
The grid always needs to be in balance to remain at a stable voltage and frequency. If you have too much supply or demand, you will destabilize the grid and it will eventually shut down to protect equipment. Negative energy prices mean the grid operator expects so much excess power on the supply side that they need to artificially raise the demand. In other words, they pay people to use – sometimes waste – energy they otherwise wouldn’t.
Batteries exist, but they won’t do you much good when they’re full.