The Home Energy Model — What It Means for Your Home

The Home Energy Model (HEM) is the UK Government's new way of measuring how much energy a home uses. It replaces the old system called SAP, which has been around since 1993. For most homeowners, tenants, and landlords, the biggest change you will notice is a new, more detailed EPC (Energy Performance Certificate) when your property next needs one. HEM won't require you to change anything about your home — but it will measure your home's performance far more accurately.

What's Changing?

At its simplest, the government is upgrading the calculator used to work out how energy-efficient your home is. The old calculator (SAP) used rough monthly averages. The new one (HEM) measures things every half hour across a whole year — 17,520 calculations instead of 12.

This matters because modern heating systems like heat pumps don't perform the same way all day and all year. A heat pump works more efficiently on a mild October afternoon than during a cold snap in January. Solar panels generate electricity at midday but not at midnight. The old system couldn't capture any of this. The new one can.

The result? More accurate, more useful EPCs that better reflect what it actually costs to heat and run your home.

What HEM Means for a Typical Home

The best way to understand HEM is to see how it would assess real properties differently from the old system. From our experience assessing thousands of properties, these three examples cover the housing types we see most often.

Example 1: A 1960s Three-Bed Semi

This is one of the most common property types in the UK — a three-bedroom semi-detached house built in the 1960s with cavity wall insulation, a gas combi boiler installed around 2015, double glazing throughout, and 200 mm of loft insulation. Under the current system, this home would typically achieve an EPC rating of around band D (score 60–65).

Under HEM, several things change:

• Orientation matters. If the living room faces south, HEM captures the free solar heat gained through those windows on sunny afternoons in spring and autumn — heat that genuinely reduces how much the boiler needs to run. SAP uses crude monthly averages that largely ignore this.
• The boiler is assessed more honestly. A combi boiler doesn't run at its rated efficiency all the time. In practice, it cycles on and off, losing heat each time it fires up. HEM models these cycling losses at half-hourly intervals, which typically results in a slightly lower effective efficiency than SAP assumes.
• Local weather data is used. SAP uses broad regional climate data. HEM uses more localised weather files, so a house in sheltered Devon is assessed differently from an exposed property in Northumberland — as it should be.
• Thermal bridging is examined more closely. The cold spots around window reveals, lintels, and where walls meet the roof are a real source of heat loss. From our experience, these junctions often account for 15–25% of total fabric heat loss in 1960s properties, and HEM gives them closer scrutiny than SAP did.

Likely outcome: This property would probably see a similar overall score, but with more detail. The Fabric Performance rating would show reasonable insulation, while the Heating System rating would honestly reflect the limitations of a gas boiler (which under the proposed new EPC format cannot achieve band C). If the house faces south, the Energy Cost estimate may be slightly lower than SAP predicted, reflecting those real solar gains.

Example 2: A Victorian Mid-Terrace

A two-bedroom Victorian mid-terrace with solid brick walls (no cavity), a mix of original single-glazed sash windows at the front and modern double glazing at the rear, a gas boiler, and minimal loft insulation. Under SAP, this property typically scores band E or low D (score 40–55).

HEM assesses this property quite differently:

• Thermal mass works in its favour. Solid brick walls are poor insulators but excellent at storing heat. On a cold morning, the brickwork that absorbed heat overnight releases it slowly back into the home, reducing the spike in heating demand. SAP's monthly averages cannot capture this effect. HEM's half-hourly modelling can, and for solid-walled terraces with consistent occupancy patterns, this can make a meaningful difference.
• Party walls reduce heat loss. In a mid-terrace, two of your walls are shared with heated neighbours. In practice, very little heat is lost through these walls — the temperature on both sides is similar. HEM handles this more accurately than SAP, which can overestimate heat loss through party walls.
• Mixed glazing is properly accounted for. This house has two different types of windows. HEM can model each window individually — the single-glazed sash at the front losing heat at a different rate from the modern double glazing at the rear. SAP tends to average these together in a way that loses the detail.

Likely outcome: The thermal mass and party wall effects could nudge the overall energy performance slightly upwards. However, the solid walls and single glazing will still be flagged as weak points in the Fabric Performance metric. The new EPC format is actually helpful here — it separates the fabric issues from the heating system, so buyers can see that the walls need work even if the boiler is performing adequately.

Example 3: A 2020s New-Build Flat

A two-bedroom flat in a 2020s development, built to the 2021 Part L standards, with an air source heat pump, mechanical ventilation with heat recovery (MVHR), solar PV on the communal roof, and high levels of insulation. Under SAP, this would typically achieve band A or high B (score 85–95).

