The Thermoelectric Generator [TEG], located on the base of the KettleCharge, uses a difference in temperature to produce electricity. This comes from having a hot side and a cold side of the TEG.
Add water to the stainless steel kettle. The water serves as the cold side of the TEG.
The flame of your heat source in contact with the baseplate is the hot side of the TEG.
As you bring the water to a boil, electricity is produced and sent to the Power Handle, leaving you with potable water and 10W of usable power.