They are not there, that's the problem, the current power plants are good enough to supply the current demand with some surplus, it's not just a switch button that would make power magically appear from some stand-by power plants, you might want to have a look at the list of all power plants in Texas
here.
There really isn't a button, which magically increases the energy.
But, just looking at this picture, I would say that they can generate +30000 MW almost instantly. It would be enough to increase energy production via Natural Gas.
Apart from solar and wind energy, and possibly hydropower, which depends on external factors, other energy sources normally have immediate production capacity if necessary.
What I see in these numbers is that they are very well prepared, as they normally only need half the energy they have the capacity to produce. Note that they have no interest in producing more energy than is consumed.
Either way, a simple increase of 1000MW, from one day to the next, is really very significant and something unwanted.