If you’re using a thermo-electric generator like the Marlow Industries EverGen Mini Harvester EHA-PA1AN1-R03 and finding the output to be a bit unpredictable, one cause of this might be your heat source. The peltier module that is used to harvest energy from the heat requires a temperature difference from the base of the TEG (thermo-electric generator) to the heatsinks at the top. If the heat source is blowing across the whole TEG, its output will be weak and sporadic. The best way to apply the heat is to place the TEG on a conductive surface and heat that surface or use a non radiated heat source.
For further information check out the following links:
Manufacturer Datasheet - https://cdn2.hubspot.net/hubfs/547732/Data_Sheets/EHA-PA1AN1-R03.pdf