Difference In Daily Total Usage

Thanks Kevin! I’m glad to hear that I have things installed correctly. That took me a while to get it done right in the beginning.

For solar monitoring I am using Enphase.

That’s a good catch on the offset of the time intervals! I moved the Sense measurements down one time slot and things appear to match a little bit better. However, as you can see some days things are almost the same, other days my utility is higher, and sometimes Sense is a little bit higher.

As you can see from the XY chart they follow the same trend, but sometimes a little different.

You can also see the % difference between the two on each given day. I would expect a +/- 1 to 2 % difference either way, but not more than that.

Your thoughts?

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Better in correlation than without the hour offset, but still not the max 1-2% difference you should be seeing.

3 thoughts:

  1. Try this form of an x-y plot, just comparing the two usage values at the same time. This type of x-y plot is going to be useful when you are comparing hundreds or thousands of readings and looking for patterns. Your comparison chart should look the one below once you get everything figured out.

  1. double check the time base that your utility uses, to understand where this offset is coming from. I have seen some cases where the utility actually does 15min usage updates and the way the intervals are defined can lead to a 45min offset with respect to Sense. The error, even after the time shift, could still be consistent with a time base offset issue, though if you don’t get the 15 min data from your utility, there might not be any further you can go along these lines.

  2. try to also figure out whether there is any way to tell when your utility is doing estimated reads of your smart meter. Together with other users, we have figured out how PG&E and NV Energy identify estimated reads in their exported data. Your time chart and associated error are consistent with estimated reads mixed in with actual reads like we saw in the NV Energy case. Example of a time comparison waveform showing many NV Energy estimated reads below - red is Sense, blue is utility reading. Dots are colored based on how many actual (vs. estimated) reads are done in an hour (4 reads of the meter per hour).

ps: Seems like you have very small solar component. In most cases with solar, I see several hours per day with negative net usage on Sense and utility.

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That’s great news… Gonna keep linking back to your original posting to help @Ray13, so you might get cc:ed a few more times.

Here is my x-y plot showing the comparison between my utility and Sense.

I actually just talked to my utility and they confirmed that the readings are done every hour. The number that shows next to 1:00pm is the amount used since the last reading at 12:00pm. If there is no reading at a given hour for communications reasons, then the amount would be cumulative for the previous two hours. For example if 12:00pm didn’t get read, then the 1:00pm reading would go back to 11:00am assuming that was the last good reading.

I’m not sure what you mean by a “small solar component”? I do know that at night when solar isn’t producing, I get a -21w value. During the day though, it is showing a positive number even if it is cloudy.

OK - that explanation from your utility highlights the offset. For Sense, the 00:00:00 (midnight+) data point is the usage for the time between 00:00:00 and 01:00:00. You should also note that that will affect your daily calculations.

That explanation also dispels the two possible correlation error sources I mentioned, though I’m still suspicious that your utility might be hiding a 15min read cycle under their hourly numbers.

The next thing I would do is plot a bunch more hours with the corrected offset using the x-y plot you just constructed and see if there is any correlation between the size of the “error” and other factors like time of day (color the dots for time of day like in the plot below). Multiple days might also highlight some larger outliers that could provide the key.

When I say smaller solar component, I mean size of solar system. My system is large enough that even in the winter, my net usage from my utility and Sense (Total Usage plus Solar Production) goes negative.

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To: @Ray13

Here is an analysis I did several years ago. Browse the whole thread. You might gain some insight from other posts.

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Thanks Kevin, I’ll go back and apply the offset now that I know that is what it is doing.

My solar never goes negative, only at night when it isn’t producing. During the day when it is producing, I can see how many Watts are going to the house and if there is anything extra, is going back to the grid. If the panels aren’t producing as much as I need, then it shows how much I am pulling from the grid in addition to what my house is using from solar.

