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Vinlander:
Solar panels that heat water directly are potentially 100% efficient whereas retail PV panels are around 15% and cost exponentially more for higher efficiencies.

Admittedly thermal methods (such as a blackened metal bottle) drop off rapidly as the water gets hotter - only about 50% efficiency as it approaches the temp of a hot beverage, but pretty quick for black instant coffee - and you could always have another black bottle for milk.

PV ones use more visible than IR (more energetic electrons).

So you might think that clear days are essential for thermal, but both IR & visible* get through an overcast, and dark clouds basically stump both systems.

I suppose the sensible way to get to 100C is a black bottle with a reflector behind it to about 50-65C and then use your PV-charged lithium battery & element -  that's if you absolutely have to use dried leaves in boiling water (or you don't even like modern coffee essences).

Cheers.

PS. * I didn't have time to find a % comparison between thermal and PV in an overcast - the answer must be out there, but buried >100 dud answers deep by both industries - is it just (pathetically normal) sloppy thinking? why would both want it kept quiet?

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