ASK THE EXPERTS: Understanding Net Metering

Intermediate

Inside this Article

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 (0)

Advertisement

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading