How many units of electricity will be consumed if we run a 1 HP Motor for 1 hour (with 75% or 50% load)?
With no load, the power consumed will not be zero, as the motor needs some power just to turn itself, against friction, air resistance, eddy current losses etc. Let's call the no load power 10% for this motor, as you used 90% as the full load efficiency.

The power used at 50% load could therefore be estimated at (50%+10%) of 0.83kW.

Actually I doubt a motor this small would have such a good efficiency at full load.