# Question about uni-directional electricity meter



## bigsmile (Sep 4, 2010)

Recently I did some research on solar panel system. I was informed that some residential meters, such as the digital meters, are "uni-directional" in the sense that they only measure the flow of energy from the grid to the load. This makes me wonder how such meter can measure the true power used. When the load is inductive, the power factor will be less than one. My understanding of the analog meter is that it's bi-directional (it can spin backward.) and it measure the true power by taking the integral of the instantaneous power (current times voltage) over time. If the instantaneous current and voltage are not in the same direction, the product will be negative, resulting in negative instantaneous power. Since uni-directional meter will not allow negative values, I can't think of how it measures the true power.


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## wildleg (Apr 12, 2009)

why would the power company care ? they prefer to charge you apparent power plus a premium if your power factor is low (if you are a commercial customer). They also prefer that you fix your own power factor so they don't have to spend the money to do it. Unless you notify them, most do not allow you to backfeed the grid, so AFAIK arrangements are usually made for that (ie. you don't just hook up a power source and start backfeeding power without making arrangements with the poco)


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## bigsmile (Sep 4, 2010)

I just want to figure out how the power company base their charge on. I am a residential user, and according to what people say, residential user are not charged for reactive power. However, I don't quite believe that a uni-directional meter can correctly measure true power.

If I find out that my utility company is charging me more than the true power I use, I will do some power factor correction. My main load is the heat pump, with I suppose is inductive load. So I think I can correct it by adding some capacitor.


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