Very rarely are any two PV systems the same, and even ones that are similar present their own challenges and nuances. Before you move forward with an installation, you need to spend some time designing the system and evaluating your options, including component choices.
After modules, inverters are the most important PV system component to consider. The recent resurgence of microinverters and the release of DC-to-DC optimizers and other “distributed architecture” products have changed design choices—but they haven’t eliminated the need for careful, proper design (see “Distributed MPPT” in HP137). This article focuses on standard grid-tied string inverters and their design parameters. For a complete list of inverter design factors, see the “Buyer’s Guide” in HP133.
Breaking down the decision process into smaller parts makes the design process easier and less risky. Each decision can build on the previous, allowing you to grow the entire design logically.
Start with the specs provided by both module and inverter manufacturers. Make sure you are using the correct specifications for the products you’ll be using—many companies make similar-looking models that have different electrical characteristics.
First, determine the PV array’s location. The available area for the modules, shading issues, the need for using multiple roof orientations, and other physical limitations will dictate the overall PV array size. These site limitations will also play a role in determining what inverter is appropriate for your location (see “Solar Site Assessment” and “Optimizing a PV Array” in HP130).
Grid-tied inverters are most commonly rated by their continuous output (AC) power capability—the watts the inverter can output continuously. In the National Electrical Code (NEC), a continuous load is defined as “a load where the maximum is expected to continue for 3 hours or more.” All circuits associated with grid-tied PV systems, on both the AC and DC sides of the inverter, are considered continuous.
This continuous power output rating dictates the PV array’s maximum power value. Grid-tied inverters will limit their power output—if you design an array that supplies more power than the inverter’s maximum, the inverter won’t be able to process all the power. Instead, the inverter will waste any excess power as heat. As is the case with all electronics, generating unnecessary heat may reduce the inverter’s life.
Deciding on how large a PV array to connect to the inverter requires predicting output over the course of a year. Generally, PV arrays produce less than their STC rating, due mostly to conditions that differ from STC—like higher cell temperatures, lower irradiance, and module soiling. When predictable system losses are taken into account, a PV system owner can expect their array to operate at around 80% of the STC rating. Since these losses are consistently present, the size of the PV array can be designed to exceed the inverter’s power rating.
Many inverter manufacturers specify simply that a PV array’s STC rating should be no more than 125% of the inverter’s continuous output rating—known as the “sizing ratio.” For example, if an inverter has a continuous output rating of 5,000 W, the maximum array size that would be connected using a sizing ratio of 1.25 would be 6,250 W (5,000 W × 1.25).
While this is a handy rule that covers a wide range of installations, it doesn’t deal with when the PV array is able to produce more power than what the inverter can process. This can happen when the array is operating in a cold and sunny environment, which will make output higher than usual. If the inverter is already operating at its maximum, the total power output will be limited.
Given that the inverter’s cost in the entire system is relatively low, most PV designers apply a more conservative sizing ratio, like 1.10 to 1.15 (10% to 15% above the inverter’s output rating). In most climates, this allows your inverter to process the PV array’s power without clipping the power output for most of the time. Some PV designers match the PV array’s STC rating to the inverter’s output rating (a sizing ratio of 1.0). This makes sense for sites with commonly cold temperatures and higher irradiance (like at higher elevations), practically removing the inverter as the limiting factor, except for brief times when the array output goes beyond STC rating, like from the “edge-of-cloud” effect.