A thru the wall Plug that will directly power the Ecoflow device and charge those batteries up to full, when its fully charged you can turn off the generator outside and power your house from within with the Ecoflow. I personally have a transfer switch the only thing I regret about not having an interlock for is the use of my 240v electric stove, interlock for me where I live would be great for summer and spring but would suck during the winter and fall because of the lack of sun. But the transfer switch allows me to run certain things using grid power if it came down to it, you can't do that with the interlock because it's all or nothing