Agreed. Also, my figures for eink power consumption are very uncertain; possibly you were able to take measurements or get good datasheets, but I couldn't.
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.
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.
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.
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.