Others answered your question. A related, but different problem is “how do you make sure a new power source you add to the grid is in sync with the grid the mom

Others answered your question. A related, but different problem is “how do you make sure a new power source you add to the grid is in sync with the grid the moment you connect it?”. That’s described on https://en.wikipedia.org/wiki/Synchronization_(alternating_current) https://en.wikipedia.org/wiki/Synchronization_(alternating_c.... Firstly, you design your generator to have the same wave form and phase sequence as the grid. Then, you power up your generator and make sure the voltage, frequency and phase angle of the electricity it generates matches that of the grid. The moment that’s (more or less) the case, you connect the grids, preferably on a zero crossing. That Wikipedia page describes a setup with incandescent bulbs that can be used to manually detect that moment, but nowadays, it’s done using electronics.