This is where HEM makes the biggest positive difference:

• The heat pump is assessed at its actual performance. SAP uses a single seasonal efficiency figure for the heat pump. HEM models how efficiently it operates at every half-hourly interval, accounting for the outdoor temperature. In a mild climate (say, Bristol), the heat pump runs at a higher efficiency than SAP assumes for most of the year. In a colder area (say, Aberdeen), the winter dip is properly captured too — but the overall picture is more honest either way.
• Solar PV self-consumption is tracked. SAP estimates annual PV generation as a single number. HEM tracks how much of that electricity is actually used in the flat (self-consumption) versus how much is exported to the grid. If you're home during the day or have a battery, you use more of your own electricity — and HEM rewards that.
• Battery storage can finally be modelled. If the development has communal batteries, SAP simply cannot account for them. HEM models the charge and discharge cycles, reflecting the genuine benefit of storing cheap daytime solar electricity for evening use.
• MVHR is properly credited. The ventilation system recovers heat from stale outgoing air and warms the fresh incoming air. HEM models this heat recovery at half-hourly intervals, capturing how it performs differently in summer (when heat recovery is less useful) and winter (when it's crucial).

Likely outcome: This property is likely to score even better under HEM than under SAP. The technologies it uses — heat pump, MVHR, solar PV, potentially batteries — are exactly the ones that HEM was designed to model accurately. Properties like this are the main beneficiaries of the new system.

Will It Affect Your Current EPC?

Not immediately. Your existing EPC remains valid for 10 years from its issue date. Nothing changes until you next need a new one — typically when selling or letting your property.

When you do need a new EPC, the assessment may eventually be carried out using HEM rather than the current RdSAP method. A government consultation published in January 2026 proposes how this will work. New HEM-based EPCs are targeted for launch from October 2026, but the existing Energy Efficiency Rating will continue alongside the new system until October 2029, giving everyone time to adjust.

Buying a New Build — What to Expect

If you're buying a new-build home from late 2026 or 2027 onwards, it will likely be built to the Future Homes Standard (FHS), which uses HEM to check compliance. This means your new home should come with:

• A heat pump instead of a gas boiler — most likely an air source heat pump (ASHP)
• Solar panels on the roof as standard
• Better insulation — thicker walls, triple glazing, and a more airtight construction
• Mechanical ventilation with heat recovery (MVHR) to keep air fresh without losing heat

The overall result is a home that is warmer, cheaper to run, and produces far less carbon than one built under today's rules. The government estimates that energy bills for FHS homes will be 40–50% lower than for conventionally built properties.

Impact on Energy Bills

HEM itself doesn't change your energy bills — it's a measuring tool, not a regulation that requires you to upgrade your home. However, there are two ways it could affect costs indirectly:

• New homes will be cheaper to run. The Future Homes Standard, which uses HEM for compliance, requires much better insulation and low-carbon heating. If you buy a new FHS home, you should benefit from significantly lower energy bills.
• Improvements you make may be better recognised. If you install a heat pump, add solar panels, or improve your insulation, HEM's more accurate modelling will better reflect the real benefit of those improvements on your EPC. Under the old SAP system, some improvements (particularly heat pumps and batteries) were poorly credited.

What Will It Cost Me?

This is the question every homeowner asks, so let's be straightforward about what we know and what we don't.

Will I Need a New EPC?

Not just because the system is changing. Your existing EPC remains legally valid for 10 years. You only need a new one when you sell, let, or build — the same triggers as today. There is no requirement to proactively obtain a HEM-based assessment.

Will EPCs Cost More?

Probably, though the government has not yet confirmed pricing. In practice, a HEM assessment for an existing home will require the assessor to collect more detailed data than the current RdSAP survey. From our experience carrying out thousands of domestic energy assessments, the data collection for HEM is substantially more involved — government testing found that a full HEM assessment takes approximately 1 hour 40 minutes compared to about 20 minutes for SAP. For existing dwellings, the proposed “reduced data” methodology should be quicker than a full HEM assessment, but it will still be more detailed than the current RdSAP process.

For context, a standard RdSAP-based EPC for an existing home currently costs between £40 and £120 depending on the property size, location, and provider. It is reasonable to expect that HEM-based EPCs will sit towards the higher end of this range, or potentially above it, reflecting the additional survey time and data requirements.

