use ECaaS?

• Only authorised accreditation schemes will have access for statutory purposes.
• Developers and researchers can use the open-source libraries (in Rust and Python) for non-statutory simulations.
• Access requests can be sent to ECaaS@communities.gov.uk

A basic web interface is under development to help users explore the model and visualise outputs before full implementation in 2025.

Impact on the Energy Assessment Industry

The days of “guesswork SAP” may be numbered. ECaaS represents:

• A paradigm shift for energy assessors
• New challenges for tool developers
• Higher accountability in reporting
• Better real-world alignment of EPCs

It's a future where calculation integrity is built-in, not added-on.

Looking Ahead: A Modular Future

Thanks to its modular architecture, ECaaS can:

• Integrate new tech (like smart meters or flexible tariffs)
• Simulate demand-side response
• Model user-behavioural flexibility
• Support future regulations without structural overhauls

As housing becomes more complex, so too must our models. ECaaS is ready.

FAQs

What is ECaaS in building regulations?
It’s the government’s official API-based energy calculation service that will be used to confirm compliance with standards like FHS and Part L.

Who will use ECaaS?
Accredited energy assessor schemes like Elmhurst and others. Developers can access the underlying libraries for non-official use.

Is ECaaS open source?
Yes. The core Rust engine is open-source and published online. A Python version exists under BRE’s development.

How accurate is ECaaS?
It simulates energy flows based on ISO-standard physics, tracking hourly weather, heat losses, internal gains, and system performance in fine-grained time steps.

When will Energy Calculation as a Service be mandatory?
With the rollout of the Future Homes Standard, likely in 2025, ECaaS will become the only way to validate Part L compliance.

Can ECaaS model unusual buildings or advanced tech?
Yes. It supports flexible zoning, advanced HVAC systems, smart controls, and even real-time battery and tariff scenarios.

Conclusion

Energy Calculation as a Service (ECaaS) is a foundational shift in how buildings will be assessed for energy efficiency across the UK. It delivers scientific accuracy and regulatory integrity becasue buildings must pull their weight on the path to Net Zero.

Everyone in the residential energy space needs to get acquainted with ECaaS because it’s already here.

Need help navigating ECaaS, HEM integrations, or compliance modelling?

Reach out to the expert team at Energy Digest, we provide tailored support, up-to-date guidance, and hands-on help to ensure your organisation stays ahead of regulation.