Monthly electricity demand is not the same although zone loads and the schedules are fixed

Dear bees and bugs, in the GH file attached the monthly electricity demand is not the same across different months although the zone loads and schedules are fixed (this can be seemed by the hourly electricity demand chart which is consistent across each day).

May I ask what might be causing this ?

This cause the problem that the net purchased electricity is not the difference between the total electricity demand and the total electricity generated. E.g. using Chicago’s epw file, the electricity generated is 4717kWh, electricity demand is 4504kWh, and net purchased energy is 258kWh.

Appreciate if you can point out any import things I miss here.

Thank you very much!

  • Ji

pv generator (886 KB)


This seems to be happening because each month is not the same number of days. Some months are 30 days long, others are 31 days and February is 28 days, which explains why it is smaller in the bar chart. So the same schedule and loads are being used each day but it’s just the number of days that is different.

If this does not account for all of the differences you are seeing, let me know and I can check out your GH file.


Dear Chris,

Thank you very much for pointing out the absence of common sense in my observation of the results :slight_smile:

However, it still begs the question that why the annual total net purchased energy (258kWh) is not equal to annual total electricity demand (4717kWh) minus annual total energy generated on site through PV (4504kWh), i.e 213kWh.

Or is the net purchased energy is calculated in different way?

Appreciate you kind advice.

Thank you!

  • Ji


No worries and I have also had several lapses in common sense going through the many outputs of E+. I don’t know about the PV issue and this is definitely a question for Anton who developed the HBgenerator components.


Hi Chris, noted with thanks! I shall contact Anton on this question.

  • Ji


Apologies for the delay in my reply,

This is a good point it seems that there is something off here, I will look into it and get back to you.



Hi Ji,

Apologies for the delay in my reply, initially I checked the read generation system results component thinking that it was a rounding error I discovered that it wasn’t.

These are actually the results coming straight from EnergyPlus and I’m not sure why they don’t add up. I am going to ask the question on unmet hours. I’m pouring through the E+ documentation but I can’t find an explaination

Ok I’ve asked a question here:…

Hopefully I am not missing anything simple

Dear Anton,

Thank you very much for your help! Hope we can get some advice soon.

  • Ji

Hi Anton,

I want to correct an error in my question:

The total electricity demand is 4504.1 kWh, and the total electricity generated from PV is 4717.54 kWh. So, the net purchased electricity shall be -213.44 kWh, i.e. a negative value.

However, the net purchased electricity as obtained using the HB Read Generator System Results component is 258.44 kWh, i.e. a positive value.