HEM Deep-Dive: PV angles

HEM engine v2.00a2 (Python); FHS v1.00a7.

At time of writing, HEM is still in development and not yet approved for regulatory compliance.

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In last week's post about PV output from the Home Energy Model, I said south facing panels generate more electricity. But then I realised I’ve been hard-wired into thinking that.

Ask any SAP assessor 'What is the best orientation for PV?'. They’ll tell you due south at 30 degrees.

But HEM isn’t SAP. So does HEM agree with SAP?

Turns out, nope!

Based on the Future Homes Standard default weather file, the best orientation is slightly west of due south... 186 degrees to be precise.

And the most efficient roof pitch for that array is 41 degrees, not 30.

After that revelation, I spent the weekend down a HEM-based rabbit-hole, trying different roof pitches and orientations. 🐇 

Graph 1 shows the output of a 4 kWp PV system by orientation (at 30 tilt). There's not much of a drop in performance when you rotate between south-east and south-west.

In this example, north facing panels generate around 40% less than south facing.

Line graph showing HEM output of a 4kWp PV system through different orientations.

Graph 2 shows the impact of the tilt / roof pitch. For south panels, peak performance is 41 degrees.

For east and west, best performance is anything below 25 degrees, and north panels work best when completely flat.

Line graph showing Home Energy Model output of a 4kWp PV system through different roof pitches.

If you're using the Home Energy Model to find the most optimal position for your PV:

☀️ The best roof pitch depends on the orientation.

☀️ The best orientation will vary depending on the weather file.

☀️ HEM allows the precise orientation to be entered (0-360) whereas SAP has eight options.

☀️ HEM allows the precise pitch to be entered (0-90) whereas SAP is limited to five options.