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Your results for many, many hours should be interesting…

My discussion on solar and negative readings really refers to the net energy, which is typically what your utility meter shows when one has solar. The meter and export will show positive energy when you have consumed more from the grid, than put out to grid from solar over the hour (net usage from the grid was positive). Conversely, the utility net reading will be negative when you have put out more energy to grid from solar, over an hour than consumed from grid (net usage from the grid was negative). My solar array is big enough that for some hours in the middle of the day, my utility hourly data goes negative (Have put out more energy to the grid than taken from it). But you might not have the same kind of net meter.

It would be interesting to hear how your utility deals with solar - that might be another piece of solving the mystery.

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My utility tracks usage and generation (the amount I send back to the grid). On my bill they subtract the generation from my total usage to come up with a new total usage and then bill me on that.

My house will will sometimes use directly from the solar as it is being generated. What it doesn’t use will get sent to the grid.

This typically happens in the middle of the day.

I am working on putting together a spreadsheet that shows what my utility says I used on a given day, and what my solar app says I imported, produced, consumed, exported, and net imported.

We’ll see what trends come from that.

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Thanks, this screen offers a little more clarity. It looks like the number you are charting for all three data sources is “consumed” electricity (Total Consumption in Sense-speak), not “net usage”, because your “net usage” would have gone negative on a day like this.

A couple notes:

  1. Your metering setup is different than mine if your utility can actually calculate your “consumed” electricity - my meter only reads the “to grid” and “from grid” data and rolls it all up into a net reading every 15 minutes.

  2. Counting the number of bars in this graph, it looks like your power company and your inverter produce reads for 15 min intervals even if you only see exported hourly data.

No biggie. Does he live in Vegas too?

Different location - utility is CMP.

CMP or Central Maine Power which has recently been rated the worst utility in the US! We are fighting them up here now because they are trying to force a new high power line from Quebec, Canada to Southern Maine to benefit MA. We don’t want it because it’s going to destroy our wilderness and be of no benefit to Mainers.

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So I am looking at my solar software (Enphase) right now and comparing it to my Sense software. Enphase is showing me 12.9 KWH produced so far today and Sense is showing me 9.8 KWH produced. Why the discrepancy?

Remember the possible 1 hour offset difference. Sense is also typically a little slower to update new aggregated hours. Enphase might update every 15 minutes. So look closely at how much time each total represents…

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I think that is the problem. Right now Enphase is showing 15.9 and Sense is showing 15.2. Sense is “catching up” so it most likely is just due to a time difference in the measurements.

One other note - Sense Solar users generally report seeing Sense solar numbers that are 3-5% less than the inverter reports. There are mixture of theories on why, but I have found that the Sense solar number when used with Sense Total Usage, matches the net usage from my utility better than the inverter data does.

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That’s better! When I use the Net from Sense and the Net from Enphase, my % difference is between 1%-3%. I can live with that.

The difference between Enphase and my utility Net is -3%-7%. Not as good and a couple days I need to look into. This is the reason I’m doing this whole exercise.

The difference between Sense and my utility ranges from -4%-6%. This seems about the same range as Enphase and my utility.

It appears that Enphase and Sense are consistent in that they both have the same range difference, which makes me believe my utility isn’t always right. The fact that Enphase and Sense have a much smaller margin difference when compared with each other, tells me that may be true!

That’s good news. One thing I’m not clear on for your setup is the data sources.

Utility - Usually, a user with solar, will have a single net meter that only provides net readings, to grid readings and from grid readings - nothing that actually reads the solar production, though I have seen one case where the utility did a double metering. Does your utility provide net usage and solar production separately ?? If so, where does it get the solar production ? Another meter or from Enphase ?

Enphase - usually a solar inverter has no knowledge of the house usage or utility net usage. Where does Enphase get the house usage from ??

I ask these questions because mixing and doing arithmetic on data from difference sources can either exacerbate differences or cancel differences depending on the biases of each source.

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@chrisseivert24 I live in Maine.