Seriously though... how complex is the Home Energy Model really?

"Is the new Home Energy Model really more complicated than SAP, or are you just saying that because it's new?"

I get asked this question more often than you'd think! And so far, I've struggled to quantify my answer.

We're going to need a bigger calculator! Home Energy Model assessments are far chunkier than what the SAP industry is used to.

But while loitering in a HEM working group this week, one of the developers gave the perfect answer....

🟠 When you create a SAP model on a typical dwelling, you need to enter around 80-100 bits of data.

That covers U-Values, thermal bridging, ventilation, dimensions, heating... all the usual things us SAP assessors love chatting about

🟠 In it's current form, HEM's Future Homes Standard wrapper requires around 400-500 bits of data.

Additional data includes volume of hot water pipes, the colour of the bricks, radiator quantities and sizes, and lots of other things that make SAP assessors run away and hide.

🟠 And if you modelled a house using the full version of HEM, you could easily be approaching 1,000 bits of data.

But there is good news... the FHS wrapper is being optimised to reduce the amount of info required. Inputs that don't impact the end results or are too complicated could be simplified or removed.

So inputs for the final version will be somewhere between current SAP (80-100) and current FHS (400-500).

To answer the question, HEM takes longer because it's more complicated. And because it's new. And because the consultation interface (now deleted) was very basic.

We're expecting news on a HEM calculation tool this summer, along with the publication of Approved Document Part L (which won't be enforced until 2026).