Get a new fan and volt meter if your going to mess with DC voltage. You do not want to put a device you have been "experimenting" with in your grow OP. And as for CELL PHONE CHARGERS, forget that crap, i doubt they can handle more than 1 fan.
Strip the wires down and put your meter on it to determine polarity (+/-), NEVER touch the bare wires together while plugged in, or within probably 30 seconds after unplugging it. Get black tape too, youll need it anyway. Usually there is a dash doing down the wire, that is usually the (+), look closely.
Polarity on a meter will those a (+/-) in front of the metered voltage, when it shows (-) reverse your leads, it should disappear. If so, the wire under the red is (+).
Just go to RS and tell them you want a 2amp (or better) 12VDC power supply, if your power supply starts running too hot, your drawing too much current and you need to upgrade. That huge power supply you showed will run all of the basic 12VDC devices you need, but you'll want to design your OP to make use of all of that current, or its just a wast of space, unless you have tons to spare.
GOOD LUCK!!