The TL-US inverter model is similar to a Japanese model that was developed after the nuclear power plant failures at Fukushima to provide a small amount of standby power in the event of a grid outage. The SPS inverter was designed specifically for charging small electronics, like cell phones and laptops. The TL-US series and TL-JP model debuted within months of each other, with the Japanese version being first to market.
Even though the technology that allows the SPS to function is integrated into the inverter, the actual electrical outlet circuit is not—it must be installed exterior to the inverter. This includes a wall-mounted receptacle, a switch, and all associated wiring. Additional labor and material costs are a consideration when integrating the SPS circuit, and depend greatly on the desired location of the SPS outlet in relation to the inverter. There is no limitation as to how far away from the inverter the SPS outlet is installed, although voltage drop will need to be considered. If the SPS circuit is located a significant distance from the inverter, wire size may need to be increased due to voltage drop.
In the Balls’ system, the SPS outlet was installed in the mechanical room, which is centrally located within the home, but about 50 feet from the inverter. The additional cost for the SPS circuit was $400, including all labor and materials. The inverter was installed in an upstairs open storage area since the mechanical room was not big enough. In many instances, however, the SPS outlet and inverter can be installed within a few feet of each other, reducing the overall labor and material costs.
The retail cost of the inverter itself is actually slightly lower than its counterpart, the 5000US, which helps to cover some of the additional costs of adding the SPS circuit. Along with a higher reported efficiency (96.5% in comparison to 95.5%), the TL-US inverter is a clear choice over the standard transformer-based model.
Unlike a typical grid-tied battery backup system, which automatically switches on the backed-up loads during a grid outage, the SPS circuit must be manually operated. Once the inverter receives a signal from the SPS switch, it initiates a 45-second SPS startup process, which includes an arc-fault circuit interrupter (AFCI) self-test. After the startup process is complete, the inverter displays that it is in stand-alone operation, which indicates that it is isolated from the grid and the SPS circuit is live. The inverter then displays the amount of power the SPS loads are consuming.
The inverter uses its capacitor bank to throttle PV array current, varying its internal resistance and acting as a current-controlling device to provide a stable voltage. Changes in insolation will cause the array output to fluctuate, which will influence how much power will be available to the SPS circuit. If the loads on the SPS circuit exceed 1,500 W or what the array can supply, the inverter will disconnect from the SPS and then try to reconnect every 20 seconds until the load size is appropriate.
At a limited output of 1,500 W, the loads that can run will most likely be relegated to communications (cell phones, routers, etc.) or lighting. That said, if adequate power is available during the day, the SPS circuit could potentially be used for larger loads, as long as the 1,500 W limit is not exceeded. That means, during daylight hours, you may be able to keep a small refrigerator operating or run a TV and entertainment system. There are, of course, many loads that would exceed the inverter’s 1,500 W capability, including space heating, power tools, and electric water heaters. With the manual nature of the SPS circuit, it’s imperative that system owners know the limitations of the SPS circuit. It may be helpful to produce a list of appliances that can be operated during stand-alone operation and post it next to the SPS outlet.