Motor Load
Watzmann, I agree with fsahmed that, since you already have the motors (and leadscrew?), it makes sense to set up a little experiment to simulate your application. There are aspects of the application that require some information that you may not have at hand, like acceleration of the load, speed of the load, and friction of your leadscrew and load supports.
Since you mention a load in mass, not in torque (which has units of force x lever arm length), I presume that the motor/leadscrew combination has to lift a 50 Kg load. If that is true, fsahmed's pulley suggestion could give you the answer you need (if you experiment with pulley diameter), but it won't tell you if you can move the load with a leadscrew.
A leadscrew advances the load some known distance for each revolution. That is the leadscrew pitch. Divide that by the steps per revolution and you have the advance per step. 50 KG is not a trivial load to accelerate, so the acceleration of the load (i.e., how fast you bring the load up to desired speed) has to be taken into consideration. The force F required to accelerate the load of mass m at acceleration a is F = ma. This force can be reduced by limiting a to the extent allowed by your application. It is in addition to the friction load and the static load.
Once you have the load moving at the desired speed, the mechanical power input is determined by the product of force and velocity (with appropriate factors to account for the units you use). For horizontal motion at constant velocity, the force is determined by the weight of the load and the coefficient of friction of the rails, rollers, slides, whatever supports the load. For vertical motion at constant velocity, the force is the weight of the load. Weight is the product of mass and acceleration of gravity. This assumes negligible friction in whatever guides the load.
Your leadscrew has an efficiency determined by the coefficient of friction of the screw on the nut (and friction of the bearings supporting the leadscrew). For a plain screw and nut, the losses can be fairly high, depending upon the materials, the finish,and the lubrication. A ball-nut leadscrew is MUCH more efficient than a plain leadscrew and threaded nut and can approach 100%.
So, you see, your question is not trivial and cannot be answered off the top of the head. An experiment may get you your answer more easily. Remember to account for deterioration of components, loss, contamination, or hardening of lubricant, etc., in your final design.
awright