If it's digital the technique used in these transmitters is extremely efficient (90%) as opposed to about 64% with the old high level plate modulated vacuum tube transmitters. That is, the older transmitters might use 78,000 watts to obtain a 50,000 watt output signal where the modern, solid state transmitters might use 55,000 watts to obtain a 50,000 watt output signal. This should give you a start point, but why don't you get ahold of the mfg and find out the facts?
If this is an existing installation you should be able take an amp reading on the electrical panel which serves the equipment. Total power= v*a* pf Then use the efficiencies in the previous post to determine how much power is being dissipated in the equipment room.