Calculation of AC power (Kwh)

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Allen
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Calculation of AC power (Kwh)

Post by Allen »

How does opticsre calculate AC power. I have a program that sums the second by second values of AC voltage X AC current / 3600 / 1000 to estimate the power in KWH. However the value obtained by this method is about 20-25% higher than the values presented by the opticsre dashboard. Thanks
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Re: Calculation of AC power (Kwh)

Post by provo »

Allen wrote: Mon Jul 26, 2021 8:01 am How does opticsre calculate AC power. I have a program that sums the second by second values of AC voltage X AC current / 3600 / 1000 to estimate the power in KWH. However the value obtained by this method is about 20-25% higher than the values presented by the opticsre dashboard. Thanks
I've noticed this also. The daily kWh "From Grid" in Optics is usually about 15% lower than what I get billed for by PGE. It varies with the power factor of the loads that day (heaters vs A/C is a big difference), but it's always lower than PGE's number. I'm very interested in others' answers to this question :grin: .
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Re: Calculation of AC power (Kwh)

Post by raysun »

Neither OpticsRe, nor the calculation in the OP calculate AC power.

(True) AC Power is measured in Watts, essentially the measure of heat dissipated by a purely resistive load, with power consumption measured in Watt-Hours. Both OpticsRE and the OP calculation label their products kilowatt-hours, but that's not what's being measured.

Both RE and the OP are measuring apparent power in Volt-Amps.

Its not specified if the V(ac) being measured is RMS or Peak-to-Peak. I don't recall seeing a spec for RE's V(ac) measurements.

The 1 second "snapshots" take a sample over what turns out to be a long period for 60Hz AC. Depending on when the sample is taken, voltage could be anywhere within the +/- limits, as could amperage. Considering Power Factor, the voltage peaks and current peaks don't usually align. True power (in Watts) is definitely not being measured.

The sum of the "snapshots" (averages for lack of a better term) is not likely equal to the average of the sums. I believe Outback uses a long-period sampling method (but haven't ever seen the algorithm) that "normalizes" the kVAH readings (even if they are called kWH).

Here's a "101" look at true power, apparent power, (and reactive power): https://www.allaboutcircuits.com/textbo ... ent-power/

Good luck with rationalizing "power" measurements. There has been a lot of forum space dedicated to the subject, but, like King Arthur of legend, the quest for the Holy Grail is unrequited, AFIK.
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Re: Calculation of AC power (Kwh)

Post by EA6LE-ONE »

Since I got the second version of emporia Vue that is calculating the wattage with the real voltage sensor, I got the power reading closer to the power generation of the radian. With the first version was only reading of the amps multiplied by fixed voltage to obtain the wattage. the power meters from power companies are probably the same: counting the amps and multiplying by a set voltage value. With the voltage fluctuating, being lower than the set voltage for calculation, I can see how you can get a lower reading on the OpticsRE than what the meter from power company reports.
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Re: Calculation of AC power (Kwh)

Post by Mike Curran »

FWIW, I noticed when I swapped my GVFXs for VFXRs, the power data increased noticeably. See this thread: viewtopic.php?f=15&t=15723&hilit=Gvfx As Ea6le mentioned (in the thread), it seems the FX models seriously understate power consumption, at least at nighttime.
Last edited by Mike Curran on Mon Jul 26, 2021 1:06 pm, edited 1 time in total.
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Re: Calculation of AC power (Kwh)

Post by provo »

raysun wrote: Mon Jul 26, 2021 10:21 am
(True) AC Power is measured in Watts ....

Both RE and the OP are measuring apparent power in Volt-Amps.
That's what confuses me, though. If I understand correctly, apparent power is always greater than true power. But the Optics-reported power is always LESS than the PGE-reported power.
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Re: Calculation of AC power (Kwh)

Post by raysun »

provo wrote: Mon Jul 26, 2021 1:00 pm
raysun wrote: Mon Jul 26, 2021 10:21 am
(True) AC Power is measured in Watts ....

Both RE and the OP are measuring apparent power in Volt-Amps.
That's what confuses me, though. If I understand correctly, apparent power is always greater than true power. But the Optics-reported power is always LESS than the PGE-reported power.
Hmmm... PGE, a private, investor owned utility with serious revenue and expense problems. Go look at the meter and see if there's a thumb on the scale. 😂

Seriously, what is being compared between the PGE meter and OpticsRE? Total meter consumption (from the bill) v.s. AC IN (From Grid)?
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Re: Calculation of AC power (Kwh)

Post by provo »

raysun wrote: Mon Jul 26, 2021 1:04 pm
Hmmm... PGE, a private, investor owned utility with serious revenue and expense problems. Go look at the meter and see if there's a thumb on the scale. 😂

Seriously, what is being compared between the PGE meter and OpticsRE? Total meter consumption (from the bill) v.s. AC IN?
Believe me, I've considered the straightforward THEFT explanation :eek: ....

