High pressure sodium lights are very efficient. The larger ones are more efficient than the best LED lights, so do not expect to get an energy savings by simply replacing HPS with LED.
The fixtures, on the other hand, may be a different story. Typical yard lights, such as the lights seen in every farm, put half of their light up into the sky. An efficient fixture puts all of its light down toward the ground. The simple test is to look at the light at night from a block away. If the fixture is bright, the efficiency is low. If you can barely see if it's on, it is efficient.
An efficient fixture puts 90% of the light down toward the ground. An inefficient fixture puts half the light into the sky, and less than half down toward the ground.
An inefficient fixture can be replaced with an efficient fixture that uses a bulb half the size (half the watts). You will get the same light on the ground, half the energy consumption, and eliminate the glare.
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