I saw a post on another forum where somebody asked if they could add another AC voltage source while grid connected. I wanted to say there would be issues. After consideration, I realized the issue would be the phase of the two power sources; they would not be in phase. Therefore, if I had a power source, say from an inverter producing AC voltage from inputs from Solar Charge Controller and Lifepo4 batteries, I would not want to introduce that power when Grid power is live; I would need to switch between the two sources. If I tried to introduce the inverter AC voltage, I would burn the inverter and risk fire or electricution. It is all due to the phasing of the alternating current... correct? That is why my inverter has an Automatic Transfer Switch integrated. If it senses steady Grid power, it is set to select the Grid power. If Grid power is interrupted, it will switch to the DC battery/solar power source and convert it to Alternating current to serve the load. The inverter chooses between the AC voltage from GRID and producing from inverter AC voltage.