The PGE value comes from a daily spreadsheet (kWh's for each of the 24 hours) downloaded from the PGE website (which agrees to the penny with their monthly bill.)

The Optics value is the "From Grid" total for the day.
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Re: Calculation of AC power (Kwh)

Post by raysun »

I've gotta confess my grid experience is zero, so others need to step in.

I'd wonder if there is a metering issue of some sort?

With a single VFXR, 120V is being fed to the house circuits, correct? Is the grid feed 240V? The inverter being fed off one leg (L1)? Where does the other leg (L2) go?
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Re: Calculation of AC power (Kwh)

Post by provo »

raysun wrote: Mon Jul 26, 2021 1:28 pm
With a single VFXR, 120V is being fed to the house circuits, correct? Is the grid feed 240V? The inverter being fed off one leg (L1)? Where does the other leg (L2) go?
240VAC coming into a 2-pole bkr. One leg keeps going into the house and the Outback system, the other leg just stops at the bkr. PGE assured me that wouldn't be any different than a house that had two legs hooked up, but where the usage was typically unbalanced. I don't know, maybe that has something to do with it.
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Re: Calculation of AC power (Kwh)

Post by raysun »

provo wrote: Mon Jul 26, 2021 2:21 pm
raysun wrote: Mon Jul 26, 2021 1:28 pm
With a single VFXR, 120V is being fed to the house circuits, correct? Is the grid feed 240V? The inverter being fed off one leg (L1)? Where does the other leg (L2) go?
240VAC coming into a 2-pole bkr. One leg keeps going into the house and the Outback system, the other leg just stops at the bkr. PGE assured me that wouldn't be any different than a house that had two legs hooked up, but where the usage was typically unbalanced. I don't know, maybe that has something to do with it.
If they are billing V * I, I wonder what the value of V would be?
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My RE system: Outback - Garage roof:
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- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied VFXR3524A-01's series stacked, replaced GVFX's (2020)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS. Tigo ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input from 14 Talesun 275W, Enphase M215 microinverters

Battery systems operate in grid-tied, battery backup mode
All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Calculation of AC power (Kwh)

Post by Mike Curran »

@provo - how far is your meter from your house? PG&E is charging you for any losses between house and meter, whereas Optics is only measuring what enters your inverter. If it's a considerable distance, and you have any high current loads, that could explain the 15% difference.
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provo
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(4 strings, total 1920W)
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One VFXR3524A
Hub 10
Mate3s
FNDC
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Honda EU3000is generator
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Re: Calculation of AC power (Kwh)

Post by provo »

Mike Curran wrote: Mon Jul 26, 2021 2:50 pm @provo - how far is your meter from your house? PG&E is charging you for any losses between house and meter, whereas Optics is only measuring what enters your inverter. If it's a considerable distance, and you have any high current loads, that could explain the 15% difference.
It's on the outside basement wall of the house . The service entrance is actually 240VAC at 100A, so the 2-pole bkr is 100A. There's also a 50A single pole bkr in the entrance box (my one "load") that carries the entire house current ~15 ft to the Outback system in the basement.
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Re: Calculation of AC power (Kwh)

Post by sbrownian »

Depending on the configuration of the meter, there are usually 2 or 2 and a half of what are referred to as 'elements' that monitor currents and voltages of all the 'legs' in the meter, both L1 and L2 plus neutral. (Neutral being the 'half' element if I remember correctly.)

The meter has the abilty to compare phase angles of the current and voltage to come up with a power factor that depending on the configuration, will be used to calculate actual KwH.

(BTW, Residential metering is usually handled differently.)

Industrial / larger power users usually have to pay for VAR useage, especially when three phase is involved..

While it may be 'watt-less' power, the generation and distribution system still has to support the volt ampere reactive currents, which shows up as I sqrd R heating in transformers, lines, generators, etc.

If only one leg / neutral is used, it still should meter correctly.

(Dang, guess I've got to dig out my intro to residential metering stuff and see how much I've forgotten..)

The calibration sheets for the interchange metering, all have entries for comparison at various phase angles, to make sure the metering is both accurate in KwH, as well as KVAR.

Every year, the two $25,000 calibration devices we use, (known in the shop as a 'Radian',) get sent off to a lab for traceable certification before the yearly meter cycle starts again.

We also have a portable rubidium based atomic clock, that with about a half hour of sync time to the GPS network, can be carried out in the field to check phasing in the system.

They guarantee it will maintain acceptable accuracy for about 8 hours after syncing.
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