Sense vs Solar app: kWh differences

Thanks @ixu.

Contacted support a few days ago and they made adjustments/reset something and now my items are back as expected.

Cheers

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My enphase system reports 100 to 350 more watt hours than sense. Being as most of us with enphase show it reporting more power produced than what sense sees right at the tap, it would be losses in the wires going from solar array to main panel. I used 10 gauge wire from my solar string to my main panel, Enphase cable only uses 14 gauge, but if installed properly you center tap, so that 14awg is okay due to super short distance. However if you only use 14 or 12 gauge from arrays to panel you will have loss in power, 10 gauge eliminates most of the losses. Although it still goes through solar string breakers, a main external mounted shut off switch, plus inverters use some power all would explain the small discrepancy and losses.

However if you are seeing a full 1k difference i would check the gauge on your wires coming from your solar array strings to the main panel, 14 is not enough unless you only 15 to 20 feet away, 12 is probably okay if within 35 feet, but 10awg gives you lots of wiggle room and can easily go over 100 feet without much loss.

I can not explain sense seeing more power than enphase reports, that extra power is coming from somewhere in those setups. But the reverse with sense seeing less than the enphase inverters is something i would totally expect. But if the difference is large, there is something in ones setup causing the losses.

I was told by Sense support that the difference in the numbers was how Sense calculated watts from the volts the solar CTs obtained. They said a variance of 2-3% was in spec. Mine is closer to 4%. For the month of October the difference was 50 kWh (Sense was lower) for solar and 1 kWh for consumption.

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They would know a lot better than me on the Sense. I am new to Sense, only about 10 days in now. However I just checked the past week and Sense has 150kWh and enlighten/enphase shows 151kWh for the week. So I can extrapolate that it will be about 4kWh a month less from Sense vs Enphase, but really there is not enough data yet to say with any kind of accuracy.

50 kWh, seems like a lot for a single month to be off. What size system do you have. I have 39 panels and I have the older 250 inverters. So that is about a 9,750 AC watts max, but I never get that, only have 260 watt solar panels, once in a blue moon they peak at 250 watts AC. Unless you have a really Large array, 50kWh seems like a lot to be off. I would certainly look into wire sizes and possible losses going from the solar panels to the mains. I forgot if you said you had solar edge or enphase, either way wire distance and size does matter, especially at lower voltages like 120 or 240. This is a bit off topic for Sense, but we are all power geeks so probably okay.

The 1kWh delta consumption on mains is awesome and very tight. I now gotta go read my power meter and see what it shows after a month vs sense. And perfect, it is Dec 1. I would love to hear back on wirte guage, distance of run, size of your solar array and type, etc. If the sense community is okay with the discussion… I will gladly provide my personal email to you, but I did not read the rules close enough to know if that is okay.

I have a 11.7 kW array that was officially activated on September 26, 2019. It’s made up of 36 Panasonic 325 panels with Enphase IQ7X inverters. So not too big.

I believe the wire gauge is 8 and it’s maybe 6 feet from the Enphase Combiner to the panel. My solar is tied in with a line side tap.

Here are my numbers from November.

Enphase Production - 1078.7
Sense Production - 1039.4

Enphase Consumption - 872.2
Sense Consumption - 872.0

Here is a photo of the tap and CTs.

Very Nice Setup, they did not make the 325watt panels when I did mine, I am a little jealous. In a good way, that rocks what you got going there though. As for the 8awg between the solar panel and main power panel, that is fine, I meant the line coming from your roof for each solar array string. My electrician tried using 12awg and told me 14awg was perfectly fine, I said no I want 10awg and I will be for the extra costs. Sure going from 12 to 10 only saved about 2-3% losses, but to me it was worth it, 14awg was out of the question.

Also for each array, per enphase recommendation, you should center tap the array, tapping it at the end of a 11 panel run will cause losses, but if you tap in the middle, then you minimize those losses on the enphase cable. Probably pretty minimal, but still it does matter if you are looking for near zero losses.

So the next gauge wire that has the most importance because it is usually the longest run is the wire going from each solar array string, down to the Joiner… My local codes required me to have a whole separate breaker box, to join the strings, as well as an single external solar panel kill switch. 8awg was great for your joiner to your mains, I see no losses there, besides it is such a short run. Oh by the way my local code also required yet another breaker fuse to go into the mains. They were way over kill, so I have to send each string through 3 breakers and a big kill switch, all generating losses. each array has a breaker, then one big kill switch outside house with the joined strings, then one breaker after string joins into main panel, then of course the whole house main 100amp breaker. I am pleased to see they let you just tap the mains, that makes so much more sense with an enphase system.

