To clear up from a hopefully unbiased source, this reference cites that LED's use 20%-30% electricity and I think much of what is repeated in the grow community originates from this bit of research unless someone can show me earlier publishings of these findings. This supports the worst case of get at least 1/3 as much wattage in LED's to match competing non-LED technology.

http://www.ledgrowlights.com/univers...nn%20study.PDF

The other benefits are listed in this PDF as well. This PDF is hosted by a seller, but it appears to be legitimate research on effectiveness of LED growing.

To break even on costs takes a number of years of electricity savings. But if you goal is to be cooler, quieter, or your operation isn't big enough that the cost is a factor, the cost isn't the motivation.

Anyway, I hope this 1/3 to 1/5 ratio on wattage is a good rule of thumb to keep in mind in answer to your question.

Also curious to who can put together the lowest cost per watt on LED lighting. It would be nice to see $2/watt.