However, the airblast cannot be treated as an explosion. Since a shockwave hit the Death Eaters, let's treat it as a shockwave instead.
E = (ρ/t^2)*(r/c)^5
Let's use the radius of Hogwarts, or 182.88 m.
t = 182.88/340.29 = 0.54 seconds
(1.225/0.54^2)*(182.88/1.05)^5 = 6.733380221441094655e11 Joules, 160.93 tons of TNT, Multi-City Block level