how do we make a net-meter (PV generation - loads)?

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EnergyPlus has a built-in way to get the net metered values: Output:Meter,ElectricityNet:Facility;. There are a few other meters you can request, such as ElectricityPurchased, and ElectricitySurplusSold. See the documentation for Meters in EnergyPlus for more details.

In OpenStudio, you can request these meters via a Reporting Measure. Here is what the energyPlusOutputRequests section of the measure.rb file should look like:

  def energyPlusOutputRequests(runner, user_arguments)
    super(runner, user_arguments)

    result = OpenStudio::IdfObjectVector.new

    # use the built-in error checking
    if !runner.validateUserArguments(arguments(), user_arguments)
      return result
    end

    result << OpenStudio::IdfObject.load("Output:Meter,ElectricityNet:Facility;").get

    return result
  end