Rooftoop solar is good for a mere 8 Watts / sqft [1] [2] when the sun's shining, and it's not a constant 8 Watts output over the day even if the weather is clear all day.
So repurposing a Sears building's rooftop area for solar power would not cover even the minimum requirements you cite (300-400 W/sqft -- x 24hrs/day) as necessary for a datacenter re-purpose.
But then there is that big, unused parking lot. If it's got, say, 30x the rooftop's area and can be fenced in, maybe then a ground-mounted solar system might supply some significant fraction of daytime energy.
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