What Is the Home Energy Model (HEM)?

Why SAP Needed Replacing, Not Just Updating

The Standard Assessment Procedure (SAP) - and RdSAP, its variant for assessing existing homes without full construction records - has been the basis of every EPC since the certificates began. It's built on a framework called BREDEM and works in monthly time steps: 12 calculations across a year, each producing an average for that month.

That approach was reasonable when most homes had a single gas boiler and no meaningful variation in how they generated or stored energy. It's a much worse fit for a housing stock increasingly built around solar panels, batteries, heat pumps, and smart, time-of-use electricity tariffs - technologies whose actual value depends heavily on when energy is generated, stored, or used, not just how much across a whole month. Government's response to that gap wasn't to patch SAP again, but to commission a genuinely new methodology.

The Core Technical Shift: Half-Hourly Instead of Monthly

This is the single biggest change. Where SAP/RdSAP calculates 12 times a year, HEM calculates 17,520 times - once every 30 minutes, for every half hour in a simulated year. Practically, that resolution is what makes several things possible that SAP couldn't do well:

• A heat pump's real-world efficiency changes with outdoor temperature through the day and across seasons - a half-hourly model can track that; a monthly average can't.
• Solar generation and battery storage only deliver genuine bill and carbon savings if what's generated lines up with when it's used, or is stored and shifted to when it's needed - modelling variable supply against variable demand needs a resolution finer than "the whole month."
• Smart, load-shifting technologies - batteries, smart meters, time-of-use tariffs - can only be properly credited in a model that can actually see supply and demand varying against each other.

This is also the direct technical reason behind the smart readiness metric that features throughout the new PRS (Private Rented Sector) MEES standard: it exists because HEM can finally measure what a monthly-average model never could.

Who Built It, and How It's Put Together

HEM was developed under government direction by a consortium including BRE, AECOM, Hoare Lea, Kiwa, Sustenic, and university researchers - not a single vendor, and not an incremental evolution of the old BREDEM-based SAP engine. It's described by those involved as a genuine fresh start: a modular architecture that separates a core building-physics calculation engine from policy-specific "wrapper" rules layered on top for whichever purpose it's being used for.

Two details worth knowing because they're genuinely unusual for this kind of government methodology: the underlying source code has been published under an MIT open-source licence, meaning it's publicly inspectable rather than a proprietary black box, and calculations are intended to run through a centralised government cloud platform (referred to as ECaaS), so that every accredited assessor's software is calling the same underlying engine rather than potentially producing slightly different results from separate implementations of the same rules.

HEM Does Two Different Jobs

HEM isn't built solely for EPCs. It has two distinct applications, sharing the same core engine but different rule "wrappers":

1. Building Regulations compliance for new-build homes, underpinning the Future Homes Standard (the updated Part L building regulations, expected around March 2027).
2. Energy Performance Certificates for existing homes, eventually replacing SAP/RdSAP entirely - this is the application relevant to PRS (Private Rented Sector) MEES and everything else in this guide.

It's worth being clear which one a given headline or document is actually talking about, since "HEM" news sometimes refers to new-build regulations rather than the EPC reform landlords in the existing rental market actually care about.

What Changes for Landlords: The Four New Metrics, Explained

Reformed EPCs calculated by HEM will display four separate headline metrics, replacing the single Energy Efficiency Rating (EER (Energy Efficiency Rating) (Energy Efficiency Rating)) that today's EPCs are built around. This is the part that actually determines what a future rating will say about your property, so it's worth understanding each one individually rather than just knowing they exist.

Fabric Performance

Measures how well the building's physical structure itself retains heat - walls, roof, windows, doors, and floors - entirely separate from whatever heating system is installed. Rated on the familiar A–G scale. What drives the score: insulation type, thickness and continuity (cavity, external, or internal wall insulation), loft and roof insulation, floor insulation, window and door glazing performance, airtightness (the subject of our airtightness testing guide), thermal bridging at junctions between building elements, and thermal mass. This is the metric the PRS (Private Rented Sector) MEES primary standard is built around - and because it's isolated from the heating system, a well-insulated house with an old boiler now gets full credit for its fabric quality, something the old single-score EER (Energy Efficiency Rating) (Energy Efficiency Rating) couldn't show clearly.

Heating System

Measures the efficiency and carbon intensity of the heating and hot water system, also rated A–G. This is where the biggest practical shift for landlords sits: reporting on the methodology consistently indicates that properties heated by fossil fuels - mains gas, oil, or LPG boilers - will not be able to reach a C rating on this specific metric, regardless of how efficient or well-maintained the boiler is, because the rating incorporates carbon intensity, not just running efficiency. Heat pumps and low-carbon heat networks are expected to reach C or above on efficiency and carbon grounds; direct electric heating with no storage or flexibility is expected to typically score D or below; electric heating paired with smart thermal storage (using off-peak, lower-carbon electricity) is expected to be able to reach C or above.

This is precisely why the PRS (Private Rented Sector) MEES secondary standard gives landlords a choice between the heating system metric and the smart readiness metric, rather than requiring the heating system route specifically - government has been explicit that no landlord will be forced to replace a working gas boiler to comply, and this metric design is the technical reason that protection exists at all.

