A news story from last October by ABC affiliate KFSN in Fresno, Calif., reported on a public meeting called by State Senator Dean Florez to hear complaints about Pacific Gas & Electric (PG&E) electric bills based on “tens of thousands” of newly installed smart meters. One citizen asked, “How did my bill go from $300 to almost $1100? It didn’t make sense.”
When I asked companies that make smart-meter chips about the complaints, they expressed puzzlement. The meters have specs, they told me: 24-bit precision and accuracy across a wide temperature range. Besides, how hard is it to digitize 60-Hz voltages and currents?
THE STANDARDS IN QUESTION
Meter standards can be found in international IEC 61036 (see “A Measure Of Opportunity Awaits In Electric Meters”). The spec includes environmental requirements such as how much power the meter itself can dissipate and how much voltage it must tolerate, along with performance guidelines such as accuracy and electromagnetic compatibility (see Table 1 and Table 2). But IEC 61036 isn’t the end of the line for meter standards.
According to a white paper published in May by the Electric Power Research Group (EPRI), the American National Standards Institute (ANSI) developed new standards with more stringent accuracy requirements in the late 1990s.1 Under those ANSI C12.20 standards, typical residential solid-state electricity meters must be twice as accurate as the old electromechanical meters. In addition, they must continue to meter down to 0.1 A (24 W), versus 0.3 A (72 W). EPRI also said that 100% manufacturing test and verification is the common practice, and utilities vary in using 100% acceptance testing or sample testing.
METER MAKERS & HYPOTHESES
Landis+Gyr, a long established, global meter supplier in Switzerland, builds PG&E’s smart meters (Fig. 1). The wireless-mesh radio link, which comes from Silver Spring Networks of Redwood City, Calif., operates in the 900-MHz band. According to Landys+Gyr, the meters cost PG&E $100 each.
Assume the chip-based meters are as innocent as newborns. Could the old spinning-disc meters have been under-measuring? Their design dates back to the 1890s, after all. The meter movement is based on a reluctance, or eddy-current motor. The familiar horizontal metallic disc rotates in a field supplied by a permanent magnet. Induced fields from ac voltage and the supplied current create a torque that rotates the disk.
Eddy currents from the disc’s rotation through the permanent magnetic field retard that rotation. The ac line voltage and the power being drawn both affect the disc’s rotational velocity, which is therefore proportional to the power demanded by the circuits connected to the meter. The disc rotations increment a multi-dial clockwork mechanism that maintains a record of energy consumption. Commonly, one disc revolution represents 7.2 Watt-hours (Wh).
Even though the design may have met specs when it was first tested, over time, the lubrication in the clockworks could gum up the works and cause low readings. Countering that hypothesis, I have at least one contrary data point. When we sized our home solar system, we based it on our typical consumption, measured using an old fashioned meter that had hung on the house for at least 30 years. The goal in sizing was to account for a 4× differential in electricity rate between peak and off-peak, while keeping our net outlay for electricity at essentially zero dollars per year.