At our office the main panel has 3 hot legs coming in. There's one 200A
master breaker at the top of the panel, then all the usual 15A, 20A breakers
for various office circuits below.

If 120V * 200A = 24,000W, would I need a 24KW 3-phase generator, or would I
need 24KW * 3 = 72KW generator (eek!) ... or is my math completely off?

I understand there's more to generator sizing for air conditioning, motor
loads, etc., just trying to get a really rough idea of what kind of beast I
would be looking at if the entire panel could be powered off the genny. In
other words a $5000 - $10,000 beast, or a $15,000 - $20,000 beast. g

-- Paul