ASK THE EXPERTS: Understanding Net Metering

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I recently had a 3.2-kilowatt, battery backup solar-electric system installed at my home in western Montana. My OutBack meter is telling me that I sold back 626 kWh for the past month’s service, yet the electric co-op I belong to only reduced my usage by 97 kWh—the reading they claim on my meter. The amount purchased was listed as 851 kWh, leaving me a net purchase of 754 kWh, according to them.

Do you have any ideas why there is such a great discrepancy? On most days with full sun, the meter displays 20 to 22 kWh “sold,” with 11 to 16 kWh on cloudy or smoky (forest fires) days. Since my system is capable of putting out much more than the roughly 3 kWh per day the utility credits, it seems like either their meter is not registering the electricity the system is feeding onto the grid, or the electricity somehow isn’t getting fed back onto the grid. Could my installer have misconnected or misconfigured the wiring?

Reg Thibodeau • Hamilton, Montana

Battery-based systems have multiple meters that report various values from different points of the system, and thus understanding the actual output of a battery-based system can be confusing. Some meters report information from the DC side of the system and, for example, show instantaneous power (kW) and daily energy (kWh) from the charge controller, as well as kWh in and out of the battery. Other meters measure power and energy on the AC side of the system.

So the first challenge is making sure you are getting the correct information from the correct meter. A quick check on PVWatts at what we would expect your PV system to produce shows us that even a batteryless 3.2 kW system should produce about 450 kWh during the best month (July) for your area. (Note: A battery-based system such as yours will be slightly less efficient and thus have slightly lower production.)

The next challenge in reading the meter that you have is knowing where the solar energy produced actually goes. Once your grid-tied battery backup system generates energy, it has two ways to be consumed at your site before the excess can be sold back to the co-op. Both of your load panels—the panel that distributes battery-sourced energy during power outages and the main service panel—will consume energy from your array before any remaining energy moves across the utility meter and onto the grid for credit.

Commonly, if there is confusion about how much energy should have been exported to the grid, the discrepancy is explained simply by realizing the difference between the kWh reported by the OutBack meter and the utility credit, which is affected by the energy being consumed by both the critical loads subpanel and the main service panel. For the sake of this discussion, we will assume the information you provided is correct; using those numbers, it would look like your house consumed approximately 1,380 kWh—626 kWh from the PV array plus 851 from utility. Subtracting the 97 kWh credit gives you 1,380 kWh. (Note: The 626 kWh displayed for the PV system’s production is measured on the DC side of the system—that is, it doesn’t take into account inverter inefficiencies or the energy used to keep the batteries topped off.)

We see this same type of confusion with grid-tied batteryless systems when a client’s loads consume most of their system’s production, and the client only sees a small credit on their bill from the utility.

The only system in which a client sees the system’s total production credited on their utility bill is when the interconnection contract is based on a feed-in tariff (FIT) incentive. Under a FIT contract, all of a PV system’s AC production is fed onto the utility side of the service and logged on a separate meter. This meter reading should closely match the kWh production shown on a batteryless inverter (although it can slightly vary depending on the accuracy of the inverter meter).

Under a FIT scenario with a battery–based system, a special production meter is required that can account for the kWh being sent from the inverter to the main service panel/grid and the kWh going to the critical load subpanel. In this case, the kWh reported by the OutBack meter will be close to the kWh total shown on the production meter. The difference represents the inefficiency of the inverters’ conversion of DC kWh into AC kWh, and any energy used to keep the batteries full.

Going back to your particular situation, if you think that your loads did not use the 1,380 kWh, checking previous utility bills (prior to the installation of the PV system) would be worthwhile. This should give you a baseline for comparison. However, if the household loads have changed since the PV system installation, then the way to test this would be to establish some means to track them independent of the sources (your PV system and the utility). This can be accomplished by measuring the loads in the two panels with a recording amp-hour meter for a day or more and calculating the load; or by obtaining a data-logging system that will collect the load data independent of the two sources. Your RE installer or an electrical contractor should set up this metering and data collection, since it requires working with equipment that can be an electrical shock hazard.

