As the solar industry continues to mature, the expectations of PV systems grow.
“It is no longer enough for a PV system to produce energy,” explained Peter Mathews, SolarEdge’s North America general manager. “It needs to be a smart system that can measure, display and monitor different indicators of a system’s performance.”
Inverters do measure the AC production from a PV system and usually display this information on screens, but Mathews said these measurements are typically only +/-5% accurate. This is where revenue-grade sub-metering systems can help.
“Installing a meter can provide higher accuracy and enable measurements to be read on a monitoring platform, which can be connected to the meter directly or through the inverter,” Mathews said.
Many states even require these more rigorous specifications on production measurements, often referred to as “revenue grade.” There are two main types of specifications: ANSI C12.1-2008 (+/- 2%) and ANSI 12.20-2010 (+/- 0.5%). Revenue-grade measurements are deemed accurate enough for billing, which is important when a system owner wants to take advantage of performance-based incentives such as SRECS, or if the AHJ or electric utility requires a revenue-grade meter. Other terms for revenue grade include revenue-grade metering (RGM) and performance based incentive (PBI).
Revenue-grade meters connect to the AC output side of the solar inverter, which converts direct current from the PV system to alternating current synchronized with the utility grid. The meter provides energy and power data by measuring both the current and the voltage at the inverter output. Revenue-grade meters are also used to measure and monitor the energy and power that is flowing into and out of the grid, which is important because some utilities limit the AC power that may be fed into the grid.
“Installers and customers alike are requiring more features from their solar installations and sub-metering systems, such as remote access to real-time data,” said Cynthia A. Boyd, director of sales and marketing at Continental Control Systems, which manufactures the WattNode revenue-grade meter. Revenue-grade meters can work with smart-inverters to provide grid support functions through measuring bidirectional power, demand, peak demand, reactive power, voltage, current, power factor and line frequency. “For example, the WattNode can provide updates quick enough to enable a smart-inverter to instantly adjust the power going to the grid to avoid exceeding the utility’s maximum power limit. Timely power reporting to the inverter has become an important element in smart-grid management.”
Meters can be installed externally or embedded into an inverter. Mathews said there are advantages to each approach. “In the current business model, contractors commonly install the inverter and then a separate external meter for revenue-grade measurements. This allows more freedom in selecting the meter type,” he said.
However, Mathews added that the benefits of using an embedded meter outweigh the flexibility installing externally offers. “Inverters with meters embedded during manufacturing offer simpler installation and seamless operation between the meter and inverter,” he explained. “This also eliminates the need for additional components and labor on site.”
SolarEdge, partnering with Continental Control Systems, is one of the first inverter manufacturers to include a built-in revenue-grade meter. The embedded WattNode meter offers high-accuracy production monitoring through the inverter display and the cloud. Once in the cloud, the data can be accessed via the SolarEdge module-level monitoring portal or third-party tools through an API. The production information can also be accessed from the inverter itself with a wired connection using an industry specification called SunSpec.
“Easy access to the revenue-grade data is also becoming increasingly important,” Mathews explained. “It is particularly useful if the information from the meter can also be easily transmitted from the inverter to the cloud.”
As inverters are managing more aspects of PV systems, such as metering, safety and grid interaction, Mathews and Boyd expect to continue to see enhanced functionality embedded into the inverter. SPW
