Question about Solar Production and Metering

[pvoveris]

I live in the LA area and have LADWP as my electric provider. I have a solar system that generates pretty much 100% of my usage and LADWP puts the excess into a bank to be used when my production is lower.

Ever since the system was new, (just over a year now) I have been trying to reconcile the amount of power I produce to what the meter shows I use.

For instance, I have one meter that measures solar production, and a net meter that shows various pieces of usage based on time and source. The solar meter shows I have produced around 13 megawatts in the last year, and according to the net meter, my total usage and production equals about 9.5 megawatts for the same time period. I keep wondering what happened to the rest of the power the system generated.

I have tried to ask the utility company and either they don't understand what I am asking, or, I am asking a silly question that can't be answered in the way I think it does.

Does my question make sense, and is there an answer?

Many Thanks,
Peter

[captaindomon]

What you are missing probably is the amount your solar system produces that you consumed, without going either direction to the meter. Here's an example:

Solar production from inverter(s): 13 MW

Net meter says you sold them: 4 MW (to power company)
Net meter says you bought: 5.5 MW (from power company)
Total net meter throughput: 9.5 MW

Your house used: 5.5 MW (from power company)
Your house used 9 MW (from your panels directly).
Total usage: 14.5 MW

Does that make sense? Your meter only measures what is going through it either to or from the power company, but some power goes from your inverter(s) directly to your consumption.

Edit: To get the full picture, you need to install a consumption meter. I use the TED Pro myself and I love it.

[Alchete]

Hmm, captaindomon seems to know what he's talking about, but I thought it was more simple than that. Maybe it depends on how your system is wired.

My production for the 7 days I had solar panels last month was 138kWh.

My net meter (and bill) read -21kWh.

So, by my math, I used 138 - 21 = 117kWh from the town.

I believe my net meter is wired to be first in the path, so it's monitoring gross solar production and gross usage of town electricity and subtracting them. My panels only purpose is to spin this meter backwards, not feed my house -- as, if the town power goes out, my solar panels turn off as well, and I have no power. This is (at least) a state law in MA, to protect power company workers who might be working on downed power lines.

[solarix]

Your net meter only measures the energy pulled from the grid (lets call that X) and the energy pushed back into the grid (lets call that Y). It doesn't measure whatever solar power is directly used by the house (lets call that Z).
In your example, X minus Y = -21kWh. X + Z = total house consumption. Y + Z = total solar production = 138kWh. Your math of 138 - 21 does not equal energy used from town. Usually, the bill will show the X and Y numbers as well as the net amount - which will allow you to reconcile everything.
The solar system feeds both your house and the grid, but depends on the grid in order to run. When the grid goes down, it can't do either. But when it is running, the solar system feeds the house first and only the excess goes out to the grid through the revenue meter.

[sensij]

Very few PV systems in the US are designed right now to push exclusively into the grid, and not supply local loads as well. Part of SDG&E's proposal for the Net Metering successor in CA would actually have this as an option, but because it doesn't allow self consumption, it isn't likely to get much traction.

It is actually kind of difficult to figure out with net metering how much consumed energy truly came from the grid, without breaking down the daily consumption into smaller buckets (15 min or so). Using Alchete's example, if all the consumption was when the sun is down, it would mean 117 kWh were supplied from the grid. If all the consumption was during hours that the PV system was generating, it would mean that 0 kWh were supplied from the grid. The actual amount supplied by the grid is somewhere in between.

I can't find a reference for it at the moment, but if I recall correctly, the amount of solar power in the US that is self consumed is relatively low, 20-30% maybe. As energy storage becomes more accessible and tariffs change away from net metering, that number could go up.

[pvoveris]

Thanks for the answers, maybe if I give my numbers from the meters it will make more sense.

Solar meter = 13,258

Net Meter

Meter
Code Type Amount
04 high peak from DWP 135
10 low peak from DWP 1740
16 base period from DWP 7650

21 solar high peak 3279
25 solar low peak 3153
29 solar base period 3228

39 DWP total 04 + 10 + 16 9525

71 net delivered & received high -3143 04 - 21
72 net delivered & received low - 1413 10 - 25
73 net delivered & received base 4421 16 - 29

74 net delivered & received -136

Now the previous year I used 11.660 from DWP

So 21 -29 add up to 9660, should this not equal 13258 from the solar meter?

Thanks again everyone.

Peter

[cebury]

Are figures 21, 25, and 29 from the Solar Meter itself? If so, then a problem (or a misinterpretation of those numbers) lies within the solar meter.

[pvoveris]

No, that is from the utilities net meter.

[sensij]

The numbers look right to me.

Looking at just the net meter data first...

From DWP (codes 04, 10, 16) = energy imported from the grid
Solar (codes 21, 25, 29) = energy exported to the grid
Net (codes 71, 72, 73) = (From DWP) - (Solar)

The fact that the solar meter does not match the energy exported is because there is some self-consumption.

self consumption = solar meter - energy exported = 13258 - 9660 = 3598.

So, the total energy consumed = self consumption + energy imported = 3598 + 9525 = 13123 kWh.

Also, note that total energy consumed = solar meter + net = 13258 + (-136) = 13122 kWh. Conceptually, this makes sense right? Whatever you generate from solar, plus the net of what you take from the grid, equals the consumed energy.

[pvoveris]

Thanks sensji, the way you laid it all out works in my head in trying to balance the numbers. And, you certainly did that.

[cebury]

Thanks sensji, the way you laid it all out works in my head in trying to balance the numbers. And, you certainly did that.

He nails it as usual.

[jeremynpross]

sensji,
I have just gotten my PTO and called numerous people at LADWP including my solar inspector looking for information to flesh out what you and pvoveris posted. Finally I spoke to LADWP Solar Customer Support and got the answers. Please verify that the following information is correct to help me and others understand the whole picture.

How to read LADWP Solar Production and Net Meters

CODE LOOKUP TABLE PROVIDED BY JOSH AT LADWP SOLAR CUSTOMER SERVICE 866 484 0433
INCLUDES EXAMPLE NUMBERS AFTER ONE DAY OF OPERATION BY CUSTOMER

Note: For customers like myself who are not on time-of-use billing, the low, high and base periods should be added up. The meter is simply designed to report based on time periods by default.
Solar Meter
04 Solar production high peak 00000
10 Solar production low peak 00000
16 Solar production base period 00039
39 Solar production total 00039

Net Meter
FROM THE GRID
04 Delivered from DWP high peak 00029
10 Delivered from DWP low peak 00049
16 Delivered from DWP base period 00195

SOLAR OVERPRODUCTION
21 Sent from Solar to DWP high peak 00000
25 Sent from Solar to DWP low peak 00000
29 Sent from Solar to DWP period 00012

FROM THE GRID
39 Delivered from DWP. Total of codes 04 + 10 + 16 = 273

NET DELIVERED FROM DWP & SENT TO DWP
71 Net delivered & sent high. Total of codes 04 – 21 = 29
72 Net delivered & sent low. Total of codes 10 – 25 = 49
73 Net delivered & sent base. Total of codes 16 – 29 = 182
74 Net delivered & sent. Total of codes 71 + 72 + 73 = 260


As sensji said, the fact that the solar meter does not match the energy exported is because there is some self-consumption.

Self consumption = solar meter - energy sent to DWP = 00039 - 00012 = 27

So, the total energy consumed = Self consumption + Net energy delivered from DWP = 27 + 260 = 287 kWh.

Whatever you generate from solar, plus the net of what you take from the grid, equals the consumed energy.