An activity monitoring device , comprising: a support structure comprising a first end spaced apart from a second end along a first axis , the support structure further comprising a first side configured to be exposed to an external environment, and a second side , opposite the first side along a second axis , the second side configured to be positioned proximate to an area of skin of a user; a processor; an activity monitoring circuit coupled to the support structure ; a non-transitory computer-readable medium comprising computer-executable instructions that when executed by the processor perform at least: obtaining sensor data from the activity monitoring circuit; and based upon the sensor data, calculating athletic measurements based upon a user's athletic movements; and at least two series-connected thermoelectric generator modules configured to generate and transfer electrical energy to the processor and the activity monitoring circuit, wherein a voltage generated by the at least two series-connected thermoelectric generator modules is received as an input signal by the activity monitoring circuit, wherein when the voltage is below a threshold level, the activity monitoring circuit operates in a first power configuration, and when the voltage is above the threshold level, the activity monitoring circuit operates in a second power configuration, and wherein the at least two series-connected thermoelectric generator modules are configured to generate electrical energy in response to a thermal gradient between the first side and the second side . a support structure comprising a first end spaced apart from a second end along a first axis , the support structure further comprising a first side configured to be exposed to an external environment, and a second side , opposite the first side along a second axis , the second side configured to be positioned proximate to an area of skin of a user; a processor; an activity monitoring circuit coupled to the support structure ; a non-transitory computer-readable medium comprising computer-executable instructions that when executed by the processor perform at least: obtaining sensor data from the activity monitoring circuit; and based upon the sensor data, calculating athletic measurements based upon a user's athletic movements; and obtaining sensor data from the activity monitoring circuit; and based upon the sensor data, calculating athletic measurements based upon a user's athletic movements; and at least two series-connected thermoelectric generator modules configured to generate and transfer electrical energy to the processor and the activity monitoring circuit, wherein a voltage generated by the at least two series-connected thermoelectric generator modules is received as an input signal by the activity monitoring circuit, wherein when the voltage is below a threshold level, the activity monitoring circuit operates in a first power configuration, and when the voltage is above the threshold level, the activity monitoring circuit operates in a second power configuration, and wherein the at least two series-connected thermoelectric generator modules are configured to generate electrical energy in response to a thermal gradient between the first side and the second side . <EOS>