It depends on the size (HP rating) of the motor used in the treadmill and the maximum body user weight of the user in Kg who would be using the treadmill.
For example, let us assume you are using 3.0HP motor Treadmill (say Model TDA350) which has maximum user weight (load) of 120Kg and assuming your body weight is 120Kg and you are run-ning at full speed (18Km/hr) for one hour. In this case, the standard power consumption calculation is as below.
Now 1 HP (Horse Power) = 746 watts
So 3 HP = 3*746 watts = 2238 watts
1 unit of Power Consumed = 1 kWh= 1000 Watts used for one hour.
So, in the above case as you run for one hour it consumes
2238/1000= 2.238 units of power.
Generally, units of electricity consumed are lower than above as actual load (user weight), speed and duration at which treadmill is used are less.
You can consider one unit of power consumed for one hour of treadmill use.
The above is the standard calculation but the quality of motor used plays a very important role in electricity consumption. If the motor used has 100% electrolytic copper and is a green, efficient motor than it uses less current and hence less power.