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Energy flow in grid connected PV system

  1. Feb 17, 2017 #1

    Let's say your house is connected to the local distribution grid, and you also have a PV-system on your house that is connected to the grid (no storage):

    How does the equipment in your house "use" the power from the PV module, and not "from" the grid? Everything is connected, right?

    If you have a power system with one power plant and a transmission line of say 100 miles to the consumer, you will have losses in the line. If you add a power plant 1 mile from the consumer in the same system, you will have less losses(?) But how does the consumer's equipment "choose" were to take the energy from? If you have less losses the current from the first power plant must be smaller.

  2. jcsd
  3. Feb 17, 2017 #2


    Staff: Mentor

    Electrons don't come with ownership labels. There is no choosing.

    The best way to think about it is that all sources, live power plants or PV, put energy into the grid. All loads, like lights or heaters, take energy out. The only accounting that happens is how much energy flows past a point, like your meter.

    In the eastern USA and Canada, any energy put in the grid contributes to energy out. Your light bulb in Key West gets a tiny fraction of its power from Hudsons Bay Canada up near the Arctic circle.

    When you inject power from your PV panels, it causes tiny power flow changes in every power line east of the Rockies. I think that's pretty awesome.
  4. Feb 17, 2017 #3
    Thank you for the answer. I've been told this explaination before, and kind of get that. The reason I wrote "choose" is that it seems to me that the meter does not measure energy consumption from pv if the pv system is "behind" the the meter. How does that part work? And how can distributed energy production raise the voltage quality in a neighbourhood with your explaination in my back head?
  5. Feb 17, 2017 #4


    Staff: Mentor

    The answer depends on whether you have met metering. With net metering, power can flow into your house or from your house to the grid. The meter simple measures how much goes past for how long.

    For example, if your house load is 1kw for one hour, that is 1kWh. Then if your PV made 0,5kWh in that same hour, the meter would measure 0.5. If the PV made 1.5 and if you have met metering, it would measure minus 0.5. If you did not have met metering, the PV would have produced only 1.0 kwh and the meter would measure zero. Without net metering, you aren't allowed to export power to the grid.

    Voltage is a completely different problem. If you are interested in that read this PF Insights article.
  6. Apr 25, 2017 #5
    Can someone explain how the power flows out of the house in terms of voltage, current and waves?

    Say the service voltage is 242V AC as measured from where the utility is connected to the house. Does the grid-tie inverter react to that and put out 243V AC, in phase, so that the net power flow is out of the house?
  7. Apr 25, 2017 #6


    Staff: Mentor

    Yes and no. The inverter controls the voltage magnitude, frequency and phase to export power to the grid. But power flow is proportional to phase angle difference, not voltage magnitude difference.

    See the PF Insights articles.
  8. Apr 25, 2017 #7
    Thank you for the welcome sign and thank you very much for the two links.
    The diagram in part 1, showing the failure of the bathtub comparison to power flow would have helped many a student in school.
    and the mantra in part 2 has opened a KISS method for understanding the relationship of power sources on the network.
    These articles will be re-read throughout the day for better understanding.
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