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My power company has very consistently reported 45kWh ± 5% per month higher consumption than my grid-tied Enphase system data over the first 6 months of my installation. Why would that be?

Is my power company, my smart meter or my solar PV system likely at fault?

I have 3-phase power and a smart meter. The 3 Enphase consumption CTs are installed in my main switchboard.

Could this be a resistive loss somewhere? If I’m calculating it right that would be about 60 Watts continuous draw.

Could it have something to do with power-factor (reactive load)? I don't have any heating, cooling or fluorescent lights in the house if that helps.

Edit:

  1. My utility typically reports between 170 kWh/month (summer) and 330 kWh/month (winter) consumption and my solar system reports almost exactly 45 kWh/month less than that.
  2. There doesn't seem to be any correlation between this discrepancy and my production, consumption, imports or exports.
  3. My export numbers are very accurate.
  4. I have 3 phase both to support my 90cm induction cooker and for future proofing (future EV, etc).
  5. Not sure how I would put together a system diagram, but happy to pop the switchboard cover and share photos if that helps.
  6. My biggest single load is a small resistive single-phase hot water system which uses between 3.5 kWh/day (summer) and 5.5 kWh/day (winter) to heat. This is on a timer in the middle of the day.
  7. I was told that for metering purposes my imports/exports are boiled down to a single number across all 3 phases (unsubstantiated though).
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  • Could you add a bit more detail? What number does your utility report, and what number does Enphase report? Could you include a simple diagram showing how the utility, PV, and your load are connected? Also, a three phase load with no heating or cooling sounds unique to me, but maybe this is typical where you are?
    – LShaver
    Dec 8 '21 at 15:59
  • Good questions, edited! Dec 10 '21 at 3:42
  • Hmm I'm sensing that this 45 kWh discrepancy may be due to some contractual requirement, assuming it's as precise as you indicate. Any chance you could share the name of your utility and the rate schedule that's on your contract? Or possibly details about the contract you have with the solar provider?
    – LShaver
    Dec 10 '21 at 6:11
  • It also works out to be pretty close to a continual 60 W, which is a standard light bulb size. Any chance there's an outdoor light somewhere, perhaps on a pole where the utility service enters your property? Could be that the utility meter picks this up but the meters on the solar system don't.
    – LShaver
    Dec 10 '21 at 6:14
  • The discrepancy is always 43-47 kWh, though I don't have the precise meter read time (only the date) so that would introduce some error. I spoke to my retailer OVO Energy here in Australia but they weren't able to shed any light. I might need to reach out to the grid operator CitiPower. The house was recently renovated and has a brand new electrical system. I'm not aware of any loads between the street and switch box, aside from the smart meter itself which is only supposed to consume ~1 kWh/year. Dec 10 '21 at 10:54
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All utilities measure real power, not reactive power, for small customers. Thus even if you have only heavily inductive loads like motors, that will not cause your cost to increase. The only case where reactive power is measured is for large users of electricity that have to pay for not only real power but also for reactive power (although the payment for reactive power is less).

If you have a solar PV array, it is possible that your power use is unevenly distributed between phases but the solar PV array distributes its power production evenly. For example the following is possible:

  • Phase 1: 60 kWh solar production, 30 kWh use, 0 kWh provided by power company, 30 kWh provided to power company
  • Phase 2: 60 kWh solar production, 120 kWh use, 60 kWh provided by power company, 0 kWh provided to power company
  • Phase 3: 60 kWh solar production, 45 kWh use, 0 kWh provided by power company, 15 kWh provided to power company

In this case, one possible way to calculate power use is the following: 0 kWh + 60 kWh + 0 kWh = 60 kWh.

Another way is the following: -30 kWh + 60 kWh - 15 kWh = 15 kWh

The difference between these methods is 45 kWh.

So read the fine print: are you being compensated if you use more electricity than your solar array produces, but some phases are heavy users of electricity and other phases actually provide power back to the power company.

Also time of day could affect you here. It's possible some times of day you are providing power to the power company and at other times you are using power from the power company. These negative and positive energy flows may not be treated in the same manner.

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  • Thanks the the quick and thoughtful response! I've done my best to add more context to the question above. Dec 10 '21 at 3:42

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