Derating Discussion

Derating is an operational mode that the inverter uses to protect itself from excessive heat. According to the UL1741 standard, the inverter must keep the temperature of the heat sink below 70°C in the US. The heat sink is directly attached to the IGBT bridge, which is where the DC is turned in AC power. As the power that is produced by the PV modules increases the amount of heat generated in the IGBT bridge also increases. As the heat sink temperature approaches 70°C, the inverter will begin to reduce the amount of power going through the bridge, and consequently to the AC grid, in a effort to protect the electronics in the inverter. This is part of the inverter’s normal operation, and should only occur in rare circumstances if the PV array is sized properly and the inverter is mounted according to SMA America’s recommendations. To properly size the PV array, please use SMA America’s PV array String Sizing Program.

The recommended practice for mounting the inverter is to install the inverter on the North side of the building. By installing the inverter on the North side of the building you can ensure that the inverter does not receive direct sunlight throughout the day for a large part of the year. Keeping direct sunlight off of the heat sink is by far the best way to ensure that the inverter will stay out of Derating mode.

The other major advantage of installing the inverter outdoors is that the heat sink can take advantage of natural airflow around buildings. The heat sink will work as designed even if the airflow is no more than 0.5 m/s. This airflow, though small, is an integral part of the inverter’s design and is the primary reason that we recommend the inverter be installed outdoors. If the inverter must be installed indoors, then we usually recommend that some kind of fan system be put in place to keep air moving over the heat sink of the inverter.

SMA America, Inc. has developed a fan kit that snaps onto the inverter heat sink fins called the Sunny Breeze. The Sunny Breeze is controlled by a temperature sensitive switch, which turns the fan on when the heat sink temperature approaches 70°C and off again when the heat sink drops below 55°C. The Sunny Breeze uses a small AC wall transformer for power and it will improve the Sunny Boy Inverter’s performance in nearly all installations. Please contact your SMA distributor for pricing and availability. The technical literature may be downloaded from the following link: Sunny Breeze Fan Kit Installation Instructions.