The rapid growth of the grid-tied residential PV industry has spurred an increase in inverter manufacturers and products. To reduce system costs and introduce handy new features, inverter technology is shifting. Transformerless inverters, more microinverters, and even a few AC modules are on the scene. This article focuses on Underwriters Laboratories (UL)-listed residential batteryless grid-tied inverters.
The table shows residential inverters that were included in the California Solar Initiative (CSI) list of eligible inverters as of November 2011. Inverter manufacturers serious about delivering and supporting products in the United States aspire to meet the incentive eligibility requirements for California, the country’s largest solar market. Technical data listed was supplied by the inverter manufacturers or found on inverter specification sheets and owner’s manuals.
Inverter specifications, and their relevance to PV system design, are described below. (For a list of all of the specifications shown in the table, see our previous grid-tied inverter buyer’s guide in HP133.)
Transformerless. Many new inverters are transformerless. While these inverters offer several advantages, neither DC current-carrying conductor is grounded. Ungrounded systems must still “equipment ground,” i.e., connect all metallic enclosures, module frames, and racking to a common grounding conductor. Ungrounded systems have additional requirements per National Electrical Code Article 690.35. For transformerless inverter advantages and requirements, see the “Less is More” sidebar.
Maximum Recommended PV Power. PV arrays typically produce less than their standard test condition (STC) rated power due to higher cell/module temperature, soiling, production tolerance, etc., so arrays are often sized so that their rated power exceeds inverter output.
Each inverter manufacturer specifies the maximum recommended array power for each inverter model. The maximum ratio between array and inverter outputs generally ranges from 115% to 125%. Determining the ideal ratio will minimize power clipping and maximize PV system output. However, there can be competing factors that can instead maximize the financial return ($/year) that a PV system can generate, and thus recommending a particular ratio has become an increasingly contentious issue (see “Array to Inverter Ratio” sidebar).
Maximum Open-Circuit Voltage (Voc). Voltage is electrical pressure, and all electronic components, including inverters, have a maximum voltage they can withstand. This specification is the maximum input voltage that the array may reach under any condition. The array voltage is dependent on module make and model, the number of modules in series, and the module’s temperature, which in turn is dependent on ambient temperature. System designers must calculate the maximum voltage to make sure it is within the limitations of the inverter (see “Determining PV Array Maximum System Voltage” in HP146).
Maximum Power Point Tracking (MPPT) Window. The MPPT window is a voltage range that the inverter can work within to find the maximum “knee” of the array’s IV curve to maximize array output as conditions vary.
An array voltage operating either too high or too low will result in less output. Just like with the maximum Voc specification, designers must design the array to operate within the limitations of the inverter’s MPPT voltage window.
To ensure an array design will be within an inverter’s maximum Voc and its MPPT window, most inverter manufacturers provide online string-sizing tools that take into account all of these factors along with their inverter requirements, to help step system designers through the array design process. PVSelect.com is another online tool for string-sizing calculations for various modules and inverters. It is helpful for comparing various designs using different modules and inverters, as it provides users with a single Web tool, rather than having to navigate between the various inverter manufacturers’ sizing programs.