Auxiliary Metering for BESS+PV Installations: Are They Necessary? By Pterra Consulting
For hybrid solar photovoltaic and battery energy storage systems (PV+BESS), a seemingly innocuous question during interconnection is: “Are two meters that much better than one meter?” This is a question that relates to both technical accuracy and economics. The economics is fairly easy to understand, adding a second meter can be expensive and may spell the difference between a viable PV+BESS project and a failed one. The accuracy aspect is not so simple and requires some explanation.
The net energy for hybrid systems can vary over the course of a day — at times delivering and other times withdrawing power from the grid. A meter, known as the main or primary meter, located at the interface between the facility and utility (the point of common coupling or PCC) measures the net energy of the facility. This meter uses an instrument transformer (IT) that has a specific error range for loadings from 10% up to multiples of 100% of the IT rating. Below 10% loading, the measurement error increases. However, at those times that the PV+BESS is charging from the grid, its load can typically be below 10% of the IT rating. Utilities point out that this higher measurement error requires that a second meter be installed rated for the auxiliary loads of the facility. The second meter is sometimes referred to as the auxiliary or sub meter. Since the auxiliary meter measures the load of the facility, it is located within the facility and far from the PCC, requiring additional, costly fiber cabling. How much more accurate will the net energy metering be with a second meter?
Sample Configuration for a Dual Sub-Meter Installation
Pterra was engaged by the New York State Energy Research and Development Authority (NYSERDA) and the New York State Department of Public Services (NYSDPS) to conduct an assessment to answer this last question. Pterra reviewed existing accuracy standards including ANSI C12.1-2008, IEEE C57.13.6, ANSI C12.20-2015 and New York State’s own guidelines (DPS 16 NYCRR Part 92), and conducted simulations of a generic 2 MW PV+BESS facility. Pterra found that the magnitude of the decrease or increase in maximum error between one and two-meter schemes were in the range of 0.25% of the total annual registry. This was much too small of a difference to justify a second meter on the basis of measurement error. Use of an estimated cost for the potential registry differential may be more economical than actually installing a second meter. The full report from Pterra can be downloaded here (link).
NYSDPS, through its Interconnection Technical Working Group, considering the findings of Pterra study with other input, concluded that “the level of inaccuracies and small amount of potential discrepancies caused by metering these loads utilizing only one meter did not warrant the separate auxiliary meter service and associated costs.” This determination was put into effect on 6/11/20 per this document (link).
Note that an auxiliary meter may still be warranted for reasons other than measurement error, such as to verify BESS controls or for variable load pricing.