Miss Elektra, you are either a liar, or very stupid. A coal fired plant went offline suddenly more than 600 miles from the Hornsdale storage batteries in South Australia. It takes 6 seconds for this to register on the instruments monitoring the grid. But the batteries kicked in in just over a tenth of a second. And carried the grid until the coal fired plant came back online.
"The coal-fired power plant holds a capacity of 560 megawatts, so its departure from the national grid was quite noticeable. Normally, the Gladstone power station hosting a backup generator would kick in, taking anywhere from 10 to 30 minutes to put power into the electrical grid. Tesla’s batteries, located 621 miles away from the Loy Yang plant, was able to accomplish this task in a fraction of a second."
Tesla's new battery backup facility at an Australian wind farm came to the aid of a failing coal-fired power plant.
www.greenmatters.com
That was 2017, and at the time, the 100 Mw plant was considered one of the biggest. However, today it would be considered a small installation. And, yes, they can supply power for hours at a time in an emergency. But their primary use is in kicking in at the peak power usage periods, thus reducing the cost of firing up slow fossil fuel plants.