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Does displaying black screen (but not turned off), consume less power than a white screen? Or is the screen just blocking light to show a black pixel and not blocking for not showing it?

And, if I go to the command line in Linux (tty3), would the monitor spend less energy?

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On an LCD monitor, liquid crystal blocks light from a backlight, provided by white fluorescent tubes or, more commonly now, white LEDs. Therefore a black pixel doesn't reduce power usage in and of itself.

If a monitor is displaying all-black, most will reduce power to the backlight accordingly, primarily to produce a deeper black, and of course also to save power.

There's a question on Skeptics.StackExchange discussing this concept.

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    \$\begingroup\$ At least on passive LCD displays, energized pixels or segments use up noticeably more power than non-energized ones; I'm not sure about TFT displays. The power difference is tiny compared with the power used by a typical backlight, but can be important in battery-powered applications. A certain portion of display current consumption is also proportional to the number of separately-scanned rows; some display controllers can save power by partially-blanking the display, but such abilities are not generally well supported (I'm not sure why). \$\endgroup\$ – supercat Mar 20 '14 at 23:06

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