One simple method for completing this load analysis is to shut off your PV system (putting the critical load panel on the utility source with the bypass switch) and monitor your consumption at the utility meter for three to five days to achieve a representative sample. Then you can calculate the monthly total.

Christopher LaForge • Great Northern Solar

Comments (3)

America Install Solar's picture

Thank You Chris LaForge!
Great explaination of kWh usage by bldg. (home) usage of PV production with net going to grid. This is a common confusion for our customers, understandably.
Also, for solar PV customers not 'selling' excess production back to grid, if they do Not use it... they lose it (their PV production) , so they must manage/schedule using the daytime PV production as much as possible...ie. use solar production directly on load circuits, (or program) devices/appliances,etc. to operate during day,
OR, have large capacity energy storage (e.g. battery pack, water, or hydrogen) to save excess daytime PV energy until needed later (e.g. evening or weekend etc.) and this will help avoid loss of PV daytime production when not interconnected to grid.

It is also, like "solarmanJD" said it Could be "slated in their(utilities) favor",
That our customer's have a right to be suspicious of meter reading errors(or?)/ subsequent excess utility bill $ due?
e.g. One on our customers (years ago & first 'net-met.' for the utility) was Billing the customer for her PV production sent to their lines!
The problem was utility billing software was not programmed to 'Credit' a customer!! (Utility clerk was demanding payment from our customer for electricity sent TO them, until utility mgmt. got it corrected) She also got a retroactive "rebate" from that utility!

It pays to be diligent and keep the 'foxes' outside!

America Install SolarMan

America Install Solar's picture

Thank You Chris LaForge!
Great explaination of kWh usage...

Also, don't forget to add up All cost of being interconnected to a utility('grid'), such as:
- min. connection fees (just to keep that old rusty transformer maintained so you can send them your solar production!)

- additional $$ (fee) to 'renewable' systems (some utilities have and others are trying...)

- D.G. liability 'insurance' (could be $million!) required by most utilities, which you must agree to prove/sign if your power to grid causes damage (v. rare & unlikely with auto-disconnect PV inverters* now!). If you have an ole' gas generator that has no auto protection, that's ok (not really), the linemen Always check the grid lines (& treat as hot') for any unexpected power from persons running non-UL listed 'gensets' which can (if not properly 'tied' to circuits) back powering AC live to the grid lines. The utility just acts like 'Solar' is now a 'special threat to line safety', when solar PV inverters are More Safe! Especially less safe (to grid/ linemen,etc.) are those ole' diesel/gas generators that most people connect (during utility power outage like 'H. Sandy') with No circuit protection (& no insurance/liability agreement with utility) is required for the millions of portable & permanent ready to 'accidentally' back-feed the grid with live AC at any time!
This is just a 'ploy' by the utilities to deceive the the avg. potential solar customer that 'solar electricity' is a new safety risk, when in fact solar electric (PV) is Less Risk to grid than the millions of unregistered (some illegal), & some completely tied to grid dangerously. The utilities claiming that 'solar is safety risk' is non-sense propaganda to mislead unknowing electric customers, about the existing high safety record (& UL & NECode requirements) that Solar (& Wind, etc.) Electric systems exceed in producing AC power for millions of homes today around the world! The benefits of correctly installed solar PV (or other sustainable) energy system are far better than the risk of electrical shock/fire hazard...otherwise no one should be using AC electricity and esp. not ole' portable gas generators!
*Solar PV Inverters/systems(All 'Grid-tied') have required UL listed "Anti-Islanding" (not back-fed AC when Grid is 'down') auto disconnect capability but the utilities state(yes, it's in writing by utilities), like they are new safety issue and now require 'homeowners' to have millions $$ liability for protection of back-feed to their grid, while other AC generators/systems do not require this (in most U.S. jurisdictions)
-

SolarManJD@DCemail.com's picture

Yea ...hear is the fun part ITS a "MONOPOLY" of course its slated in their favor...who's watching the" Hen House" the "sly Grey FOX"...
or do you really believe the so called "Public Service Commission" is really a "Public Service"...wrong the directors are all retired Utility directors and executives....what a laugh...

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