Will I Need to Make Changes to My Property?

No. HEM is an assessment methodology, not a building regulation that requires you to upgrade your home. You will not be required to install a heat pump, add insulation, or make any other changes simply because the EPC system is being updated.

The only scenario where HEM could indirectly require action is for landlords who are close to the Minimum Energy Efficiency Standards (MEES) threshold. If your property is currently just above the minimum band E and HEM assesses it less favourably than RdSAP did, you might need to make improvements to maintain compliance. See the landlords section below for more detail.

Selling or Renting Your Home

If you're selling or letting a property, you need a valid EPC. Once the transition to HEM-based EPCs happens, your property will be assessed using the new methodology. Here's what that means in practice.

Impact on Selling

The more detailed EPC format could work in your favour if your home is genuinely energy-efficient. Instead of a single letter grade that lumps many different factors together, the four separate ratings will show buyers exactly where your home performs well (and where it doesn't).

Research already shows that homes with higher EPC ratings tend to sell for a premium. A 2023 study by the Department for Levelling Up found that properties rated EPC band A or B commanded a price premium of around 5–14% compared to equivalent band D properties. By making EPCs more detailed and trustworthy, HEM could strengthen this price link between energy efficiency and property value.

Which Properties May See Better or Worse Ratings?

In our experience, most properties will see a broadly similar overall picture. The biggest shifts will be for homes at the extremes — those with modern low-carbon technology that SAP undervalued, and those with hidden issues like severe thermal bridging that SAP overlooked.

What Landlords Need to Know

If you let a property, HEM matters to you for two reasons: the Minimum Energy Efficiency Standards (MEES) and the potential impact on tenant demand.

Under MEES, it has been illegal since April 2018 to let a property with an EPC rating below band E in England and Wales (with some exemptions). The government has previously consulted on raising this to band C, though no date has been set for this change.

The shift to HEM-based EPCs could affect your rating in either direction. If your property currently sits at band D or low E, a change in either direction matters:

• If your rating improves (for example, because HEM captures the thermal mass benefit of solid brick walls), you have more headroom above the MEES threshold.
• If your rating worsens (for example, because HEM more accurately models the cycling losses of an old gas boiler), you could find yourself closer to — or below — the minimum standard.

On the positive side, any improvements you make to rental properties will be better recognised under HEM. If you install cavity wall insulation, upgrade to a heat pump, or add solar PV, the EPC will more accurately reflect the benefit — which is useful both for MEES compliance and for attracting tenants who are increasingly aware of energy costs.

Timeline for Sellers and Landlords

• October 2026: New HEM-based EPCs are targeted for launch. Both old and new EPC formats will be available.
• October 2026 – October 2029: Transition period. The existing Energy Efficiency Rating runs alongside the new four-metric format.
• October 2029: The old single-rating format is expected to be retired. All new EPCs will use the HEM-based format.

Your existing EPC remains valid throughout this period for its full 10-year life. For more detail on the transition, see our How EPCs Change guide.

What to Do Now

There is no need to panic or rush into expensive improvements. Here is our practical advice:

• If you're selling soon (before October 2026), the current EPC system still applies. Get your EPC as normal.
• If you're selling after October 2026, you may have the choice of old or new format during the transition period. It is worth waiting to see how the new system settles before making any EPC-driven improvement decisions.
• If you're a landlord near the MEES threshold, consider investing in cost-effective improvements (loft insulation, draught-proofing, boiler controls) that will benefit your rating under either system.
• If you're planning improvements anyway, go ahead. Heat pumps, solar PV, insulation, and battery storage will all be better credited under HEM than under the current system.

Retrofit and Home Improvements

If you're considering energy efficiency improvements to your existing home, HEM's more accurate modelling is broadly good news. Improvements that genuinely reduce energy use will be better recognised:

• Heat pumps will be properly credited for their dynamic performance rather than assessed using simplified averages
• Solar PV will be assessed based on how much electricity you actually use on-site (self-consumption), not just the total generated
• Battery storage can finally be included in the assessment — the old system couldn't model it at all
• Insulation improvements will be reflected through the dedicated Fabric Performance rating
• Smart controls and demand flexibility will be recognised through the Smart Readiness metric

In short, if you invest in making your home more energy-efficient, HEM is more likely to reflect that investment accurately on your EPC than the old system was. From our experience, the biggest gap in the current system is around heat pumps and solar PV — many homeowners who've invested in these technologies have been frustrated by how little credit SAP gives them. HEM should fix that.

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