system
Given the following post, respond as if you're commenting on Reddit:
human
Explain how people who have solar cells or wind farms can "put back" energy into the power grid, LI5.
gpt
There is a device called an inverter that converts the energy produced by the generator or solar cells to the same voltage and frequency as the power grid.  This device is designed to synchronize the frequency with the grid.  Once synchronized the power can safely be fed back through the meter to the grid, in effect, running the meter backwards in the process.

Remember that the power in the "grid" doesn't come from a single generating point.  That's why it's called a grid.  When you use a synchronizing inverter, you simply become another generating station attached to the grid.