HEM Deep-Dive: PV performance

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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I am forever telling people how the Home Energy Model is far more complex and detailed than SAP... even if they don't care 😂

Anyway, now, I can show you in graph form! 📈

The following shows output data from SAP and HEM calculations which predict how much electricity is generated by a 4kWp PV system.

I modelled this four times, changing the orientation of the PV through north, east, south and west. The house design has a 35 degree pitched roof, so HEM is uses the true 35 degree pitch but SAP rounds down to 30 degree.

The graphs run from January to December, so the biggest output (as you would expect) is in the summer months. PV output is reported as a negative value, so the lower the line, the more being generated.

The first image is from SAP:

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

You can count the 12 sections from its monthly calculation, and see how it treats east and west identically. (The east line (blue) is hiding under the west line (yellow).

The second image is from HEM:

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

This shows daily totals throughout the year. The imperfections in the curve are caused by whatever data is stored in the weather file overcast days vs blue skies).

But wait.. there’s more!

The above graph shows daily totals, but the PV module on HEM calculates hourly. So for every point on the above graph, you can go down another level to see the hour-by-hour generation for any day of the year.

This third graph shows the PV output for July 7th (for no other reason than it’s today!).

Line graph showing HEM output of a 4kWp PV system through different orientations - July 7th

You can see how east facing panels get the morning sun and west facing gets the evening sun. Across a years worth of data, south facing panels should perform better, but on this particular day it looks like due west panels perform better.

You can also see a big dip in performance late morning with smaller dips through the day. These are caused by a reduction in solar radiance recorded in the weather file (in other words, clouds).

Repeat this graph 365 times, and you get the daily totals (shown in the second graph).

You can, in theory, influence the performance of the PV by loading an alternative weather file into the HEM engine. We're expecting a pre-saved weather file will be locked in to Part L calculations in England, with a separate file for Wales.

Scotland is still considering the idea of using regional weather files for Section 6 compliance. These would also be locked in, and chosen by the software based on the site location.

But if you step away from Building Regulations wrappers, you could load in any historic or predicted weather file and see how this changes the PV output.

This granular level of detail tells the HEM engine how much power is being generated so it can decide how much is being used in the house, how much can be sent to batteries, and how much to export.

I'll be doing more nerdy stuff with HEM through the summer. You have been warned! 😄