FHS Launch - Mandatory solar panels

One part of the Future Homes Standard has grabbed the media’s attention more than most: Mandatory solar panels on all new homes. But how will these new regs work in practice?

Most homes will need a substantial amount of PV to comply with the Future Homes Standard, but within the limitations of the dwelling design.

Approved Document Part L (2026) includes an equation to work out the minimum onsite generation required to comply.

Take the ground floor area (as defined by SAP Conventions), multiply by 0.0889 to get the kilowatt peak of a notional PV array, set the panels to south east at a 45 degree tilt, and that gives us the kilowatt hour (kWh) target.

(0.0889 = 0.40 (40% of floor area) x 0.22 (efficiency of the notional panel))

Then, refer to equations in SAP Appendix M & U that are too big to post here.

🌞 For example, a dwelling with a 50sqm floor area needs to generate 3,566 kWh annually 🌞

New homes can generate electricity by any means, but let’s assume for simplicity’s sake that you’re not installing wind turbines or waterwheels. 🌊 

Get your friendly neighbourhood PV designer involved early so they can work out the max fit design for the house.

The SAP assessor will then model the proposed PV details, using the true orientation and roof pitch, which will calculate the actual generation.

🏠 For our house, let’s say we have 4.2 kWp, south facing.

That gives us an annual output of 3,627 kWh, which is higher than 3,566, so that meets the functional requirement. ✅ 

🏠 Maybe we have an identical house next door that’s east/west facing. We add our 4.2 kWp to that plot as well, but the annual output drops to 3,050 kWh (caused by the less favourable orientation). That doesn’t meet the requirement, so we’ll need an additional panel or two. ❌ 

There will be plenty of examples where a max-fit PV design doesn’t meet the functional requirement. In these cases, the failure is noted on the BREL report, but the Building Inspector will allow it providing the PV designer shows they can’t physically fit more panels on the plot.

Also, SAP targets will be relaxed so the plot isn’t penalised for not having enough PV (will explain more in a later post).

Important notes: there's no expectation to change the roof design to fit more panels on, nor use panel shapes and sizes that fit perfectly, nor to rotate your dwellings, nor to come up with innovative ideas for generating more electricity. Solar thermal cannot be used as an alternative; the technology has to generate electricity for the benefit of the occupants.

There are two exceptions where PV will not be mandatory.

🟢 Buildings that contain dwellings and are taller than 18 metres,

🟢 and plots where the available roofspace is so small, the max-fit calculation is less than 720 kWh annually.