No electrical outlet... what's my best option?

So I'm ready to get started with setting up a grow room in my attic, but right now I really only have one major hangup: electrical outlets.

The attic has a 20 amp circuit dedicated to it, but no one has installed any outlets. There is a 12/3 NM-B cable that comes out of the insulation in the wall and goes to a switch box with two switches. One of them controls a light socket, the other controls a small fan that can be turned on to vent hot air out of the attic, both of which are along the same 12/3 cable, hardwired rather than plugged into outlets. There don't appear to be any other wires attached to the attic circuit.

I have seen little outlet things you can screw directly into a light fixture, but unless I'm mistaken those are 2-prong-only, as a light bulb has no ground wire. Since I really don't want my grow op to be ungrounded, I guess I'll have to try my hand at some layman electrician work, which sounds a little scary to me. I imagine I'll have to take the cover off the box for the light socket (after turning off the breaker, of course!) and swap the wires onto an electrical outlet, and then screw that in place instead. Presumably the two are fairly easily swapped out, with just a screwdriver, right? I did some searching on here but didn't find any good tutorial for such a project. The best I could find on the wider internets was this: Electrical Wiring in the Home: Can I replace a light fixture with an electric outlet?, silver screw, brass screw

You can just buy a 50 ft orange drop cord and run it to your attic. If you would go through the trouble to ghetto rig a lightbulb you could just use a orange cord , but stealth it out. like paint it white and run it cleanly to your attic. That's what I would do. boom.

That is an option I didn't mention, but since I have a dedicated 20 amp circuit for the attic, I'd really like to make use of it, rather than putting more strain on other circuits. There's also not a grounded outlet near the attic hatch (it's an old house and only a few outlets are 3-prong), so I'd probably have to run it out a window or through the air ducts, either of which will look a little obvious.

From what I understand, replacing a light fixture with an outlet is simply a matter of unscrewing a plate, switching the three wires (black, white, and bare/green) from the fixture to the outlet, and then screwing the new plate on. That seems like the better option to me. I mean I took a semester of Circuits before I changed majors way back in college, so I vaguely understand all the principles. I was hoping someone on here might have experience with this sort of thing and be able to confirm that this is as easy as it looks to be. Anyone?

What is the total draw your planing on pulling that's where i would start, (fans,Lights< ect)
and with an attic fan based on size and if you plan on utilizing it, (normally 10-12in 6 amps)
it needs to be in your figure before adding amps to you circuit. (20 amps now total)
an attic is a long draw from any receptical.

My advice,
figure out when you want run your grow, night better house consumption low.
go to power box get breaker out, and go to local hardware store and get 30amp replacement,
no larger because romex will get to hot during operation and it will end up causing a fire or
costing you double to run your grow cause of distance and draw.
pull light fixture out (remove breaker or turn off),
take with you and ask for a 30amp/120 gfi protected circut like in a kitchen,
ask how to install ( very simple) and their you have it electrical 101 and with safety.
cost approx 35bucks.

let me know i need to put a receptical for my new 12in in attic before summer ill do quick vid and post on you tube for ya.

@ 120 1000,hps draws 10amps with your distance
@ 240 1000, hps draws 5 amps

food for thought the longer the draw the the hotter the more it cost....

Thank you for the detailed reply! I very much doubt I'm going to need more than 20 amps -- right now I'm planning an all-CFL scrog or sog as the first grow to test out the chamber, then maybe upgrading to 400-600 hps if everything else seems to work out okay.

As far as draw on the circuit, the attic fan appears to be burned out (also looks like it is 60 years old), so I was planning to just run my exhaust duct to the vent where the attic fan is currently installed. That means for my first grow I'm probably looking at no more than 300-ish watts of CFLs, plus an exhaust fan (still haven't decided what size), plus a new light fixture for the attic (which I will plug into my outlet). Even at the largest I can imagine going in the future, I'm looking at about half the wattage of HPS from your example, and no attic fan.

So should a 20 amp outlet suffice?

Just thought I'd post a quick update. I was easily able to swap out the light fixture for an electric outlet, and it all seems to work fine now. I don't think I clipped off quite enough of the wires, so it was a bit of a tight squeeze to get the plate on at the end, but I managed.

Next step: building the grow cabinet. I'll update once I make some progress on that front. Unfortunately I don't think I can post photos yet, but I will try to do that at some point.

20 amps should be fine looking forward to your pics..... be safe...!