That is the problem, there is NOT enough power on site to run a datacenter. The article quoted one location with 127ksqft and 5MW. 5MW is tiny for that footprint. To give you an idea, you need to roll at least 300-400w/sqft of power/cooling to be viable, calculate in UPS and other losses, and 5MW powers around 5000sqft of datacenter space which is only a couple hundred racks of servers and storage. They're probably going to need 40-100M to retrofit that space, so I'd say the math is not on their side...
This reads like a realestate person proposing this, not a datacenter person...
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.
This reads like a realestate person proposing this, not a datacenter person...