You can go online and bring up a calculator for wire losses, that is how I decided on 10awg for each string.

So three questions…

  1. Is your 3 solar array strings center tapped (if not there will be losses from the voltage rise there)
  2. what gauge wire from each solar array to your joiner?
  3. what is the length of the run from your solar array to your joiner?

The length of the run will most likely be more than you think, because it is mostly likely weaving down walls and around bends, but it all depends on your setup, it might be a really short run, mine are about 50feet.

All 3 of the arrays terminate in a Enphase IQ Combiner box with each having their own breaker. The Enphase Production CTs are in the Combiner with the IQ Envoy. The output, with it’s own breaker, feeds a disconnect box and from there to the main panel.

Responses to your questions.

  1. Yes, center tapped
  2. 10 gauge hot lines and 8 gauge for ground. Ground is 8 throughout the system.
  3. There are 3 soladecks on the roof and they are each connected via conduit in the attic. They are in a line from west to east. The east is the furthest and it’s maybe 70 feet away tops.
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For the month of October (I can’t use November because I had a full day where my Sense was offline) Sense says I produced 760 kWh and the SolarEdge dashboard says I produced 784 kWh. That’s about a 3% difference.

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My October numbers

Enphase Consumption 1300.90
Enphase Production 1373.36

Sense Consumption 1299.30
Sense Production 1323.20

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Ken2, That sounds like a really nice install and setup. Max output of 320VA for you IQ7X and 11 panel strings, that is peaking at 3,520VA, so I divided that by the 240 volts to get 14.67 amps. Now since there are two legs of 120, each carrying max 14.67 amps. I ran an online voltage loss calculator on 120 volts, 70 feet, 14.7 amps and it is less than 2%, which is idea. So bottom line is your system install sounds idea and should not be be an issue at all.

Ken2, I am not a professional electrician or anything, so take what I say with a grain of salt. However with your new November numbers and your setup, it is close to 2% difference. Being as you have a ~1.7% voltage drop/loss from your solar power station to the mains, and your new November numbers are showing ~3.7% difference, that would lower the difference based on install losses to only 2% delta.

I will be curious to see if the delta changes in the heat of the summer, when the longer 70 foot run of wires are warmer and have more resistance. Also note if you underestimated your length, and really it is 100feet, based on all the turns and twists it took to go through walls and following floor joists, the voltage drop is more like 2.45% instead of 1.7%, which would skew the results. I was blown away with the lengthy of my run, I would have guessed it to only be 30 feet but it was double that when I actually measured it to calculate voltage drop.

Billparn, thanks for the feedback. I’m very happy with my system so far. I actually spec’d all the components I wanted myself. I had a great installer.

The 70 foot run is pretty close. They went in the attic with a 100 foot coil of conduit and came out with extra.

My understanding is the Enphase production (solar) numbers are measured at the CTs in the Combiner box that is 6-8 feet away from the panel where the Sense solar CTs are installed. I’ll double check that, I could be wrong…

UPDATE - My assumption was incorrect. The Enphase production numbers are coming from the micro inverters. So there is a loss from the roof to the tap in the box. Bill, thanks for the insight, I learned something new.

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Your welcome Ken, i love trying to figure this stuff out. So i think i may have a better explanation of full loss now.

Since voltage drop calc show a 2.05 volt drop at peak 14.67 amps per leg. That is ~30watt/hours per leg per string. 2.05 volts x 14.67amps = 30.0735 watts.

So multiple that by 6, because each of the 3 strings has two 120 lines. And we get 180.441 watts per hour loss at peak

Assume 10 hours peak per day, for simplicity.

10 hours x 180.441/hr = 1,804.41 watts per day loss
30 days x 1,804.41 watts = 54,132.3 watts per month loss.
54,132.3 watts / 1,000 = ~54kwh

Which is amazingly almost exactly the difference you are seeing. Weather of course reduces that number, but still thought it was pretty neat to almost perfectly work out.

So if i am correct and volt drop calc i am using is accurate. Going to 8awg on the long runs would reduce monthly loss to ~34kwh (1.29 voltage drop) and going to a whopping 6awg (.81 voltage drop) would reduce monthly loss to ~21kwh per month.

But is it worth it to change?

My power runs a little over 12cents a kwh times 50kwh loss = $6 month loss, $72/yr. So going to 6awg, would reduce loss to $2.52 per month or $30/yr. so it would only safe $42 per year, look over 20 year life span and that is only $840 savings …

So in my opinion, No not worth changing nor even doing from beginning, i would keep the 10awg for sure. :slight_smile:

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