A Review of The FHS 'Wrapper' for the Home Energy Model
The Future Homes Standard (FHS) consultation includes proposals for a wholesale revision to the underpinning calculation method by which the energy efficiency of new homes is evaluated.
I have already written about the Home Energy Model (HEM) in an earlier blog. It is planned that this will replace the Standard Assessment Protocol (SAP) currently in use by Energy Assessors to calculate whether a new home design specification meets building regulations. An FHS version of the Home Energy Model will be used to demonstrate compliance with the building regulations, by preloading the Home Energy Model calculation with a set of assumptions and inputs and defining the outputs it needs to provide, collectively called a 'Wrapper' for the HEM.
The consultation document on the Future Homes Standard wrapper can be found here.
In this blog I go through the most significant changes the wrapper introduces compared to the current version of SAP.
Occupancy
When a new home is built you probably don't know how many people will live in it and even if you do it will change over time, so calculations for building regulations require a standard occupancy.
The number of occupants is an important factor - affecting the amount of energy used for lighting, appliances and hot water use.
In SAP the standard occupancy was taken to be a function of the total floor area of the building, but for the FHS this will change to be driven by the number of bedrooms, apart from 1 bedroom dwellings which will have an occupancy driven by floor area.
In the graph above the dots show data from a national survey and indicate that SAP 10 occupancy (wide yellow line) is not a good match. the coloured horizontal lines show the new occupancy level based on the number of bedrooms.
A higher occupancy will increase the hot water demand (although this is offset by other changes, see below), and electricity demand for lighting and appliances.
Hot Water Demand
As in SAP 10.2, hot water demand is driven by occupancy, but demand per occupant is lowered based on new evidence from a 2021-22 study of 45,000 combi boilers in UK homes, which suggests that measures taken to reduce hot water use (for example low flow taps and shower heads) have had an effect on hot water use.
For contrast typical consumption in SAP 10.2 is 120 litres per day for a two person household and 160 litres a day for a three person household. This reduction of nearly 20% will be offset by the higher occupancy for three and four bed homes.
Weather
The FHS standard is consulting on the use of regional weather data. Historically housebuilders have been against this as it means they cannot build the same house all round the country (or rather they can but would need to meet the regulations in the most arduous location and therefore over-provide in others).
Another possible change is to use 'future' weather files based on Met Office climate projections relating to the assumed use period of the standard (2025-29).
Electricity Demand
The wrapper also contains assumptions on electricity use for lighting and other appliances (dishwasher, tumble dryer, fridge, freezer and electronic goods).
The FHS wrapper has to support the 30 minute time resolution of the Home Energy Model so both lighting and appliance electrical use has been split into daily profiles. For now, these profiles are aggregated and averaged whereas in real life electricity demand is more 'peaky' with kettles being boiled for only a few minutes and freezers cycling on and off. This smoothing will over-state self consumption of electricity for solar PV.
Compared to SAP 10.2 the demand for lighting is increased, reflecting the fact that homes have more and brighter lighting than in the past, although the efficiency gains from LED bulbs more than offsets this.
Emissions Factors
In the FHS, the emissions associated with mains electricity has fallen substantially from the value used in SAP 10.2, reflecting the decarbonisation of the generation mix providing UK power, and also a change in approach to predict the carbon intensity of the grid for the time the standard is in use rather than fixing it at the consultation value.
The big difference in the emissions factor for renewable generation is due to an 'accounting change'. As explained in this document
Fuel factors within the Home Energy Model: FHS assessment, the emissions factor for on-site generated is deducted from the emissions factor for grid electricity when calculating a reduction in dwelling emissions and primary energy.
This differs from SAP 10.2 where the onsite generated energy was a negative value and was multiplied by the renewable emissions factor to get a saving to take away from the total emissions or primary energy for the period.
A key factor to note is that solar generation exported to the grid produces the same benefit to carbon emissions and primary energy as solar generation used in the home. The logic is that this exported unit of electricity is preventing the need to generate electricity at the grid factor and the benefit accrues to the dwelling. This is in stark contrast to Scottish building regulations which since the 2023 revision have deliberately excluded the benefits of exported energy from contributing to the assessment of the dwelling performance.
Another feature of this approach is that energy storage in batteries produces no benefit on emissions or primary energy scores. In fact the addition of a round-trip efficiency to energy stored in a battery for later use actually reduces the benefit of battery storage compared to export. This will remain the case until the model can take account of the fact that grid emissions and primary energy varies during the day and a strategy of avoiding export by storing surplus solar generation for use in the evening will not only reduce energy bills but also result in a net reduction in carbon emissions and primary energy use because renewable energy is generally less of the grid mix at this time of day .
Conclusions
The addition of a time of day value for grid emissions and primary energy would be a welcome addition to the FHS Home Energy Model, but apart from this omission, the changes introduced compared to SAP 10.2 look benign from the point of view of the solar industry.
