Depend`s on what led`s your buying mate
you can finish a pretty big size plant under 50w of led but with auto`s you could try going as low as 30w per plant but remember most led lights are rated as the power of the leds fitted to them and not the actual power the leds are being driven at.
Another tip, keep auto`s under low light for the 1st week or so
I really pushed the last batch I did and they went into flower so quick that they did not get a chance to grow as big as I`d have liked.
Saying that I was testing how well a light set up I had would induce flowering so I guess the test was a success