You would also need to find a suitable location. If the reservoir is really far away, you’ll be losing too much energy. Think of transmission losses, but for water in a pipe. The reservoir would need to be pretty high as well, so a flat desert won’t work for an application like this.

Ideally, you would have a solar farm in the desert and use the excess energy to pump salt water to the top of a small mountain that sits right next to the ocean. With this setup, you would have a stable source of energy, which you could send to the grid. When the reservoir is full and energy demand is low, you could dump the remaining energy into desalination.

You could also use some of that energy to produce hydrogen and oxygen from water. During peak demand hours you could used a fuel cell to make electricity from the hydrogen.

You’ll also get oxygen as a byproduct, which could be used for a bunch of different chemical processes to get some additional revenue.