What does it even mean for a TV to be energy efficient? A TV's job is to convert electricity into... light, I guess? It's not light like a light bulb, nor would you want it to be. You wouldn't light your home with TVs.
So is it something like "only produces light where needed on the screen" or "uses the least energy when turned off" or "does image processing for x format with the least energy" What are they measuring here?
Yes,measuring the energy consumption and efficiency of the conversion from electricity to light is part of the label. The light bulb example is not as unsuitable as you seem to assume. Incandescent light bulbs only convert about 5 percent of the energy consumed to light, the rest is converted to heat. LEDs, in contrast, are 6 to 8 times more efficient.
Your "does image processing..." part is also important. Different processors consume vastly different amounts of energy. A hobbyist example would be trying to run something on a battery powered ESP32 with or without deep sleep.
I meant it's it's physics job to produce light. If it does that efficiently, great! That's one thing you should want.
It's human job is to convey information. If you're measuring efficiency that way, it would be most efficient to have the dimmest screen possible (using the least energy) as long as you can still tell what's happening on screen. If that's already happening, then surely a screen twice as bright as you need it to be would use twice the energy and be half as efficient, even if it physically is equally as efficient making light at any brightness.
You could be using half the energy to do the "same" job.
It's the electricity usage when running the TV. There is e.g. quite a difference between LED and OLED.
TVs is also quite a large part of electricity usage in a household. Maybe 75 - 150W running a few hours a day. You have to keep in mind that not many people have PCs, NAS, etc. running 24/7.
So is it something like "only produces light where needed on the screen" or "uses the least energy when turned off" or "does image processing for x format with the least energy" What are they measuring here?