A solar plant with fixed panels can produce 100% power only one hour a day (during local apparent noon), and can effectively produce usable (80+ maximum) power only from 10:00 (AM) to 14:00 (PM) in the 3 winter months, and from 09:00 to 15:00 during the hottest 3 summer months.
At least - in the desert, and at that relatively far south latitude, and with that little pollen and rainfall, only dust will cause further efficiency losses. Then again, central southern AZ is a long way from a huge power demand as well.
Again, putting up this much money for a (heavily-taxpayer-subsidized) solar plant is merely a political stunt.
I don't think they plan to use fixed panels. With movable panels, they will be able to track the sun as the Earth rotates, giving a longer maximum power. Also, some energy will be stored as heat, perhaps from the midday maximum, to be used later in the evening.
It is trivilally simple to provide more reflection surface than is required for maximum collection efficiency, but it makes sense to do so to extend the productivity time. Excess or unneeded panels can be cycled through maintenance procedures when they are not immediately required for energy production.
It is only about 50 miles as the crow flies from Gila Bend to Phoenix, Arizona- I guess that is the intended power demand.