Introduction

In this article, we return to the semi-detached home from our last case study. 

We model a heat pump with the Home Energy Model, and consider the carbon, economic and comfort impacts of running a heat pump with a variable tariff and solar PV.

The Home Energy Model is ideal for modelling heat pumps

While the MCS room-by-room methodology is designed to size heat pumps (by quantifying peak heating demand), annual running costs are estimated using heat demand from an EPC (using the MCS Performance Estimate calculator). For reasons we outlined in our last case study, the SAP methodology used for EPCs (in the form of rdSAP) produces inherently less realistic estimates of space heating demand. More importantly SAP produces monthly outputs, which cannot be used to analyse the value of variable tariffs or self-generation.

The Home Energy Model is capable of detailed heat pump specification. The model can identify if a heating system is sized to meet demand (with a dynamic methodology to identify peak demand), and estimate total annual running costs. The Home Energy Model methodology may be more robust than MCS, as it models the dynamic impact of thermal bridges and intermittent heating instead of using rules of thumb, and because it can use predicted peak heating demand (whenever this occurs) instead of simply assuming that the coldest outside temperature represents the design day.

The Home Energy Model also produces additional useful outputs compared to MCS, with half hourly energy consumption newly enabling analysis of the value of variable tariffs and self-generation, and temperature metrics enabling analysis of comfort impacts.

The modelling process

We modelled the following decarbonisation scenarios:

For Scenario 1, we modelled a high-temperature heat pump replacing the current combi boiler. This was lower disruption than other options we could have modelled, such as a low-temperature heat pump (which could trigger distribution system and radiator upgrades) and/or a hot water immersion heater. 

We also modelled switching the gas hob to electric, so that the home could fully move off gas and avoid paying the standing charge.

The Home Energy Model requires more detailed inputs than SAP for heat pumps. Importantly, it will use lab data on heat pump efficiency across a range of external conditions, and low and high temperature applications (according to EN 14825) instead of SAP which uses a single annual efficiency. This can enable more realistic modelling, and better match heat pump systems to specific use cases. The Home Energy Model also adds detail on modulation control specification, and auxiliary power (with parameters for a circulation pump, crankcase heater, stand-by and off-mode).

This data is not currently available in the Product Characteristics Database (PCDB). The Home Energy Model team has started engaging with suppliers to update the PCDB and Appendix Q process. While the exact way new technologies will be recognised is not yet clear, the direction is integration directly into the model rather than a bolt-on spreadsheet as done today.

For the purposes of this case study we identified a heat pump that suited the peak demand (6.11kW at 6pm on 6th January), and researched the required information. 

Scenario 2 represents the switch to a variable tariff, in this case the Octopus Cosy Tariff. We used controls to restrict heating and hot water system usage to hours with discounted tariffs, as seen below.

There were other ways we could have modelled controls, such as preventing heating system use in hours with peak tariffs (4:00PM to 7:00PM), or using historical data for tariffs that better reflect wholesale prices.

In Scenario 3, we modelled solar panels to maximise the potential of the south-facing roof space. We identified that 6 panels of 615Wp could fit on the roof, for a total 3.69kWp installation. In Scenario 4, we roughly sized the battery based on the maximum exported energy on a peak day (~16.4kW). There were other approaches we could take to this, including testing many batteries and choosing the design with highest return on investment.

The results

Installing the heat pump means ~69% immediate annual CO₂ emissions reductions (using DESNZ carbon factors). As the electricity grid decarbonises, this will mean a 92% cumulative reduction in CO₂ emissions between now and 2050. 

For Scenario 1, most of the financial benefits are from not paying the gas standing charge (£128/yr). If this wasn’t avoided, the total energy bill of the home would be a modest £832/yr (only 3% lower). 

The comparable running costs of a heat pump and boiler reflects the current gap between electricity and gas tariffs. The residents of this house paid 4.23x more for electricity (24.3p/kWh) than gas (5.7p/kWh).

With the Octopus cosy tariff but no change in heating controls, the running costs of the home would be £655/yr (7% lower cost than the current tariff). Changing controls to only run the system on discounted tariffs further reduces running costs £574/yr (-33% vs baseline, or -18% vs a heat pump alone). 

Restricting operating hours may impact heating efficiency and result in a less comfortable home (assessed by temperatures outside a range). The Home Energy Model allows these impacts to be effectively evaluated. In this case we found that switching schedules had a negligible impact on internal operative temperatures. If we had found an impact, we may have selected a less restrictive heating controls schedule.

When we modelled solar panels in Scenario 3, approximately 29% of generated energy was consumed (offsetting the need for mains electricity), with 71% exported at 12p/kWh (eg, the best rate available that doesn’t require a supplier switch, from ScottishPower). 

This encouraged us to model an electric battery to improve self-consumption. A 15kW battery enabled 61% of generated energy to be consumed and 39% exported, but had limited economic benefits. This may be because displaced electricity was already low cost with the cosy tariff (at 11.7p/kWh). The electricity standing charge (£161.33) also acts as a floor to potential savings.

Conclusions

The Home Energy Model is an important tool to facilitate the electrification of home heating. It not only enables heat pumps to be sized, but also enables precise evaluation of the economic impact of flexible tariffs and self-generation, to maximise economic, carbon and comfort impacts. This can help more homeowners understand how they can save money with a heat pump, and make the switch.

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