A thing I didn't mention is that there are actually eink applications that have lower update rates than this. Store price tags, for example.
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https://www.pervasivedisplays.com/wp-content/uploads/2021/12...
Pervasive Displays has an oscilloscope measurement of 3.75mA on their 5.9-second updates for their 1.54" eink screen. (Page 14). That's 22.125 mCoulombs of electrons.
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https://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/LS0...
Sharp 1.2" memory-LCD (a bit smaller) has 12uW to 50uW on static images. Assuming 3.3V, that's 15uA worst case (0.015mA).
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22.125 mCoulombs / 0.015mA == 1475 seconds per update break-even point, or ~24 minutes.
So yet another number, but yeah, memory-LCD is just really good. Remember that the 1.2" screen from Sharp is a bit smaller though, so its not a perfect apples-to-apples comparison.
Either way, its a much longer period between updates than I was expecting.
There's a lot of different eink and memory-lcd screens that could explain the difference between your calculation and mine. But all else considered, 15-minutes vs 60-minutes isn't much of a difference at all.
For most applications of updates-per-second (or at worst, a few seconds per update), low-power monochrome seems best served by Sharp's memory-lcd.