Wow... Bunch of ignorant responses so far.
Indoor growing can bitch slap the fuck out of the sun/outdoors. A hardy plant can probably handle up to 20,000-25,000 lm/sq ft. This is twice the PEAK sun output here on earth. That's peak output, it's a sine wave, basically. The average is between 6000 and 7000 lm/sq ft(varies depending on your 'artificial' day). That's all you need of a constant radiant output to match outdoors using 16/8, 18/6, or 12/12. You could go lower with longer days(eg. 20/4). All that really matters is the total light collected over the day(obviously your other environmental factors have to be in check).
Plus you can CO2 dope, and perfectly control the environment. No cloudy days, no fucking bugs! Etc. Indeed, the finest weed is grown indoors.
Then again, no doubt the sun is a hell of a lot cheaper, and gives great results. As long as nature doesn't fuck you over.
It takes much more than light to grow weed, just remember that.
Genetics, care, light. That's how I see growing in stages of importance. You obviously need the plant. It needs to be taken care of(whether by nature or you). Lastly, the light. It is by no means unimportant, just least important. It should be something you basically 'forget' about, besides paying the bills for uhm.
You can focus on the lights all you want, if you don't focus on the genetics and the care, you'll still end up growing nothing special.
