I've wrestled with this too, as at-anchor I have decided to use Bebe Owl lights at deck level (one each side of the boat, used mainly in crowded anchorages to ward off drunken dinghy operators who don't look up) and a Dr LED masthead replacement bulb in a Hella fixture. All combined use lees than a third of what a single 10w masthead uses. The Owl lights are brighter than "approved" 2nm incandescent lights by quite a bit -- but they are not "certified". The Dr LED Polar Star 40 light is "approved" when used in an Aqua Signal series 40 fixture but not in a Hella.

I asked Dr LED:

"I realize you haven’t USCG certified the polar star 40 in a Hella fixture, but I want to know if you know of any reason it would not produce the same test results?

Answer:(nl)It will have the same result but we do not want to pay for every single fixture out there."

I feel pretty comfortable that I would not be found negligent if I walked into a courtroom with a 12v battery and the three fixtures, along with a 4th "approved" fixture equipped with an incandescent bulb and requested them to dim the lights while they are all powered on side-by-side for comparison. You can also bet your sweet bottom that if someone rammed my boat while anchored I'd be demanding the local authorities run a sobriety test on the offender, because those Owl lights are so f-'n bright I've been asked by nearby boats to turn them off when anchored for fireworks.(nl)