Time of day
Here in California we have a lot of solar and wind. We have lots of spare power for some of the time each day. Solar and wind are lousy sources for base load and for peak load. However, they are great for any load that can be varied depending on available supply. The big problem with solar and wind is is energy storage, but it's quite easy to store fresh water. Another way to look at this: the desal plants can effectively act as load buffers, allowing the entire energy system to act more efficiently. From a rate-paying perspective, This usage pattern should get the lowest electricity rate. Alternatively, the power company could build the plants and sell the water.
Salt pans have produced salt commercially at the south end of SF bay for more than century. Since a desal plant discharges high-salinity waste water, it might be possible to sell this waste water to the salt company. I doubt that the total amount of salty waste for a statewide system would have a market, but some would.
Also, using solar and wind power for this purpose would also get help get the environmentalists on board.