Solar meters allow for the display of real-time PV energy production data. Photo: Solar-Log.
Solar meters can refer to pyranometers, which are used to measure solar radiation flux density (W/m2), or devices used to measure the kWh production from a PV system. This article is about the latter.
Solar meters collect PV yield production and local energy consumption to monitor and analyze PV plant performance. Solar meters often come with a monitoring function to alert plant owners of issues with PV plant performance, letting them quickly resolve issues and maximize return on investment. Plant data is transferred to a monitoring platform that provides a concise presentation of PV yields, monetary savings and plant performance.
“Solar meters record electricity readings using integrated current transformers (CTs),” said Anne Nelson, marketing manager at Solar-Log. “Solar Meters can be cellular enabled, which allows for a dependable communication from the plant to the monitoring platform, where the installer and customer can view performance remotely via the web.”
To measure electrical power (kW)—the rate of production—two aspects are measured: current and voltage, according to John Browne, applications engineer at Continental Control Systems. The current represents the amount of electricity (electrons) flowing through a conductor. Voltage represents the pressure pushing the electricity through the conductor. Current is typically measured by sensing the strength of the magnetic field produced when electricity flows through a conductor. A current sensor installed around a conductor is the current transformer (CT). The voltage is measured directly by the meter via two or more connections to the electrical service. The meter multiplies the current by the voltage to calculate apparent power (VA), and compares the current and voltage signals to calculate power factor (Pf). The apparent power is multiplied by the power factor to calculated real power (P). Real power is integrated over time to calculate real energy (kWh).
Meters, which do the measuring, can be internal or external to the inverter. All inverters include an internal meter, but frequently these meters are not revenue grade. External meters can be installed along the AC line between the inverter and the main distribution panel. This allows one meter to measure output from multiple inverters, which can create significant cost savings, as well as a collection of more precise PV yield data if the meter is “revenue grade.” Some solar meters can connect directly to the inverter through an RS485 or Ethernet cable, providing additional data for plant monitoring and diagnostics, Nelson said.
A revenue grade meter meets the ±2% reporting accuracy requirement in accordance with the American National Standards Institute (ANSI). Most meters embedded in solar inverters do not meet the ANSI certification requirements which means that these readings are much less accurate. In the U.S. PV market, many solar financing or incentive programs require revenue grade metering precision.
“In order to reduce field installation labor, the industry is heading towards integrating the meter into the inverter at the factory,” said Browne.
Even aside from the financing and incentive programs that require revenue grade metering, more and more, solar meters are becoming a necessary component of solar PV plants, Nelson said. Meters allow PV plant owners to monitor their plants and ensure the plant is performing correctly. Meters also provide the production information that is necessary for plant owners to calculate a solar PV plant’s return on investment.
