Reviewing the new solar energy calculation in the Home Energy Model

The procedure for calculating the energy performance of homes is set to change with the implementation of the Future Homes Standard in 2025. The new calculation, known as the Home Energy Model (HEM), will replace the current Standard Assessment Procedure (also known as SAP) and is likely to affect Energy Performance Certificates (EPCs) as well. 
 
So, what can we expect from the new method? And how will it affect solar PV specification? As part of the industry consultation of the scheme, which ended in March, the government published a paper explaining how PV electricity generation and self-consumption would be calculated using the Home Energy Model. The BRE also developed a sample calculation tool. These gave a preview of what we can expect in 2025 and provided insight into how solar specification may change.  

In this blog, we’ll share what we gleaned from these resources so you can start thinking about what the updates will mean for current and future projects. 

What is the new Home Energy Model calculation? 

The energy output of solar PV systems is calculated using the method set out in BS EN 15316-4-3:2017: Energy performance of buildings – method for calculation of system energy requirements and system efficiencies.  

The energy output of a PV system is calculated using the hourly procedure (Method 6). The calculation is:  

Eout = Solar energy ⋅ efficiency at STC (standard test conditions) ⋅ system performance factor 

 
The performance factor is usually less than 1.0 and is affected by factors including power losses due to the temperature of the PV cells rising above 25°C, inverter losses, age degradation, soiled panels and wiring resistance.
 

Solar PV test conditions in the Home Energy Model 

As part of the consultation for the Home Energy Model, case studies were provided to validate the calculations using real-world examples and lab tests. The results were then compared to the SAP 10.2 and Microgeneration Certification Scheme (MCS) calculations. 

While variations between the different calculations were apparent, the similarities helped validate the proposed HEM methodology and indicated that HEM provided more accurate results. Let’s look at some of the variations and what they mean: 

• Panel ventilation – The amount of ventilation around a solar panel is a new variable not considered in SAP 10 or MCS calculations. Because solar PV electricity generation decreases as the panel's temperature rises, panels with open back faces (such as roof-mounted systems) should produce more energy than a similar panel with less ventilation. These are deemed to be “moderately ventilated” panels, while roof-integrated solar panels (such as the Marley SolarTile®) are considered “unventilated”. The difference in energy generation between these two ventilation methods, however, is thought to be between three and five percent, which is overall negligible to a roof PV system. 

• Orientation – The ideal orientation for UK solar panels is south-facing. As a panel’s orientation moves from south to north, its capacity for electricity generation decreases. However, the effect may not be as pronounced as previously thought since reflected or diffused light can still be used to generate electricity. The Home Energy Model has been adjusted to consider diffused and reflected light – it predicts around 64% more energy generation than MCS or SAP 10. This is a significant disagreement between the models and could indicate a problem with the Home Energy Model, so more research may be required. 

• Angle – In the UK, the ideal angle for a solar panel is between 35 and 45 degrees from horizontal. The HEM's results were closely aligned with SAP 10, with both models predicting a lower energy generation than MCS.  

• Location – The variation of solar yield was compared across five locations in England: Plymouth, Norwich, London, Birmingham and Manchester. Based on the dataset, the results for HEM were very similar to SAP 10.  

• Shading – This can come from various sources, such as trees and other buildings, and can change throughout the day as shadows move with the sun. 

The government’s Home Energy Model consultation document provides additional details on the case study houses and their results. 

Understanding energy demand in the Home Energy Model 

Of course, the energy output of the solar panels is only part of the calculation. There’s also the question of energy demand. The HEM is modelled on half-hour timesteps, which are intended to account for real-time variation in the property’s PV generation and energy demand. Generated electricity is assumed to be allocated a priority of: 

1. Household demands 

2. Battery storage 

3. PV diverter 

4.Exported to the grid 

Meanwhile, consumed electricity is assumed to be taken from solar first, then battery storage then the grid. 

Compared to SAP 10, HEM predicts that self-consumption drops more quickly as the size of a solar installation increases – at least in scenarios with no battery storage of solar PV diverters. However, the government's report admits that a small dataset was used to establish the instantaneous self-consumption curve and that electrically heated dwellings were not included. Also, the use of half-hourly timesteps may introduce some degree of error. The report concludes: "This area would certainly benefit from further study in future." 

Preparing for the FHS and Home Energy Model with Marley 

While the Future Home Standard and Home Energy Models are still in development, the Marley team is working hard to keep up with the news and updates around the new standards. Our solar products, including the Marley SolarTile®, inverters, battery storage and EV chargers are all designed to provide a high level of efficiency and performance and our team is here to provide technical advice and calculations. To find out more, visit our dedicated renewable solutions page or contact us today.