solar energy has the huge downside of being dependant on weather and time of day, while mining requires constant 24/7 energy. At night the powergrid would overload. The only way to sustain the mining requirements are coal and nuclear plants.
Fortunately, this isn't really a big issue:
1) you can distribute your mining plants on several sites on Earth, using daylight on different longitudes. This would obviously increase the incidence of hardware costs in the profit equation, but at good sites with high insolation you would have also smaller electricity costs than grid-connected farms.
2) there are solar electricity generation methods like "
concentrated solar power" providing the possibility to store energy so they can also work at night. The Andasol plants I mentioned, for example, work with this principle.
3) if CSP isn't possible, batteries may be an option - their costs are also falling.