Small discrepancy between utility meter and PVOutput values

Questions
#1

I just went outside to check the values on my utility’s meter and compare those with the values I see on PVOutput.

The values of my meter, since the last reading, are the following: 166 kWh export, 164 kWh import, 3 kWh net (probably a rounding factor)

The values on PVOutput for the same period (using customized monthly view, using Nov 6 as start date since that was the day my meter was read last) showed 166 kWh export, 156 kWh import, 10 kWh net.

It seems that over the past 16 days there is a small discrepancy of about 7-8 kWh between what my utility says I imported, and what PVOutput calculated for import based on my generation and consumption, but I am not entirely sure why this is.

It seems to me that if the export is the same at both utility meter and on PVOutput for the same period, shouldn’t the import then also be the same, seeing as the export is calculated based on my generation and consumption?

It’s not a biggie, but I am trying to understand it nonetheless :slight_smile:

Does anyone have any insight in this?

#2

My data seems to follow the same conclusion.
the meter import reads higher and export reads lower vs my pvoutput
im guessing mine is losses, difference in accuracy of meters and the fact their meters probably are sided towards them.

#3

Retailer smart meters comply with an accuracy standard that is pretty good. Off the shelf systems may or may not be as good. Accuracy isn’t usually as good at low loads.

It’s not just meters. My Fronius Solarweb and PVO report different consumption values when supplied data from the same Fronius smart meter*. Over a month PVO is usually under by 2-3kWh. I put that down to the missing data in PVO for the final interval of the day.

It may also be data gaps.

  • BTW - there is excellent consistency between my retail smart meter and the Fronius smart meter.
#4

Thanks both :slight_smile:

One of the things I was wondering as well was a possible data gap. But I kind of dismissed it because when we look at a date range on PVOutput, it starts a day at midnight. The utility meter reader comes by sometime during the day, so if there is a data gap caused by PVOutput’s midnight and the actual time the meter guy comes by, then I am inclined to believe the gap caused should result in a higher output shown on PVOutput’s side instead of the utility’s meter).

I also considered a simple loss, the distance between my utility meter and the CT clamps where I measure my consumption is roughly 20 feet. It would mean I’d lose about 0.5 kWh a day. I have no idea if that’s a reasonable loss over that distance, but if it is, it would explain the discrepancy.

Meter accuracy is also indeed something to wonder about and I agree that the utility’s smart meter should comply with standards and should be calibrated correctly. It could be caused by my consumption meter not measuring correctly, but when I look at the exported value, which is the same on both PVOutput and my utility meter, I believe it does a decent job since the export is calculated between what I generate and consume during generation.

#5

You have someone come and do a meter read? Once a human is involved in data recording, then you have another potential source of error.

Before our retailer smart meters were installed, my meter reader made an error (we had 4 meters, 3 phase + controlled load) and it resulted in a quarterly bill error of ~$400. As I was fairly new to this house, I had no way of knowing if the bill was reasonable or not (as by the time the bill arrived some 4-5 weeks later the meter reading was already beyond the meter read value). Tip - if on old meters, then take your own photos of meters on/around the day of the expected meter read.

When the next quarter came by, the bill was significantly less than expected. I put it down to seasonal variations, use of heating/cooling variances. It wasn’t until another two years went by that I realised it was actually a meter read error as those two quarters didn’t follow the seasonal consumption patterns at all.

#6

I have 3 meters at my house. One for each of the PV systems (2) for production and a net meter. My electric provider gives me a monthly report of these meters on my bill. I have been using CTs to record total house usage as well as the PV production and this uploads to PVoutput. I also keep an excel spreadsheet to calculate net power usage and I read the Net meter daily. After doing the calculations vs Net meter readings, they have correlated well +/- 1-2 kWh. After reconciling the CTs monthly numbers with my monthly electric bill of kWh used based on these 3 meters, I would say that the utility meters are within 1-2 % of the CTs on a monthly basis.

#7

I calculated my last quarterly bill using my Solarweb data and it was just 34c different compared with the actual retailer’s bill. I’d say my Fronius 3-phase meter aligns pretty well with the retailer’s 3-phase meter.

#8

Wow! Assuming the 34c is a small portion of your quarterly total, the Fronius meter could be considered a revenue grade meter!

#9

This is very interesting! My Powerwall 2 was installed in July and my old mechanical meter generally recorded Zero peak power demand per day, except, rarely, when the battery ran out of charge. The Powerwall usually supplies all the Peak demand. I have a dual Peak and night rate tariff.

I had a new Smart meter installed on 30th October, with a remote display showing current power consumption to 10W resolution, and immediately noticed that there was a fairly constant demand of 60-100W. That doesn’t sound very much, but it’s 1.5kWh per day, over 500kWh Peak tariff per year = >£150 - 20% of my electricity bill!

We have done fairly extensive testing (by turning everything off in sequence) and it appears that the Smart Meter is recording accurately and that the Powerwall itself is responsible for the demand. I am in discussion with Tesla Energy at present. They told me that the Spec was for a maximum daily demand of 500Wh so mine is way out of spec. I’ve just sent the requested 7 days of meter readings to them and am waiting with baited breath for the response!

