The bottom portion of the building is a bit blocked, but since I'm assuming the bare minimum for a high rise like this seems to be, it should be fine
The building is 523 pixels tall and 362 pixels wide, a discrepancy of 0.69216061185
A high rise is at least 20 meters tall, making the building 13.843212237 meters wide, I'll assume it's also that long, 20 times 13.843212237 times 13.843212237 makes for a volume of 3832.69050077 cubic meters
Assuming 80% hollowness, that makes the actual volume 766.538100155 cubic meters, 766538100.155 centimeters
Times 25700 for the vaporization of stone is...
1.9700029e+13 Joules, 4.70841997466 Kilotons, Small Town level+
Huh, I guess they're back in that tier
Undyne also catches the building as it's falling on them, so mass and GPE is worthwhile
766.538100155 times 2400, the density of concrete, is 1839691.44037 Kilograms, Class M
GPE is mass times 9.81 times the height of the center of mass above the ground. In this case, it fell 45 degrees, about halfway down, meaning it would be 5 meters
So, 1839691.44037 times 9.81 times 5 is
90236865.1501 Joules, 0.02156712838 Tons of TNT, Small Building level
yeah kinda expected
Edit[]
There's no real reason to assume vaporization and that's not the default assumption, so pulverization should apply instead
766538100.155 times 214.35 is...
1.64307442e+11 joules, 39.27042113 Tons, City Block level