I have asked this same question here before but have never been able to come up with a good answer.  Five years later I am trying again.

In 2011, I put 5.1 kW of solar on my south facing roof with an 8 pitch - close to ideal in my Mid-Atlantic location.  It makes me about 6500-6700 kWh per year if you look at the installer's website and the readout on the side of the inverter.  I receive SRECS for this amount of production.

I have a net-zero meter - runs in reverse to put excess production back into the grid. 

Here's the problem - My system website tells me how much power I have used and how much power I have made.  I then do the math to see what my kWh usage will be when the bill arrives.  But when the bill comes, it is consistently 150-200 kWh HIGHER than what my system tells me it should be.  It says I am making LESS than the system tells me.

The power company has been here and has no ideas.  The installer came and installed a couple of additional ammeters in case there was a line or two that was not captured, but other than they they shrug their shoulders.

Anybody have any ideas why there could be this consistent disparity?  I get SRECS for the higher number, but utility credit for the lower number

Thanks for any ideas.

Ed

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Hi Ed, one possibility is a calibration issue with the inverter meter, but that seems highly unlikely with such a large discrepancy. I've had several clients describe the exact same problem and it always turns out to be a misunderstanding as to how net metering works or the way the utility presents the data on the bill.

Keep in mind that the utility (net meter) doesn't know how much power the array produces, nor does it know your home's total consumption (more on that in a minute). The utility bill credits you only for power you send back to the grid. In a true net-meter scenario, that value is necessarily less than what the array produces. What's missing are the kWh's you consume in real time.

In my experience, households with an array sized close-to-net-zero typically consume between 25% and 35% of the array's output in real time (higher for arrays sized closer to base load). The discrepancy you describe appears within that range. Assuming that explains what you're seeing, it's not surprising that a utility rep didn't understand how this works. But your solar dealer should have been able to explain this!

Even though the amount of solar generation consumed in real time isn't tracked by either system, you can easily calculate this amount for a given period by subtracting the power you sent to the grid (net meter 'received' channel) from the array output (inverter readout).

Likewise, neither system tracks total consumption, but you can calculate this amount by adding the power you pull from the grid (net meter 'delivered' channel) to the array output (inverter readout) and then subtract the power you send to the grid (net meter 'received' channel). Some inverters offer an optional consumption monitor (requires installing a CT on the mains).

BTW, my understanding is that SREC's require a revenue-grade meter. Is your inverter readout considered revenue grade?

David

Thanks for the thoughtful answer.  Yes, I have a revenue grade meeter.

I think I have taken out of the equation the factors you mention above.  My electric meter tells me how much I have used net.  The array website tells me two things - my actual usage, and the amount the array has produced.  A simple subtraction should equal what the meter says.

I understand that it will never match exactly and I would have no problems with a dozen or two kWh difference, but to be consistently off by almost 200 kWh ($25 per month at my 12.5¢ rate, $300 a year) seems a little high to me.

Ed

Indeed, I'm not surprised you're already up to speed on how this works. But the size of your discrepancy instilled a wee bit of doubt ;-)

You wrote: "The array website tells me two things - my actual usage, and the amount the array has produced. A simple subtraction should equal what the meter says."

Ok, so your solar dealer must have installed a pair of current transducers (CT) on your mains that monitor actual usage, right?  Since you're using that reading in your validation calculation, and since the other data points are from revenue grade meters, the most likely suspect is clearly the actual usage as reported on the website. 

CT-based power monitoring introduces several potential sources for errors. The CT's themselves are typically accurate within a few percent but an accurate voltage reference measurement (RMS) is also necessary, as well as power factor (phase angle). But even a sloppily implemented CT power monitor shouldn't be that far off.

Other possibilities: the power monitor uses a single CT and doubles the reading, instead of measuring current on both legs. This misses imbalances between the loads on the two 120V legs. OR, the CT's are installed at the wrong location. This can happen if you have another sub-panel or branch circuit upstream from the CT's. The whole-house CT's should be installed upstream (typically above) the main breaker at the utility meter.

There's an easy way to sort this out. Instead of using the website-reported 'actual usage', calculate actual usage based on readings from the revenue grade solar meter and the utility's net meter (see next to last paragraph in my previous comment for the formula). Take the readings yourself at midnight at the beginning and end of a one-day or multi-day test interval. You can do this retroactively at monthly billing intervals, but keep in mind that the utility's readings were probably not done at midnight, or even the same time of day, which introduces small (although offsetting) errors.

If I had to guess the inverter is telling you the how much DC you are making with the PV. The inverter is probably eating those kWh converting it to AC. 

