As an important technology of developing a large scale superconducting magnet, we are working at power leads of a supercritical helium (SHE) force-cooled superconducting coil. For evaporated gas cooled mode one has only been required to find an optimum dimensional ratio of lead, but for SHE force-cooled one must design a power lead with both an optimum flow rate and the optimum dimensional ratio. The optimum condition of the lead is evaluated by energy consumption of a refrigerator at room temperature. We have found the optimum gas flow rate through the lead to obtain the minimum energy consumption. Comparing evaporated gas cooled with SHE force-cooled power lead, we concluded the latter is superior to the former from the energy cost.