Wind and solar in general are horrible for power systems because they aren’t consistent. So what ends up happening is that power companies keep fossil plants on standby to pick up the slack for when the sun goes behind the clouds or the wind dies down. A fossil plant can’t just instantly ramp up its power production though, so the plant’s boilers have to be run at a high enough output that they can pick up the slack on short notice. So effectively what ends up happening is that fossil plants end up being run horribly inefficiently and wasting a lot of fuel just because wind and solar aren’t constant. This is not only horrible for the environment and tends to offset a lot of the environmental benefits of solar and wind power, but it also makes the fossil plants look like they are a lot less efficient than they actually are because they are wasting so much fuel that isn’t going into actual electrical production.
It also takes a lot of fuel to start up a power plant boiler. You don’t just flip a switch and turn it on. So even if you are only running the plant at night to cover the loss of solar power, you still end up wasting a lot of fuel.
There are methods of storing energy from wind and solar, but they only work in some areas. For example, you can use wind power to pump water up into a higher level reservoir. Then, when you need electricity, you use hydroelectric power from the reservoir. Your ultimate source of power is the wind farm, but the reservoir means that you can have power 24/7/365 (as long as you don’t exceed the capacity of the wind farm to fill the reservoir).
Similarly, a different method of using solar power is to heat something like salt until it melts, then use the molten salt to heat water into steam and use that to power a conventional steam turbine. As long as you put enough heat into your salt that it stays molten through the night, you can effectively have solar power 24/7/365. This only works in very sunny areas like the Southwestern U.S.
This is also true. While solar and wind power are horrible for overall power generation, in specific use cases like this they aren’t bad at all.
It should also be noted that in many areas (i.e. the Southwest and Northeast parts of the U.S.), the power system is strained during peak daylight hours, especially at certain times of the year (summer, when everyone is running their AC) so adding solar to these areas is actually quite helpful. At night when solar isn’t producing energy, the demand is also less, so the lack of consistency from solar isn’t quite as much of an issue.