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Lithium-air battery would work. Additionally, you can use a sunlight-readable reflective display, which in principle only needs as much energy as an LCD, which is negligible.

...or imagine a display that includes a photovoltaic panel taking up most of the area of each pixel to power the device using ambient light, with a tiny electronically steerable LED that points in the direction of whoever is watching (and if multiple people are watching, just switch rapidly between them). The display would appear very bright but use so little energy that it could be powered by ambient light. In principle.

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