The mining rig consumes around Watts. This number can be used to calculate the ethereum mining profitability. When buying the power supply units make sure you have around 150-200 Watts extra for each unit. For example if you plan to use one power supply for 3 graphic cards of 150 Watts each, make sure your power supply is at least 650 Watts. This amount is necessary to cover eventual peaks in consumption(it can happen especially when you start running the miner). If the power supply does not provide the extra power, the gpu might freeze.