One thing occurs to me - PVOutput grid flow shows continuous very small demands and exports, usually <10W. I understand that this is the Gateway calibrating things. Tesla also told me that the inverter and internal systems sometimes draws power from the grid. Why it can’t use it’s own power I’m not sure since it will operate off grid with no problem. However, the Smart Meter only records Import. It can theoretically record Export as well but that hasn’t been activated on my meter. In any case, it would be impossible to separate out the Solar export, which we get paid for, (assumed 50%) and Powerwall checking export, which we wouldn’t get paid for. So the meter may be recording the import side of the Gateway balancing algorithm stuff and not the return export.

Maybe the old meter was not sufficiently sensitive to record very low power demand although it should surely have recorded a couple of light bulb’s worth of power?

#10

This was last quarter’s comparison:

METER COMPARISON Fronius Retailer Difference (%)
GRID IMPORT kWh 962.6 967.7 +5.1 (+0.5%)
CONTROLLED LOAD kWh 506.4^ 499.6 - 6.8 (-1.3%)
GRID EXPORT kWh 2,532.1 2,529.8 - 2.3 (-0.1%)
DAYS 92 92

^ Not Fronius metering, calculated by other means

So my Fronius (3-phase) meter was within 0.5% for imports and 0.1% for exports when compared with the retailer’s smart meter. I’m pretty happy with that.

#11

whats the best way to adjust the data. my generation is correct so i need to increase my consumption on PVO to get a better reading comparable with the meter reading.

#12

My TED5000 system allows me to adjust the kWh (Use or generated) individually to the 0.01 (1%) level for each CT. I used a spread sheet looking at daily net meter and individual PV readings comparing them to the individual CT readings on TED5000 and adjusted the kWh calibration after 1 month of readings. Perhaps your upload program has the ability to be calibrated. My original PV inverter (Fronius) was 2% too low, my new PV inverter (Solar Edge) was 10% too high and my consumption meters were 1% and 3% too low. After adjusting, my readings correlate to my monthly bill within 1%.

#13

[quote=“wattmatters, post:5, topic:2563, full:true”]
You have someone come and do a meter read? Once a human is involved in data recording, then you have another potential source of error.[/quote]
They used to with the old meter, unsure if they still do. The new one is a smart meter that sends data back to them all of the time. The data on their meter I’ve seen is the same as what we get billed from, so I am not too worried about that :slight_smile:

#14

This is a very interesting thing to read. I never considered this, but this could very well be the source of my discrepancy.

Here’s how my setup basically is:

solar

And there’s indeed always power on the end of the solar fuse box, where aside from the fuses and the generation CT’s also the Enphase Envoy is located. The Envoy always needs power, and like you mentioned, the inverters probably as well since they communicate whether they’re still online or not.

It would make perfect sense if that’s where the discrepancy is coming from.

Thinking about it, I guess the only way to truly solve this, is to put the consumption CT’s where the utility meter is located, so before it makes a split between the home and the PV installation. However, this is all maintained by the utility, and by our state’s law I’m not allowed to do anything with that, which is why I went with the main fuse box inside the home. There’s another box outside, installed by the PV installers. I’m not sure what is in there, but if the actual split is made in there, I could place the CTs there I guess.

edit: Hmm… I am saying this, but I just did a calculation with a full month of generating (the calculation above was after a few weeks), and the meter/bill shows a difference of 6 kWh import in the utility’s benefit, and a difference of 7 kWh export in their benefit. This means they metered (and billed, if I didn’t generate more than I use) 13 kWh net more than I did. It’s a discrepancy of roughly 2% during a full month.

Is it possible the Enhpase Envoy and/or inverters use solar, unmetered, when I am producing solar? If that is the case, then I guess that’s where the discrepancies are coming from. In that case I’d never fully be able to get rid of the discrepancies, at least not on the generation side but thankfully it’s only a small number :slight_smile:

#15

Here’s part of the reply I got from Tesla energy:

“…the system does not seem to record any import from the grid except the tolerated 0.4/0.5kW that basically is the amount of energy that the inside inverter uses to keep the modules DC to AC and AC to DC ready to be used.”

They also mentioned that the meter might be recording reactive power, something I had thought about and am currently investigating. My smart meter is capable of recording reactive power but domestic customers should not be charged for it. However, the Powerwall appears to have a very low power factor and if my meter has been incorrectly programmed it might explain why I’m being charged for 1.5kWh per day of peak rate energy that the old meter didn’t log. Fuller details in another thread. Here’s part of a typical peak period Powerwall api instant download for “site” demand (= what’s being taken from the grid- should be next to nothing if the Powerwall is supplying the house):

“instant_power”:7,“instant_reactive_power”:-177,“instant_apparent_power”:177.13836

The meter should be logging 7 watts, not the 80W on the meter remote instant readout, which doesn’t tie in to any of the above figures.

#16

In my continuing investigations into why my Smart Meter appears to be over-reading or logging reactive power, I have noticed that the “site” values are sometimes positive and sometimes negative. e.g.
instant_power":-8,“instant_reactive_power”:307
The “Load” values are always positive.

The api downloads “instant” readings. Does it depend on where in the AC “sine wave” it takes the reading, i.e. could it hit and return a value in the the negative part of the cycle? Or is it actually recording export when there is a negative value?

We do know that the Tesla Powerwall both imports and exports small amounts of power while supplying battery power to the house and maintaining the AC-DC inverters “'ticking over”.