Once grid power is restored, the SPS switch must be manually turned off for the inverter to restart its grid-tied operation. Otherwise, the SPS circuit will keep the inverter in “stand-alone op” mode, preventing grid-tied operation, and therefore preventing the rest of the array’s solar electricity from being used in the home or any of it from being fed to the grid.
Even though the SPS technology has limited applications in comparison to a battery-based grid-tied system, the ease and low cost of adding it to a grid-tied design, including the cost of the SMA inverter and the associated wiring, makes it hard to imagine why anyone wouldn’t want the added benefit of accessing otherwise unused solar power during a grid outage.
Deciding whether or not the SPS suits your needs as a backup power source is another question. Because the Balls don’t have critical loads that need to run during a grid outage, or frequent power losses associated with their utility service, the SPS circuit is adequate for their needs. On the other hand, the SPS circuit may be too limited for those who want a backup power system that can run at any hour of the day and is not limited by the solar irradiance available at the time of the grid outage. For those who already have a grid-tied PV system, but have always lamented over not being able to use their solar energy during a grid outage, it may be possible to upgrade their system by replacing their current inverter with the new TL model. The feasibility of this upgrade will depend on the system capacity, array configuration, and, of course, cost considerations.
Whether it’s the 3, 3.8, 4, or 5 kW model, each TL inverter can redirect 1,500 W of solar power. If a system owner wants more backup capacity, they may want to consider dividing their array into subarrays and using separate TL inverters. The Balls’ 5 kW array is wired into two strings of 10 PV modules. Instead of installing one 5 kW TL inverter, with each string feeding a separate MPP input, each series string of 10 modules could have been tied into a separate 3 kW TL inverter, giving them two 1.5 kW SPS circuits. Of course, the cost of installing two 3 kW inverters would be more than the cost of one 5 kW inverter, but, given the right circumstances, it is a design option worth considering.
Orion Thornton is the owner of Onsite Energy, a Montana-based solar-electric design and installation company. He is a NABCEP-certified PV installation professional and a contracted instructor for Solar Energy International (SEI). Orion lives in a net-zero energy home powered by solar electricity, solar thermal, and wood heat.