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Since everyone else gave a joke answer I’ll take a stab in the dark and say the upper limits would be the availability of hydrogen and physical limitations in transforming heat output into electricity. The hydrogen is the most common element but 96% of it is currently produced from fossil fuels. After that, it would be how well you can scale up turbines to efficiently convert heat to electricity.
Never used the Roku stick, but I used their high end wired Internet model for a long time and it was great for what it was. I haven’t tried any of the stick devices because I always questioned the performance they would have.