Significant difference btw 2 methods to calculate net purchased electricity

There seems to be a significant difference (about 20%) between the two methods to calculate annual total net purchased electricity as shown below, using both Singapore and San Fransisco weather files.

The 1st method is just to read the net purchased electricity output directly.

The 2nd method is to calculate the difference between total electricity demand and electricity produced by PV generator system.

May I ask:

  1. why is there such a huge difference?
  2. which method is producing the correct result?


Hourly difference in net purchased electricity btw the 2 methods:

Hourly difference in net purchased electricity btw the 2 methods:

net purchased energy error (576.2 KB)

BTW, the PV related output variables added to the workflow are shown below because the option to report them on the EPOutput component is not available anymore:

Output:Variable,*,Generator Produced DC Electric Power, hourly;
Output:Variable,*,Generator Produced DC Electric Energy, hourly;
Output:Variable,*,Generator PV Array Efficiency, hourly;
Output:Variable,*,Generator PV Cell Temperature, hourly;

Appreciate your kind advice on this issue.


… let me reply myself …

I checked the EnergyPlus HTML output file, and it seems there is a Power Conversion in the calculation of net purchased electricity:

So, the correct workflow for method 2 should be the following:

now the hourly difference between the two methods is quite small:

I assume the Power Conversion is related to the inverter efficiency which is specified as 90% here, i.e. only 90% of the electricity generated from PV will be converted to AC electricity going to utility or the grid, and 10% will be lost in the process.

Is this interpretation correct?

net purchased electricity issue (584.8 KB)

1 Like

You’re right that the total energy production is for DC electric energy so the conversion efficiency of the inverter is not considered . As you can see in the attachment that the produced DC electric energy is equal to the total electric generated, which is 60,174 kWh for San Francisco.

Thanks, @MohammedAlshayeb.

Yes, no PV cell temperature related results are reported even if I changed the simulation as hourly, tried different PV integration modes, and add surface temperature related output variables…

I assume it might be related to the PV integration mode as shown in the link of E+ document, and additional input information might be needed to get this value:

Do you want to open a new post for this question?

Yes, I will open a new post for the PV cell temperature outputs.