An "energy efficient" machine doesn't use much electricity. It's built to save energy. Newer light bulbs, air conditioners, televisions, etc. are usually more "energy efficient" than older ones.

The word "efficient" can also be used in these compounds:

a fuel-efficient vehicle

a time-efficient method

If you're wondering why "fuel-efficient" and "time-efficient" are hyphenated, it's because they appear before a noun. Look at this example:

Their new cars are more fuel efficient.