As a remedy (rather power-saver-option) the phone incorporates a double display. When in active use the AMOLED screen powers up in all glory and in standby mode the E-ink display gets active. This essentially means that the E-ink uses very little energy and the degree of transparency of the display shows how much battery is remaining. The more transparent the display, the less battery remaining.
So besides ultra-low power consumption, the transparent screen offers an intuitive expression: “You’re low on juice so better recharge the phone soon!” Apparently the E-ink uses power to change the degree of transparency and doesn’t add to the battery load.