Does the utility program calculate / make provision for 'line loss'. You would have to find out if this is the case from one of the utilities programmers. Most, other than the programmers, will not know if this is the case. 

Might be a stretch.

Michael

Since the utility already checked the net meter, it sounds as if it's the inverter itself is either having a problem or its not communicating correctly with your monitoring system. See if you can get data directly from the inverter; ask your installer how to go about this or the inverter manufacturers' tech support.  Or, perhaps you need an upgrade for your monitoring system, which again your installer can help you arrange to download. 

Just a few ideas, and hope this helps. Let us know if resolved.

Frustrating! Can't help you there but I have a question. Do you use any other energy? gas or oil? If not, we can get you the zero energy hero award! 

I have run into this twice. Both times it was the way the utility calculated and billed for electrical usage. They just made it confusing. Hidden charges built into the payout calculator. 

otherwise,

I was thinking; line loss, inverter/controller/sine wave stabilizer/conditioner loss.

Does the equipment feel hot? Heat is loss of energy/efficiency. How long is the run between the inverter and power meter, what gauge wire? Kinda feel this is a stretch-tho for that amount of loss. But, look into the inverter. It might just not be as efficient as others. 

Have you activly measured the power? I would use several ammeters to confirm the power coming in and out of each line and the equipment at the same time. Calibrate them with a battery to confirm they read close to each other. 

@Daniel, Ed said he has a revenue grade SREC meter, which presumably is the one he's using for  his calculations (right, Ed?).  A revenue grade meter is necessarily on the output side of the inverter, so it would already reflect the DC wiring and inverter related losses.

I would begin by asking your power company for the hourly numbers for the billing period. Depending on the metering system they should be able to give you the hourly kWh you purchase from them and the hourly kWh you sell to them. What they will not have is the hourly kWh you are consuming from your array. Compare their numbers to your numbers. I know the utility meter is required to have not more than 2% error. You may wish to ask for a meter accuracy test or a meter change. My experience is that this is a waste of time because the utility meters are exceptionally accurate. Another factor is knowing what time the meter is read. If you are assuming the time you may be missing some hours that you are purchasing kWh from the utility (overnight). Some other factors could be rounding errors. My best guess is that the error is in how your system website is calculating the power you are using.

Thanks all - I have been out of town with grandkids for the last 7 days (loads of fun but boy do I need a nap).  I appreciate all of the answers and I am really looking for an answer, so let me go over it again and try to fit your comments in

My system was installed by one of the largest installers on the east coast that does mostly commercial.  They have many 10+ acre systems under their belt.  One of my best friend's in town is a policy wonk for them and he wrote the MD community solar bill for the state.  Technically, I have the A-Team working for me.  I have 2 400 A breaker boxes, and each has a CT.  The installer sent a crew back to check everything and they installed 2 or 3 additional CT's - which actually sounded like a duplication.  I have about 9-11 feet of wire between the inverter and the outdoor meter with the breaker boxes mounted between.  

I have an inverter that sends information back to a central location and I get that information in any form I want - and the SREC are calculated from this information.  I get 6 or 7 a year ($4.64 for the last one).  Their website can tell me usage or generation by year, month, week, day, hour, or any combination of days.  The 6,500 or so kWh it says I am producing per year is about right for my 5.1 kW system in central MD.   When I do the math of subtracting generation from usage for the dates covered by the utility meter, I get a number.

The utility at first had just a straight digital meter that read out the usage.  About March of this year they installed a new meter that allows them to tell me the total usage, and also the amount they bought back from me - in other words how much the meter has gone forward and how much it has gone backward.  The end result of this new meter - the reading - will be the same as on the old meter.  This is the number I am comparing my inverter numbers.

When I compare the utility bill to what the the website says - here is where I get the discrepancy. The real head scratcher is that I am being charged for one amount, but being given credit for another higher amount.

Mick - if the inverter were eating power, it would be happening to everyone with the same inverter

Michael - there is less than average line length, so less than average line loss, and this would be a tiny fraction of the totals

Nancy - I have confirmed the website numbers with the readout on the inverter.  In addition, the total output on an annual basis is about what I expect it to be.

Brett - the house is all electric, LEED Platinum without certification, Energy Star with all ancillary programs without the actual certifications, HERS 24 with solar, NGBS without certification - point being I am going to live in this house long enough that certifications won't help me increase it's value - I have the bills which are what sells efficiency in my experience.  Thanks for the offer.

Daniel - I think this is all covered

Bruce - the error is always off in the same direction and time of day errors won't matter over the long run

And finally - David - I have not managed to get the one-day meter readings as I have been out of town.  I will try for that soon.

Thanks to you all and keep the ideas coming

Ed Minch

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