Not to be rude, but that is not the kind of question you should be asking if you are considering building a water chiller, even with the help of an uncle. Voltage has no relation to the power drawn in this case, what you are asking is how much power will the compressor draw? Voltage is not a unit of power, nor of energy, rather it is a unit of electrical potential. Not to be rude, but as power and energy/enthalpy are central to the concepts of refrigeration, you may want to double check as to how much you are willing to put into this system, including reading about basic thermodynamics.
However, to answer your question:
Google the compressor, it will display the power draw at various evaporator temperature, it will likely be less power draw than the air-conditioner (before modding), as power consumption increases with evaporator temperature (generally)
Please keep us up to date with your project, I hope I did not come off to strongly, I just felt the need to say something!
To answer your question in a very inaccurate way... assume a couple hundred WATTS
Perhaps also an easy way would be to do the following:
divide the ambient temperature by the temperature you plan to achieve, subtract that value from one (yeilding the maximum possible efficiency) then divide the load you wish to dissipate by that value...