The minimum amount of work needed to pump some amount of heat Q from a temperature T0 to a higher temperature T1 is W = Q*(T1/T0 - 1). For example, if your ambient heat sink is at 20C (293K) you need at least 2.8W of electricity to run the cooler for every 1W dissipated at 77K, or 28.3W for 1W dissipated at 10K. This is the thermodynamic lower limit, and practical heat pumps will be less efficient in general. In practice it might be something like 4x and 50x, respectively.