Assuming a 12 pound brisket has about 2 pounds of water in it, and assuming the heat transfer from the ambient air is relatively constant as the brisket thaws (to a first approximation this is OK) then the energy required to thaw the brisket from an initial temperature is given by:
specific_heat_of_solid_water*mass_of_solid_water*(32F-initial_temperature) + specific_heat_of_fusion_of_solid_water*mass_of_solid_water
If you do the math, the energy required to completely thaw a brisket starting from -20F is about 409 kJoule. The energy required starting at +20F is 325 kJoule.
The energy required to increase the temperature of ice is much less than the energy to melt ice. Therefore, it's most important to make sure the brisket is completely frozen and to make sure you have plenty of ice in the cooler to suck up all of the heat transfer from the ambient.
