Based on your interest in "decent resolution", I suggest you avoid pico projectors. Their actual image quality is fairly poor, and probably will remain so until the technology matures.
What you need to do is pick a specific power target, say, <= 100 watts at 120v, and then begin looking.
That said, an LED backlit television will probably be the best mix of power efficiency, space efficiency, and picture quality. The Vizio VF551XVT is a 55" display which, when calibrated for low power usage, can use just under 100W.
LED backlighting is much more efficient with power than CCFL (the most common backlighting), but even then the brightness and backlight intensity can make up to a 100% difference in the power consumption of the set.
The Sony KDL-46EX700 can get down to about 65W if 100W is over the limit.
Unfortunately the best displays in terms of power efficiency, such as OLED, have never been successfully marketed as a real-world television display.