The Future Homes Standard (FHS) will make solar PV panels mandatory on all new homes from 2027, with the government announcing that panels will have an area of at least 40% of the floor area of the building.
In this article I'll take a look at how the calculation works, what amount of solar it might result in and how exceptions are dealt with when roofs don't have enough room. I'll also share an early estimate of how many houses might not be able to meet the requirement in full.
How Much Solar?
The actual requirement for the amount of solar is, quite rightly defined in terms of the total rated power, which means that if you use higher performance panels, you can reduce the area or conversely if the panels you use are of low power rating then a greater area must be installed.
The actual target is to install solar PV that has the same annual output as a solar array with the following characteristics: an area of 40% of the ground floor area, a specific power of 0.22 kWp/m2, facing SE or SW at a pitch angle of 45 degrees and with little or no shading.
So for an 85 m2 semi detached home, arranged across two floors we might have 42.5m2 of ground floor area and a target solar installation of:
40% x 42.5 x 0.22 = 3.74kWp
This figure could be met for example with ten 405Wp solar panels, or nine 445Wp panels. It's worth noting that the current average installation we see under Part L 2021 is between four and five panels per house, so the Future Homes Standard represents a rough doubling in the quantity required.
Any amount by which the solar provided exceeds the target counts towards achieving the overall energy targets for the building, so there are benefits to going higher if the roof can fit more.
What About Complex Roofs?
The regulations makes allowances for roofs that are an interesting shape or that have features that limit the amount of solar that is possible, and this is outlined in Appendix B9. The developer must provide evidence to the building control body to show that it is not possible to install sufficient solar on the roof to meet the requirement.
Roof diagrams showing the roof with and without solar panels showing that the installation provides the maximum PV capacity and annual generation possible for the roof orientation and layout must be provided.
The diagram showing the maximum possible roof area for solar PV has to be made using specific guidance given on the minimum allowable offset distances from other roof features.
An example of a suitable diagram showing the maximum solar possible on a house is shown at the top of the page.
The target for solar PV then becomes a system that produces an amount of energy each year that matches that of a system with the area of solar in the drawing, a specific panel power of 0.22kWp/m2 and the orientation and angle of the actual roof pitches. This figure is then also used in the Notional House specification so there is no penalty there for having to use a smaller solar system.
At present there are no restrictions on how close to roof edges a solar panel installation can be made (subject to having a sufficient wind resistance to install in an edge zone and a sufficient fire rating to install adjacent to a party wall). Consequently it might be possible to provide a B9 drawing showing a maximum solar area, and install something different - so long as it matches or exceeds the annual power generation
For example this could be achieved by using higher efficiency panels than those in the standard, and covering more of the most advantageous roof pitch, while dropping solar off the less advantageous roof pitches, to meet or exceed the annual energy yield of the B9 drawing.
However, on the same day that the FHS was revealed, the Health and Safety Executive
opened a consultation on Approved Document B - the building regulations for fire safety. This includes what would become mandatory provisions for minimum offset distances for solar panels, so reducing the possibility to use tighter margins to outperform the regulated max-fit.
Something that has been missed in the guidance is to define what size of solar panel should be used in the assessment. This really needs to be clarified because otherwise people will get very different outcomes depending on what panel size is chosen.
 |
| How different panel sizes give different maximum-fit outcomes |
I have discussed this point with colleagues at the Future Homes Hub and the plan is to develop an industry guide in conjunction with representatives from building control bodies to fill in some of the missing detail from Appendix B9. This way we can hopefully have a consistent approach across industry.
What Proportion of Homes Will Fit 40%
As part of our work at
Viridian Solar in support of our housebuilder partners, the design team has performed studies on sample developments to assess which house types can meet a 40% requirement and which cannot.
So far we have seen around 80% of houses on a typical site can fit the full requirement of solar panels on the best elevation. Of the houses that could not fit the requirement, on average one half of the requirement could be fit on the best elevation, so around 20% of the ground floor area.
Under the approach in Appendix B9, these homes would need to use all the possible other elevations as well as the best elevation to maximise the solar generation possible, including North facing elevations if necessary. Depending on the size of the less advantageous roof pitches, this might result in a greater number of solar panels being used due to the penalty that comes as panel orientation moves further away from South facing.
Of course, this analysis was done before the final regulations were published, and so was using our own manufacturers' guidelines on offset distances which are tighter than those in the FHS. It is likely that a higher proportion of houses will struggle to meet the 40% target using the Appendix B9 guidance. No doubt the design team will continue to be working with housebuilders re-evaluating their house types to the final version of the standard. I will post an update when we have got more data from this work.
