yes, that's right. so there's a refresh rate crossover point at which e-ink actually uses less power. my calculations from the very uncertain information i have is that it's around 20 minutes. that is, if you update the display once every five minutes, the e-ink display will use significantly more power than the memory-in-lcd display updating 60 times a second. there are real limits to the utility of a computer that needs several minutes to redraw its display; though i wouldn't venture to say that it's useless, you can't do anything similar to a conventional gui on it
The hisense e-ink phones are known to have week long battery lives (A5 and A9) so I'm thinking not all e-ink is the same. I know there's e-ink nerds out there and forums dedicated to it but I don't actually know the different types (here's an overview I think: https://en.wikipedia.org/wiki/Electronic_paper). Maybe the good stuff is harder to find
the amazon swindle is known for a month-long battery life, so a week is easily believable. but you're talking about how to solve a different problem
if batteries are an option, e-ink is a super-low-power option because it uses 100 milliwatts or maybe 10 milliwatts at a cellphone size, while a conventional backlit color lcd uses 1000. but to run off solar panels, indoors, my power budget for the whole zorzpad is 1 milliwatt, and the screen can only have a fraction of that