HEM's half-hourly simulation also means heat pump efficiency is calculated dynamically against real outdoor temperature variation through the year, rather than SAP's fixed seasonal averages - generally a more favourable and more accurate way of crediting a well-installed heat pump than the current methodology.

Smart Readiness

An entirely new concept that didn't exist under the old EER (Energy Efficiency Rating), also rated A–G, assessing how capable a home is of interacting with smart energy systems and the wider electricity grid - recognising that a home's real energy value increasingly depends on flexibility, not just efficiency. What drives the score: smart heating controls (internet-connected thermostats, zoned heating, weather compensation), a smart meter, battery storage (capturing solar generation or cheap off-peak electricity for later use), smart or managed EV charging, general demand-side response capability, and solar PV with smart export/self-consumption management. A property combining a heat pump, solar panels, a battery, and smart controls would score highly here; a property with a basic timer-controlled boiler and no smart meter would score poorly, even if its fabric performance is excellent.

Energy Cost

The odd one out - rather than a letter grade, this is displayed as an estimated annual running cost in pounds per year, calculated using standardised fuel prices (held constant across every assessment at a given point in time, so properties are being compared on a like-for-like basis) combined with HEM's half-hourly simulation of the property's actual energy use. What drives it: heating and cooling system efficiency, fabric performance (better insulation reduces the energy needed in the first place), and how much a property can self-consume solar generation or use stored battery electricity rather than buying from the grid at full price. Worth flagging clearly to tenants and prospective buyers: this is a standardised estimate for comparison purposes, not a promise - actual bills still depend on the occupant's tariff, usage patterns, and how the property is actually lived in.

How This Actually Changes What a Rating Looks Like

Reporting on the transition period consistently describes new EPCs continuing to show both the legacy EER (Energy Efficiency Rating) A–G rating and the four new metrics side by side for a period - plausible given the January 2026 government response confirms the EER (Energy Efficiency Rating) will be "retained as a legacy metric" for grandfather rights purposes, though we haven't confirmed an exact end date for that dual display beyond "for a certain period." After that transition period ends, there's no longer a single overall score at all - a property is described by its four independent metrics rather than one headline letter.

The practical effect for landlords: a property's story can genuinely look different under the new system than it does today. A solid, well-insulated period property with an old gas boiler that currently scores a mediocre EER (Energy Efficiency Rating) might show a strong fabric performance grade and a weak heating system grade under the new metrics - information the single EER (Energy Efficiency Rating) never separated out. That's a large part of the point of the reform, and it's also exactly why the PRS (Private Rented Sector) MEES standard was deliberately built around a primary fabric standard plus a landlord's choice of secondary route, rather than a single flat target - it's designed to work with what these four metrics actually reveal about a property, not against it. This is covered in more depth in our EPC 2030 Deadline Explained guide.

The Timeline Update: Pushed Back to the Second Half of 2027

This is worth flagging clearly, because it's changed since earlier guidance was written, including our own PRS (Private Rented Sector) MEES overview. Government's January 2026 policy response described October 2026 as the target for launching reformed EPCs, while explicitly flagging that timeline as ambitious. As of March 2026, government has confirmed - in the official HEM:EPC methodology documentation - that this launch has been formally moved to the second half of 2027.

The reasons given cover essentially the full scale of what's needed to actually retrain an entire assessment industry: training and upskilling thousands of practising energy assessors, redeveloping the Domestic Energy Assessor qualification itself around the new methodology, publishing complete methodology and transition documentation, getting assessor software and its user interfaces through approval, and running assessor feedback sessions and beta testing before a genuine national rollout. Government has said it will work with industry and the devolved administrations to agree a specific revised launch date and a shared implementation plan.

What This Means Practically, Right Now

No property can get a HEM-based EPC yet, and won't be able to for a meaningfully longer stretch than originally expected. In the meantime:

• Today's EER (Energy Efficiency Rating)-based EPCs remain fully valid, and are the only route to grandfather rights before the 1 October 2029 cutoff.
• The delay arguably strengthens the case for pursuing EER (Energy Efficiency Rating) C now rather than waiting to see what a reformed EPC eventually says about your property - there's simply less runway than there used to be between reformed EPCs launching and the 2030 deadline arriving.
• It's worth checking back on the confirmed relaunch date once government publishes it - expected to follow the "Summer" 2026 timeline they've set for agreeing it with industry.

Key Takeaways

• HEM replaces SAP/RdSAP, calculating energy performance every half hour across a year (17,520 periods) instead of 12 monthly averages.
• That resolution is what lets it properly credit heat pumps, solar, batteries, and smart tariffs - the basis of the new smart readiness metric.
• It's open-source (MIT licence) and runs through a centralised government cloud calculation service for consistency across assessors.
• It does two separate jobs - new-build Building Regulations compliance and existing-home EPCs - sharing an engine but not the same rules.
• The launch of HEM-based EPCs has been officially delayed from October 2026 to the second half of 2027, confirmed by government in March 2026.
• The 1 October 2030 MEES deadline is unaffected by this delay - only the timing of reformed EPCs themselves has moved.

Frequently Asked Questions