An electronic reader can run for weeks while a tablet lasts a day. The difference is not battery size but a display that consumes nothing while showing a static page.
The image is made of physical particles
Each pixel contains microcapsules holding charged particles suspended in fluid, typically white particles carrying one charge and black particles the opposite.
Applying an electric field pulls one set toward the viewing surface and the other away, which makes the pixel appear light or dark.
Once the field is removed the particles remain where they are, held by the viscosity of the fluid. The image persists with no power applied at all.
Power is consumed only by change
Because the state is physical rather than electrical, the display costs energy only during a page turn. Reading a page for ten minutes costs nothing.
Conventional displays must refresh continuously, driving every pixel many times per second whether or not anything moved, and backlighting adds substantially more.
This is why a device with a comparatively small battery can operate for weeks, and why its battery life is measured in page turns rather than in hours.
Reflectivity replaces backlighting
Electronic paper reflects ambient light in the same way printed paper does, rather than emitting light toward the reader.
That removes the largest power draw in a conventional display and is also why the screen is readable in bright sunlight, where emissive displays are overwhelmed.
Front lights added for darkness illuminate the surface from the edge rather than from behind, and they are optional rather than fundamental to the display working.
The trade is speed and colour
Moving particles through fluid takes time measured in fractions of a second, which is fine for turning a page and unsuitable for scrolling or video.
Ghosting occurs when particles do not fully clear their previous positions, which is why devices occasionally flash the whole screen to reset every pixel.
Colour requires giving something up
Adding colour generally means placing filters over the pixels, which reduces the amount of light reflected and produces a muted appearance compared with the black and white version.
Alternative approaches use additional coloured particle types, which increases the time needed to arrange them and slows updates further.
Either way the underlying property remains the same: the display stores its image mechanically, and that is what makes the power consumption so unusual.