A system comprising: a generator including: an output stage configured to generate energy to treat tissue; and a controller coupled to the output stage and configured to control the output stage; and an energy delivery device (110, 130) coupled to the generator and configured to treat the tissue, the energy delivery device including: an electrode configured to deliver the generated energy to the tissue; a plurality of temperature sensors (430, 440) including first and second temperature sensors, the at least one temperature sensor being configured to be in thermal communication with the tissue and at least one temperature sensor being configured to be in thermal communication with the electrode, wherein the controller comprises: a signal processor configured to estimate mass of the tissue, and a thermal resistance scale factor of the tissue and the electrode, based on the sensed temperatures of the tissue and the electrode, wherein the thermal resistance scale factor accommodates for a ratio of heat conductivity between the tissue and the electrode; and an output controller configured to generate a control signal to control the output stage based on the estimated mass and the estimated thermal resistance scale factor. a generator including: an output stage configured to generate energy to treat tissue; and a controller coupled to the output stage and configured to control the output stage; and an output stage configured to generate energy to treat tissue; and a controller coupled to the output stage and configured to control the output stage; and an energy delivery device (110, 130) coupled to the generator and configured to treat the tissue, the energy delivery device including: an electrode configured to deliver the generated energy to the tissue; a plurality of temperature sensors (430, 440) including first and second temperature sensors, the at least one temperature sensor being configured to be in thermal communication with the tissue and at least one temperature sensor being configured to be in thermal communication with the electrode, an electrode configured to deliver the generated energy to the tissue; a plurality of temperature sensors (430, 440) including first and second temperature sensors, the at least one temperature sensor being configured to be in thermal communication with the tissue and at least one temperature sensor being configured to be in thermal communication with the electrode, wherein the controller comprises: a signal processor configured to estimate mass of the tissue, and a thermal resistance scale factor of the tissue and the electrode, based on the sensed temperatures of the tissue and the electrode, a signal processor configured to estimate mass of the tissue, and a thermal resistance scale factor of the tissue and the electrode, based on the sensed temperatures of the tissue and the electrode, wherein the thermal resistance scale factor accommodates for a ratio of heat conductivity between the tissue and the electrode; and an output controller configured to generate a control signal to control the output stage based on the estimated mass and the estimated thermal resistance scale factor. an output controller configured to generate a control signal to control the output stage based on the estimated mass and the estimated thermal resistance scale factor.